Warming Up to Chillers

Chilled water cooling systems, also known simply as “chillers”, are a popular choice for commercial air conditioning repair and institutions such as schools and hospitals. A chiller is a compressor based cooling system that is similar to an air conditioner except it cools and controls the temperature of a liquid instead of air. Large commercial buildings that require a substantial amount of cooling often use water chillers because they are cost effective and there is a reduced hazard by not having refrigerant piped all over the building. The chiller will provide a stable temperature, flow and pressure once it has been programmed by a user for their individual needs. Harmful particles are kept out of the system by an internal strainer. Air-cooled chillers, meanwhile, can be located in open spaces like car parks, roofs or ground level areas and have the advantages of a relatively low installation cost and low maintenance. They can also do without a plant room, cooling tower or condenser pumps. The advantages are no heat buildup in a room and no need for ventilation. Noise level is greatly reduced since there is no fan operation.

  Both a chiller and a cooling tower are used to remove heat from a liquid, which is used as a coolant in large devices like power stations. A cooling tower removes heat from the water that is discharged from a condenser. The discharged water is then recycled back into the plant to be used to cool the system again, or discharged into the environment. Chillers absorb heat from a coolant, which is fully contained in a cooling system. The chiller then transfers heat to the air around the chiller unit. Though chillers and cooling towers perform similar functions, they vary according to their types and components used, and the nature of the equipment they cool and power. A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream (such as air or process water). As a necessary by product, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. The industrial chiller is a cooling system that removes heat from one element (water) and transfers it into another (ambient air or water). The other main components to a chiller are a temperature controller, a recirculating pump and a reservoir. Operation and setup is simple. Fill the reservoir with fluid to be recirculated, typically water or an ethylene glycol/water mix. Install plumbing between the chiller and the application and provide power to the chiller. The controller regulates the chiller’s functions. A portable chiller is a liquid cooling system on casters that can be relocated from one application to another with relative ease. It can be used to cool one or more heat generating devices. Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional facilities.

Need for Chillers

Equipment Protection

  The most compelling reason for a chiller is the protection it provides to our valuable processing equipment such as spot welders, injection molding equipment and other applications. A chiller commonly represents a small fraction of the cost of the processing equipment, yet it provides solid protection of our investment, 24-hours-a-day, seven-days-a-week for years and years to come.

Increase Production

  The speed and accuracy of production will increase as we maintain a constant and proper cooling temperature in the equipment. A chiller will reduce the number of rejected parts while increasing the number of parts produced per hour.

Chiller Types

Air-cooled Chiller

  These chillers absorb heat from process water and can be transferred to the surrounding air. Air-cooled chillers are generally used in applications where the additional heat they discharge is not a factor. They require less maintenance than water-cooled units and eliminate the need for a cooling tower and condense water pump. They generally consume approximately 10% more power than a water-cooled unit as a wet surface transfer’s heat better than a dry surface.

Water-cooled Chiller

  These chillers absorb heat from process water and transfer it to a separate water source such as a cooling tower, river, pond, etc. They are generally used for large capacity applications, where the heat generated by an air-cooled chiller creates a problem. They are also considered when a cooling tower is already in place, or where the customer requires optimum efficiency of power consumption. Water-cooled chillers require condenser water treatment to eliminate mineral buildup. Mineral deposits create poor heat transfer situations that reduce the efficiency of the unit. Water chillers can be water-cooled, air-cooled, or evaporative cooled. Water-cooled systems can provide efficiency and environmental impact advantages over air-cooled systems.

Chiller Designs

  One chiller cannot control every heat load. Some chillers are designed to cool to very low temperatures while others are designed for only mid-range applications. Some designs can support very high flow rates of fluid while other may be designed for just a trickle of fluid. The same issues apply with ambient temperatures. Some chillers use refrigerant suited for a high ambient temperature environment while other refrigerants are formulated for cooler conditions. The customer must also consider the fluid being cooled. Distilled water or di-ionized water requires different conditions than tap water. Di-ionized and distilled water can cause the breakdown of metal they come in contact with. In cases like this the chiller is designed with no brass, copper or mild steel components that would come in contact with the water, instead, plastic or stainless steel are used. This eliminates the corrosive effects of the fluid.

• Refrigeration Compressors - are essentially a pump for refrigerant gas. The capacity of the compressor, and hence the chiller cooling capacity is measured in kilowatts input (kW), Horse Power input (HP), or volumetric flow (m3/h, ft3/h). The mechanism for compressing refrigerant gas differs between compressors, and each has its own application. Common refrigeration compressors include Reciprocating, Scroll, Screw, or Centrifugal. These can be powered by electric motors, steam turbines or gas turbines. Compressors can have an integrated motor from a specific manufacturer, or be open drive - allowing the connection to another type of mechanical connection. Compressors can also be either Hermetic (welded closed) or semi-hermetic (bolted together).

• The condenser is a heat exchanger which allows heat to migrate from the refrigerant gas to either water or air. Condensers can be air-cooled, water-cooled, or evaporative. Air cooled condenser are manufactured from copper tubes (for the refrigerant flow) and aluminum fins (for the air flow). Each condenser has a different material cost and they vary in terms of efficiency. With evaporative cooling condensers, their coefficients-of-performance (COPs) are very high; typically 4.0 or more.

• Evaporators can be plate type or shell and tube type. The evaporator is a heat exchanger which allows the heat energy to migrate from the water stream into the refrigerant gas. During the state change of the remaining liquid to gas, the refrigerant can absorb large amounts of heat without changing temperature.

Latest Developments

• In recent years, application of Variable Speed Drive (VSD) technology has increased efficiencies of vapor compression chillers. The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll technology compressors.

• The expansion device or refrigerant metering device (RMD) restricts the flow of the liquid refrigerant causing a pressure drop that vaporizes some of the refrigerant; this vaporization absorbs heat from nearby liquid refrigerant. The RMD is located immediately prior to the evaporator so that the cold gas in the evaporator can absorb heat from the water in the evaporator. There is a sensor for the RMD on the evaporator outlet side which allows the RMD to regulate the refrigerant flow based on the chiller design requirement.

Chiller Applications

Use in air conditioning

  In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handling units or other types of terminal devices which cool the air in their respective space(s). The water is then re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. A typical chiller for air conditioning applications is rated between 15 and 2000 tons, and at least one manufacturer can produce chillers capable of up to 5,200 tons of cooling. Chilled water temperatures can range from 35 to 45 F (2 to 7 C), depending upon application requirements. When the chillers for air conditioning systems are not operable or they are in need of repair or replacement, emergency chillers may be used to supply chilled water. Rental chillers are mounted on a trailer so that they can be quickly deployed to the site. Large chilled water hoses are used to connect between rental chillers and air conditioning systems.

Use in industry

  In industrial application, chilled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries. They are often used in the plastic industries, injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers, and in hospitals, hotels and campuses. Chillers for industrial applications can be centralized, where a single chiller serves multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both centralized and decentralized chillers, especially if the cooling requirements are the same for some applications or points of use, but not all. Water-cooled chillers are typically intended for indoor installation and operation, and are cooled by a separate condenser water loop and connected to outdoor cooling towers to expel heat to the atmosphere. Air-cooled and evaporative cooled chillers are intended for outdoor installation and operation. Air-cooled machines are directly cooled by ambient air being mechanically circulated directly through the machine's condenser coil to expel heat to the atmosphere. Evaporative cooled machines are similar, except they implement a mist of water over the condenser coil to aid in condenser cooling, making the machine more efficient than a traditional air-cooled machine. No remote cooling tower is typically required with either of these types of packaged air-cooled or evaporative cooled chillers.

Advantages

  Chillers are a popular choice for commercial air conditioning repair and institutions such as schools and hospitals. If you are in the process of making decisions about your future cooling needs, then chilled water cooling should be one of the systems on your list. Below are a few advantages that chillers offer over other cooling systems and a few disadvantages that you should also keep in mind. The advantages offered by chilled water cooling systems:

  Safer for humans: Safety should always be a primary concern when making decisions about the environment in which people live, work and play. Chilled water systems are fundamentally safe due to the use of non-toxic, chemically-stable water as the refrigerant; chillers don’t require that potentially-hazardous refrigerants be circulated throughout a building in close proximity to occupants.

  Cost effective: A chilled water cooling system can cut energy costs by up to one-half if it utilizes the latest in high-efficiency equipment. Water is better at absorbing heat than air, and this fundamental fact of physics means that it will always have an advantage in this regard. Not only that, water is plentiful and cheap; eliminating the need to use costly refrigerants can contribute greatly to the overall cost savings.

  Sheltered from elements: The operational machinery for chilled water cooling systems, except for cooling towers, is typically installed in a mechanical room, basement or other interior space. This means these complex components, such as evaporators and condensers, are less exposed to the elements than systems that are mounted on rooftops or in exterior locations. Less exposure to rain, ice and heat can extend the lives of these components by several years.

  Quiet Operation: Another advantage offered by chillers is they operate at much quieter levels than air cooling systems. The flow of water through the system is less susceptible to the expansion and contraction that causes air to affect mechanical components such as ducts and vents. This degree of quietness is important for building occupants, particularly in sensitive environments such as hospitals and schools where noise would otherwise be unhealthy or distracting.

Disadvantages

  Cooling Towers: Chillers utilize external cooling towers to transfer heat to the atmosphere, and these structures can be costly to build. They don’t need to be located immediately adjacent to the building that holds the operational machinery, but they do utilize valuable real estate which adds to the cost. Cooling towers are also unsightly, and the water vapor generated during operation can be uncomfortable for those who pass by these structures.

  Enhanced Maintenance Needs: Since chilled water cooled systems use water for transferring heat, this exposes the water to a variety of conditions that can create scaling. Scaling is an accumulation of deposits on metal, and this can cause corrosion as well as decrease system efficiency. To control the problems associated with scale, the water used in chillers must be treated to remove impurities that can lead to scaling. In addition, periodic inspection and cleaning of the chiller’s internal machinery and components will be necessary. This necessitates downtime for scheduled maintenance and added maintenance costs.

  Less Effective in Humid Environments: Chilled water cooling systems don’t work as well in climates with high prevailing humidity. Higher levels of humidity raise the wet-bulb temperature, which is an indicator of how efficiently water absorbs heat. An increase in wet-bulb temperature corresponds with increased operating costs as well as lower comfort levels due to the higher ambient humidity. Chillers can create a cold, clammy feeling for occupants if the humidity is too high. In this scenario, air cooled systems are much better at extracting moisture from the air.

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AUTHORS CREDIT & PHOTOGRAPH

Dr S S Verma
Department of Physics
S L I E T
Longowal, Punjab

Source: http://coolingindia.in/blog/post/id/14314/warming-up-to-chillers

Retrofitting to improve performance

Air cooled chillers are a preferred choice in designs where availability of water is scarce and the weather is hot and dry for a majority of the year. Air cooled chillers offer many advantages such as the lesser number of system components (lowering maintenance costs), compact design, effective use of open space, lower operating costs etc. Traditionally, Air cooled chillers were used for smaller applications with lower heat loads, but in the last 10 -15 years, with the advances in centrifugal compressor technology, Air cooled chillers are being used in larger, more critical installations. Multirole compressor configurations have allowed the designers to offer designs for a wide range of loads thus further optimizing on the cots. The Air cooled chiller also scores better on the overall life cycle costs since the operating costs are lower due to absence of the condenser water systems and cooling towers.

  Since condensation of the refrigerant is undertaken using the ambient air, the heat exchanger is a key component of the Air cooled chiller and the performance of the plant is directly affected by the performance of the heat exchanger. Deterioration of the heat exchanger effectiveness will have a direct impact on the capacity of the chiller to provide desired cooling to the work space. Unfortunately, the performance of the heat exchanger is dependent on a number of factors which all contribute to lowering heat transfer rates as the chiller operates over time, thus lowering the system efficiency as the plant ages. System performance can be dramatically improved by replacement of the condenser cooling coils when system performance has deteriorated beyond acceptable levels. This article showcases a case study of the retrofit of the condensers coils of an air cooled chiller and the benefits that accrued to the owners

System overview

  The air cooled chiller that this case study is based on is part of 3 chillers system for air conditioning a large office space which has 24x7 operations. The building is located in the northern part of India, which experiences long hot and dry summers and a short monsoon season. The plants are located on the terrace of the building and the location of the building is such that there is adequate air flow across the terrace, without any large buildings obstructing the flow patterns. The key characteristics of the chiller are listed in table 1. The operations and maintenance of the chillers is undertaken by a dedicated onsite team and the systems are maintained to a high standard.

Need for retrofit

  The chillers were installed in 2007 when the building became operational. As part of the M&M teams annual maintenance shutdown activities in late 2015, performance assessment of the 3 chillers was undertaken. The chillers physical condition, efficiency, operating parameters etc. were analyzed from the system logs as well as onsite visual assessment. While the typical life of Air cooled chiller is 12 – 15 years, the performance of the chillers was observed to be below the design parameters, with higher condenser pressures and lower EER (Energy Efficiency Ratios) or IKw based on calculations. The reasons for the deterioration of the performance was analyzed and the problem identified as the ineffective heat exchange across the condenser, leading to higher system pressures and consequent lower efficiency. The heat exchanger fins were seen to have a high level of deterioration as well. The cause of the higher level of deterioration of the heat exchange surface was attributed to the ambient conditions. The building was located in a zone where the last 4 – 5 years had seen a large number of building construction activities, leading to higher dust levels and increase size of suspended particulate matter in the air.

  Since the condensers performance effects the overall chiller performance, a decision was taken to replace the condenser coils for one of the chillers due to the limited time available for shut down provided by the Maintenance team.

Business case for the retrofit

  Condenser coils replacement involves both time and financial outlays. While the performance of the chiller was not as per design, the overall air conditioning system was delivering required thermal comfort to the occupants and workspace due to the redundancy and design of the system. However, the energy costs of the system had been increasing steadily over the past 4 – 5 years. There was thus a case to undertake corrective action to lower operating costs.

  Chiller energy performance was reviewed from the BMS system data and annual energy consumption and correlated costs documented to create the baseline. With the help of the chiller manufactures, cost estimates were developed for the heat exchanger replacement and the projected efficiency gains calculated. The new heat exchanger coils evaluated enabled higher heat exchange rates due to the advances in fin design that had taken place since the chiller was installed. The key costs associated with the chiller heat exchanger replacement were identified as

• Cost of the heat exchanger

• Import costs as the Heat exchanger coils had to be procured from the          OEM’s international locations

• Labor and material costs during the replacement phase

• Refrigerant top up costs

• Testing and commissioning costs post replacement

  The potential efficiency gains and subsequent lowering of energy consumption as a comparison to the cost of the replacement were analyzed. The resulting Return on Investment (RoI), based on a simple Net Present Value (NPV) approach was assessed to be approximately 18 months. The relatively short payback period and high efficiency gains helped get a sign off from the management for the replacement.

Why heat exchanger performance fails

  A Heat exchanger (HE) as the name suggest, is used to transfer heat across a surface. When the transfer is between two liquids, sensible heat is transferred, while if it there is a phase change, latent heat transfer takes place. Most HVAC applications involve sensible heat exchanger.

  The rate of heat transfer across a heat exchanger is given by the formula Q = UAΔtm where

• Q is the rate of heat transfer

• U is the Overall heat transfer coefficient

• A is the surface area across which the heat transfer takes place

• Δtm is the temperature difference between the two liquids

  In the above equation, U depends on the properties of the material and the distribution of the heat exchange surface such as cross flow or counter flow arrangements. The larger the areas A, the more the heat transfer that can occur and hence, HE design focuses a great deal on how to increase transfer area without increasing overall size. The temperature difference is a function of the system design as it depends on the inlet and out let temperatures required for the application in mind. Another important parameter in HE operations and design is the “approach Temperature. Refer to figure 1. The approach is the difference between T2 and t1. A smaller approach will need a larger surface area to get the desired heat transfer.

Figure 1: Heat Exchanger Temperatures...

  With the above overview of HE theory, we can now correlate why a heat exchanger’s performance can deteriorate during its life. The key reasons are

- Lowering of U: The heat transfer coefficient changes due to various factors but mostly due to fouling of the surfaces. The scales reduce the velocity of flow and hence the heat transfer rate in addition to the actual heat transfer. Scaling occurs due to poor treatment of the water or due to high temperatures in the system than designed. Other causes of increase in U dust and microbiological growth over the fins in air cooled chiller and algae growth in the headers and condenser plates of water cooled chillers.

- Reduction of surface area: In air cooled chillers, the fins across which heat transfer occurs are fragile and tend to get damaged over a period of time and use. The damaged sections, due to the reduced surface area are not able to contribute to the full extent to the heat exchange and the overall transfer rates reduce.

- Temperature difference: When the HE is selected, the plant load is used as the basis. If during the life of the plant, there is a significant change in the heat load, the HE will be operating at higher temperatures which will result in lowering of the efficiency.

Results of retrofit

  The retrofit was carried out in the month of Dec which is a typical lean month for HVAC operations. Post commissioning and operations, the plant’s functioning was compared with the other air cooled chiller in the system during the summer months. This helped the O&M team to quantify the actual gains due to the change in the heat exchanger. Table 1 lists the comparison of the operating parameters:

  The replacement of the chiller resulted in a savings of approx. 8 – 9 % on the energy consumption. In addition, the overall system efficiency was improved as the time taken to cool the workspace reduced. Indicated Kw, which is a direct measure of the energy output to the energy input for chiller was brought down from 0.998 to 0.58 due to the change of the heat exchanger coil.

Conclusion

  As air conditioning systems age, the performance of the plant starts to deteriorate from the design point, leading to higher powerconsumption and operating costs. The failure rates also increase due to the plant operating at off design values. While replacement may not always be an option, component level replacements that can increase the efficiency of the plant can be evaluated to keep costs low. There is an investment requirement for such changes as replacement of the HE coils, but with advances in technology, the new components tend to be significantly more efficient and hence, the returnof Investments period reduces. O&M team should thus identify opportunities such as the case study shared above to improve system performance as well as reduce operating costs.

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AUTHORS CREDIT & PHOTOGRAPH

Aneesh Kadyan
Director - Operations 
CBRE South Asia Pvt Ltd. 
Asset Services - India

Rajender Kasba
Mechanical Engineer
Working as DGM - 
Asset Services
(For a leading real 
estate services firm)

Source: http://coolingindia.in/blog/post/id/13377/retrofitting-to-improve-performance

Different Type Of Chillers & Their Application

A chiller is a heat transfer device that uses refrigeration system to remove heat from a process load and transfers the heat to the environment. Chillers may also be seen as cooling machines of choice to condition industrial, commercial, and institutional facilities.

  They are used to lower the temperatures of all kinds of equipment and processes such as: robotic machinery; semiconductors; injection and blow molding machines; welding equipment; die-casting and machine tooling; paper and cement processing; power supplies; power generation stations; compressed air and gas cooling systems; medical imaging machines; chemical, drug, food and beverage production; even simply to cool potable water to desirable levels. Whether for office comfort, keeping data server centres from overheating, or specialized industrial processes, water temperature control plays a vital role in many of the behind-the-scenes activities that affect our everyday lives. 

  A chiller consists of a reservoir that is filled with a fluid such as water or ethylene glycol/water mixture which is continually circulated. In a typical building application, chilled water is circulated to air-handlers or now the increasingly used chilled beams in order to transfer heat from air to water, or stated the other way, transfer cooling from the water to the building air. The schematic diagram of a chiller plant is shown in figure 1.

Figure 1: Schematic diagram of a chiller Plant

Types Of Chillers

  Chillers can primarily be classified as absorption chillers and refrigerant compression chillers, based on the refrigerant cycle on which they work. 
Cooling processes are significantly different in the two types of chillers. Absorption chillers use a heat source such as natural gas or steam to create refrigeration or cooling effect. Refrigerant chillers use mechanical compression and are the most common. Refrigerant compression chillers consist of four main components - a compressor, an evaporator, a condenser and a valve metering system. Basically, a refrigerant gathers heat, and then uses an evaporator heat exchanger to remove that heat. 
There are two main types of refrigerant compression chillers, air and water. Air condensers are cooled by utilizing the air, whereas water condensers are cooled by using water sources. Water cooled chillers are generally located within the building and use cooling towers, a pond, or river located near the building to reject water’s heat from the condenser.

  Chillers with condensers cooled by air operate essentially the same as those cooled by water regarding the refrigerant cycle and the steps along the way. The cooling medium on the condenser is of course air instead of water. Air cooled chillers are intended for outdoor installation and operation.

  These reject heat to the atmosphere by mechanical means such as circulation of outdoor air by a fan directly through the machine’s condenser. These types of condenser cooled units do not require a cooling tower as is common with water cooled chillers since the air rejects heat to the atmosphere. Based on compression method of the refrigerant in its vapour phase, chillers can also be classified into four categories. The compressors may be reciprocating, centrifugal, rotary screw and rotary scroll type.

  Reciprocating compressors possess a crankshaft and pistons. The pistons compress the gas and the gas is heated. The hot gas is discharged to the condenser. The pistons have intake and exhaust valves that can be opened on demand to allow the pistons to idle. A few examples of this would be in an office or school, but not necessarily in a hotel or an apartment building. 
Common capacities range from 20 to 125 tons but can even get up to 450 tons. Centrifugal compressors operate much like a centrifugal water pump. They contain an impeller that compresses the refrigerant. Centrifugal chillers can provide a very high cooling capacity in a compact design. They have the ability to vary capacity continuously to match a wide range of load fluctuations with near proportional changes in power consumption. This provides tight temperature control and energy conservation. 

  The capacity can range from 150 tons up to 2400 tons. Rotary screw or helical DNA type compressors have two mating helically grooved rotors. As the rotors rotate, the gas is compressed by volume reduction between the two rotors. These helixes require high tolerances to fit perfectly, thus driving up the initial cost.

  Capacity is controlled by a sliding inlet valve or variable-speed drive (VSD) on the motor. Capacities range from 25 to 450 tons with the largest capable of 800 tons. Rotary scroll compressors use two spirals to pump and compress the refrigerant. Commonly, one of the scrolls is fixed while the other orbits eccentrically without rotating within the other fixed scroll.

  This motion traps and compresses pockets of fluid between the scrolls. This design and operation makes them the most efficient of the four compressor types. The capacity of single refrigerant loop scroll compressor ranges from 2 to 25 tons. Typical chilled water cooling temperature ranges between 39-45 °F. The classification of chillers from various aspects has been shown in Fig.2.

  For proper heat transfer between the circulated water to be cooled and the refrigerant, it is important to maintain sufficient chiller water flow. The commonly recommended range of chilled water flow velocity is between 3 and 12 feet per second. Therefore, it is very important for a chiller to maintain this flow for proper efficiency and corresponding energy usage as well as maintaining long-term performance.

Absorption Chillers

  Absorption chiller is a machine which operates based on vapour absorption refrigeration cycle. This cycle consists of four major heat exchangers, (generator, condenser, evaporator and absorber) with two kinds of solution, (refrigerant and absorbent). During this cycle high pressure will prevail inside generator and condenser, while inside evaporator and absorber there will be low pressure. The cycle starts with input waste heat in the generator. As a result of this heat input, the solution in the generator will be separated into refrigerant and weak solution. The refrigerant in the vapour form will enter into condenser and will change into liquid. The solution part will enter absorber, since there is a pressure difference between condenser and evaporator, the refrigerant will flow inside evaporator and will absorb heat from cooled water that is in circulation inside evaporator. Consequently, the temperature of circulated water decreases and then it is used for air-conditioning purpose. The evaporated refrigerant will then enter absorber where it will be mixed with weak solution, the mixture will then get the liquid state and finally it will enter generator and the cycle is repeated. The schematic diagram of the vapour absorption refrigeration cycle has been shown in Fig. 3.

Vapour Compression Chillers

  The schematic diagram of chiller based on vapour compression refrigeration cycle has been shown in Fig.4. Refrigerant gets vaporized by taking heat from chilled water in evaporator thus serving its prime purpose. Refrigerant comes out of evaporator as vapour but on other side chilled water is produced. Thus, heat is added to refrigerant at constant pressure but is extracted from chilled water. Both refrigerant and chilled water don’t get mixed and are separated by some solid wall in between them in evaporator like shell and tube design. Refrigerant vapours come out of evaporator and then compressed by chiller compressor to high pressure and temperature. Compressor requires energy input for its working and hence electric energy is supplied to it. Refrigerant vapour rejects heat to outside cooling liquid or air. Refrigerant in condensed or liquid form coming out of condenser is expanded in expansion valve and its pressure and temperature are reduced to level of evaporator so that above cycle is repeated again.

Applications

  A chiller can be realized as a refrigeration system that cools water.  Air conditioners and dehumidifiers condition the air while a chiller, using the same refrigerating operations, cools water, oil, or some other fluid.  This chilled solution can then be used for cooling in a wide range of operations. Some of the most common applications are as follow:

1. Plastics Industry:  Cooling the hot plastic that is injected, blown, extruded or stamped.  
2. Printing Industry-Cooling warm rollers due to friction and ovens curing the ink, along with ultraviolet lamps also for curing purposes.
3. Medical Industry: MRI Systems-The hospital MRI units need to be cooled to operate properly.
4. HVAC Industry: Large scale air-conditioning systems pump this chilled water to coils in specific areas of high rise buildings. The air handling systems for each area open and close the water flow through specific area keeping the air of the rooms at a desired temperature.
5. Laser Cutting Industry: Technology has created machines that can cut out very specific steel products with the precise use of a laser cutting machine.  These lasers run at very high temperatures and must be cooled to run properly.
6. Brewery Industry- The cooling of the kettles in fermentation has become an upcoming industry where chiller have been used to keep the kettles and storage area of beer at cold temperatures.

Future Scope

  New formulations of lubricants and refrigerants blended with nano particles could yield increased energy efficiency for chillers.

  Carbon dioxide has been used in some supermarket refrigeration equipment. However, the high operating pressures with CO2 are a concern. 

  Hydrocarbons as refrigerants offer the possibility of good efficiencies. HFOs will become the new mainstream refrigerants of choice for chillers.

  The development of oil-free centrifugal compressors, where magnetic bearings replace the use of oil for lubrication has seen even greater increases in efficiency and lower operating costs.

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AUTHOR CREDIT & PHOTOGRAPH

Madhu Sruthi Emani
Student
Indian Institute of Engineering 
Science and Technology

Bijan Kumar Mandal
Department of 
Mechanical Engineering
Indian Institute of Engineering 
Science and Technology, 
Shibpur,Howrah

Source: http://coolingindia.in/blog/post/id/11394/different-type-of-chillers--their-application

Cooling The Tube

St. Paul's is a London’s underground station located in the City of London financial district that takes its name from the nearby St Paul's Cathedral. The station is on the Central line, between Bank and Chancery Lane stations. During summer, commuters passing through this station need proper cooling. They were being kept cool in last summer (2015) by an innovative fan chiller system, which pumped cold air onto the eastbound Central line platform.

  At the heart of the new system - a first for the Tube network and designed and built by LU's cooling team – is a ventilation fan that pulls fresh air in from the street. The air is then cooled by a water chiller system which circulates 16 litres of cold water every second around the pipes in the ventilation shaft, cooling the air by up to seven degrees before it is blown onto the platform. The huge fan is capable of moving the equivalent of 15 double-decker buses full of air every minute.

New Tube for London

  In the past it had been challenging to lower the temperature on the Central line as traditional cooling systems had proved prohibitively expensive and difficult to install within the 115 year old tunnels and stations.

  The New Tube for London will bring the first walk-through air-cooled trains on to the deep-level Tube network, which includes the Central line - with the first trains due to be introduced in the 2020s.

  Until then, LU continues to seek innovative solutions to reduce temperatures during the summer months on the deep-level lines. Other measures currently aimed at cooling the Tube include:

• New air conditioned trains on the Metropolitan, Circle and Hammersmith & City lines, with 40% of the network covered by 2016
• The capacity of the underground St. Paul's station ventilation fans network has been doubled and eighty three fans have been restored
• Portable fans installed within ticket and concourse areas, to increase air circulation at a number of stations
• The creation of a Cooling Innovation Centre to explore new efficient and environmentally friendly methods to cool the Tube.

Big Engineering Challenge

  David Waboso, LU's Capital Programmes Director, said, “We know travelling around London during the summer months can be uncomfortable which is why we are always looking for innovative ways to tackle the temperature. Cooling the Tube is a big engineering challenge, but we're making significant steps forward and, by the end of 2016, 40% of the Underground network will be served by air conditioned trains. Projects like the installation of a fan chiller system at St Paul's Tube station demonstrate how LU has some of the most skilled, creative people in the business when it comes to developing entirely new ways to cool London's Tube.”

  In addition, a larger fan chiller system is being installed this summer (2016) at a mid-tunnel ventilation shaft on the Victoria line between Walthamstow Central and Blackhorse Road Tube stations.

Major Improvement

  The work is being done to coincide with the period of major improvement works on the Victoria line this month, which will enable the operation of 36 trains per hour next year (2016). The new fan chiller system will be ready for next summer and mean more comfortable temperatures for passengers at the northern end of the Victoria line. LU's cooling team is developing proposals to continue the expansion of this innovative technology in the coming years.

Source: http://coolingindia.in/blog/post/id/10034/cooling-the-tube

Saving Cost Through Chiller Replacement

 

High efficiency turbomiser chillers installed at North Somerset Council’s headquarters in Clevedon are saving the organisation more than £1000.00 a week in energy costs.

     Cool-Therm installed two Turbomiser TMA 400kW chillers at the council’s building at Castlewood earlier this year, working closely with the council’s M&E and energy management department.

     The high efficiency, oil-less chillers, which run on virtually frictionless magnetic levitation bearings, replaced three aging Hitachi machines that were approaching the end of their expected operational life.

     The existing chillers, rated at 569kW each, were considered to be oversized for the application following major changes to the building’s occupancy and usage, resulting in high maintenance costs, poor control and reliability. Cool-Therm carried out a turn-key project for the client involving the safe removal of the existing chillers, replacing them with new Turbomiser machines. The changeover was successfully completed while maintaining continuity of cooling to the building, so that it could continue to function as normal.



     The project took two months to complete, and involved the staged removal of existing units and installation of new chillers with major work completed out of office hours to minimise disruption on site.

     Crane lifts posed a particular challenge due to the location of the building near the sea front, with high winds and unpredictable conditions affecting roof-top working.



     The Cool-Therm team worked closely with Steve Hodges, Principal Mechanical, Electrical and Energy Engineer, North Somerset Council, to ensure the existing chillers were removed safely and the new Turbomiser craned accurately into position. 

     Accurate placement was important as the new Turbomiser chillers were manufactured with connection positions designed for hook-up to the existing fixed on-site services.

     Due to the proximity of the site to the sea, and the risk of metal corrosion from onshore wind and salt-laden air, the heat exchange coils on the chillers were treated with a heavy duty Heresite protective coating designed for use in harsh environments. The chillers, which have an ESEER rating of more than 4.9:1, are equipped with a MODBUS gateway, enabling their performance to be monitored via the internet and any alarms to be interrogated and diagnosed remotely.



     Following the installation, the council reports that the chillers are saving in excess of £1000.00 in energy running costs a week.

     Steve Hodges said: “The Turbomisers offer a proven high efficiency solution, and the results to date confirm the anticipated savings. We are very pleased with the high quality approach and professionalism of Cool-Therm in delivering the turn-key package, and look forward to the savings that will continue to accrue over the life-time of the plant.”

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Ken Strong, Managing Director, Cool-Therm

Source: http://coolingindia.in/blog/post/id/8411/saving-cost-through-chiller-replacement

Air Handling & Air Distribution in HVAC System

Air conditioning is the process of conditioning of indoor air to maintain its temperature, humidity, velocity and cleanliness inside the room or desired location at a desired level for human comfort or any industrial process. Air handler or air handling unit (AHU) is one of the most essential and major parts of an air conditioning system. AHU is a large metal box that consists of a blower, heating or cooling elements, filter racks or chambers, sound attenuators and dampers. AHU is basically used to condition indoor air with their heating, cooling, humidification and de-humidification functions in a HVAC system.

Basic Components of an Air Handling Unit

  An air handling unit consists of few important components. Those are as follows:

Casing

  Casing is the insulated enclose of an AHU which keeps all the system components of an AHU safe inside and prevent the heat gain or loss from it. A schematic diagram of a typical air handling unit is shown in Fig. 1.

Duct

  Two types of ducts are used in an AHU. These are the supply air duct and return air duct. The cool and conditioned air is supplied to desired locations from the AHU by the supply air duct, while the hot air from the room is again returned back to the air handling unit through return air duct. There is one main supply duct that is then divided into various small ducts those lead to all the rooms that are to be maintained at a specific condition. Similarly, the return ducts from all the rooms also end into one main duct. Insulating materials are used to cover those ducts to prevent the heat gain or loss from these ducts and the ducts are designed in such a fashion that the distribution should be equal to all the rooms and wastage should be minimal.

The Mixing Chamber

  In this section, the fresh air from the environment is mixed with return air from the room. This is done to minimize energy consumption by taking advantage of the heating/cooling ability of the return air. In this system, fresh air is only required to supply the air change rate needed for comfort.

  Damper openings are generally controlled manually or by servo motor. But the main issue is to allow air to circulate freely and facilitates installation in closed ducts. Thus, dampers are designed to allow synchronous control. The three-way mixing chamber has been designed for similar purposes. However, this system includes a fan for return air. The desired portion of return air is given back to the system and the remainder can be exhausted.

Filters

  Filter is one of the very important components in an air handling unit. Filters are basically used to remove particles and contaminants of various sizes from the air. These air filters are usually placed at initial stage of an AHU to keep the downstream components clean.Generally, filters are placed in two or more successive stages with a coarse grade filter in front of a fine grade filter. Sometimes, final filtration medium is also there for further cleaning of the air.

  Different types of air filters are used in an AHU and their performances are listed in table 1. The type of air filter being used is very much dependent on the application of the system.

  Air filters may also be classified according to their applications such as:

• Panel Filter

  These types of filters are flat and rectangular in shape and provide a low efficiency filtration. The high velocity filters are arranged vertically whereas the low velocity filters are arranged in V shape. Typical air velocity through these types of filters ranges between 2-3 m/s.

• HEPA Filter

  HEPA Filters are very efficient and is able to achieve efficiency up to 99.97%. These filters are efficient in removing minute particles and airborne bacteria from the air. It is usually used in clean room applications such as semiconductor production floor, operation theaters and places undergoing critical processes.

• Electrostatic Filter

  Electrostatic Filter is used to remove particles from the air by using highly charged electrodes that ionized the air. Bag Filter is able to remove dust particles and is thrown away after use. Roll Filter is used for high velocity filtration where the used part is rolled up automatically or manually.

Fig 2: A Heating Unit                                      Figure 3: A Cooling unit

Cooling / Heating Arrangements

  Temperature control is one of the major factors in an air handling unit. The perfect temperature for human comfort generally varied between 18°C – 23°C. So, for human comfort this temperature must be maintained by an AHU. Therefore, a cooling or heating or arrangement for both purposes is provided with the AHU. Heating and cooling is, generally, done by either direct type heat exchangers or indirect heat exchangers. Direct type heat exchanger includes burning of gaseous fuel in the air stream for heating or evaporator for cooling purpose. Electrical heater or heat pump can also be used to serve the purpose. While, in indirect heat exchangers hot water or steam or chilled water in pipe line can be used. These pipe lines are manufactured from copper and fins are provided which is made from copper or aluminum. Typical heating and cooling units have been shown in figure 2 and figure 3 respectively.

Humidifier / Dehumidifiers

  Another very important parameter while designing an air handling unit is humidity. A humidity comfort level in the range of 45% - 55% relative humidity (RH) should be maintained. The humidity of the air sometimes goes very low causing discomfort to the occupants during the peak season of winter or the vice versa during summer. That makes the manufacturers to think about the necessity of humidifier in air conditioning system. Figure 4 shows a pictorial view of a humidifier. The humidity of the air can be increased or decreased according to the requirement using the humidifiers or de-humidifiers. Various types of humidifier are commonly used in an AHU. Those are:

• Spray Type

  Spray type humidifier has a header and spray nozzles that spray water with a pressure of 15 psi or more.

• Steam Pan Type

  Steam Pan Type humidifier has a pan and a heating coil to heat up the water of the pan. The evaporation of water caused by the heating will increase the humidity level of the surrounding air.

• Steam Grid Type

  Steam Grid type humidifier has tiny holes on the pipe to distribute the steam that flows through it. In this case, the water that is heated up to produce the steam to be supplied to the grid is conditioned to prevent odor being discharged to the room.

Fig 4: Humidifier

Fans

  Another very important component in an air handling unit is fan or blower. The hot return air from the room is first sucked and then blows it over the cooling coil where the hot air is cooled and then that is sent to the room to be conditioned. This is done by the fan or blower arrangements in an AHU. In general, two types of arrangement of fan are there in an AHU: draw though arrangement and blow through arrangement. The return air is sucked through the filter, the cooling coil and humidifier in the draw through arrangement. While passing through the filters, humidifier and cooling coil, the air gets conditioned and then it is sent to the required location. But, in case of the blow through arrangement the fan absorbs the return air and blows it over the air filter and the cooling coil. The air then flows to the rooms to be air conditioned. The draw through arrangement is used more commonly due to its compactness. The fans that are used in AHU are basically of centrifugal types. Figure 5 shows the pictorial view of fan unit used in an air handling unit.

Fig 5: Fan unit

Difference between Air Handling Unit and Fan Coil Unit

  Though both air handling unit (AHU) and fan coil unit (FCU) serves the same basic function of cooling in a HVAC system, there are few differences between those two units. Those are:

• AHU is generally bigger system than a FCU.
• AHU is more complex than FCU.
• AHU is generally used in bigger establishments
• AHU systems needs ducting whereas, FCU does not need any duct work.
• AHU system takes outside air into the system whereas, FCU just recycles air.
• AHU is used for filtering, heating or cooling and humidification or dehumidification of air while FCU just cools or heats air. 
• AHU is less noisy than FCU.

Fig 6: Grills and Registers

Air Distribution System

  After transmitting conditioned supply air from the air handling unit to the room, it has to be distributed to the conditioned space. So, it is very important to design the air distribution system properly. It is found that sometimes the efficiency of an air distribution system becomes low in the range of 60 – 75% due to the poor design. There is a scope of improving the efficiency upto 80% or more with proper installation of distribution system. Proper designing and installation of the air distribution system can save money up to 50 – 200$ per year. Moreover, efficient distribution system also reduces the equipment size.

Types of Air Distribution Devices

  Different types of air distribution devices are being used in HVAC systems recently. These are:

• Grilles and Registers

  Grilles are the outlets for supply air or inlets for return air whereas, registers are the grille with a volume control damper. Figure 6 shows the front view of a supply air grille with horizontal and vertical vanes that are basically used for deflecting airflow. Grilles have a comparatively lower entrainment ratio, greater drop, longer throw and higher air velocities in the occupied zone compared to slot and ceiling diffusers. The performance of the grilles are specified in terms of core size or core area, volumetric flow rate of air, effective air velocity, total pressure drop, throw and noise levels. They can be mounted either on the sidewalls or in the ceiling.

• Ceiling Diffusers

  A ceiling diffuser consists of concentric rings or inner cones made up of vanes arranged in fixed directions. Ceiling diffusers can be round, square or rectangular in shape. Square and rectangular ceiling diffuser has been shown in figure 7. A square diffuser is the most commonly used diffuser for supplying air. The supply air is discharged through the concentric air passages in all directions. The adjustable inner cones or the deflecting vanes are provided to change the air distribution pattern. These types of diffusers are normally mounted at the center of the conditioned space and those can provide large entrainment ratio and shorter throw for conditioned spaces with low head space.

• Slot Diffusers

  Slot diffusers are made up of plenum box with single or multiple slots and air deflecting vanes. These types of diffusers are mounted on either side of walls or in the ceiling. Linear slot diffusers are mounted on the sidewalls of the conditioned room. These are used for supplying both supply air and return air. These diffusers are particularly suitable for large open-spaces as long as 30 meters in length that require flexibility to suit changing occupant distribution. Figure 8 shows the photograph of conditioned space with linear slot diffusers mounted on the ceiling.

Fig 7: Ceiling Diffuser

Fig 8: Slot Diffuser

Types of Air Distribution Systems

  Two types of air distribution system are commonly used for maintaining the room conditions. Those are:

• Constant Volume

  Constant-volume systems are operated at a constant airflow rate and only the temperature varies to maintain the zone set point. Constant-volume units can be used in single-zone or multi zone applications. This type of system can use single duct or dual duct for the distribution of air. A single-duct system provides ventilation and cooling to the conditioned space. If heating is required then a heating unit is provided in the terminal unit or a separate system for heating may be introduced. A dual-duct system can distribute both hot and cold air by using a single fan to move air through both cooling and heating coils in the air handler. The supply air can be distributed by separate duct to the desired locations depending on the zone requirements. Another constant-volume system is the multi zone unit. The multi zone unit supplies air to several zones from a centrally located air-handling unit. Individual zone requirements are met by mixing cold and warm air through dampers in the air handler. The conditioned air is then distributed to the zones via single ducts.

• Variable Air Volume (VAV)

  A variable air volume (VAV) can vary the air flow at a constant temperature. In a VAV system one supply duct distributes supply air at a constant temperature. As the supply air temperature is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served. This type of system has many advantages over constant volume system. In this system, the supply air flow rate can be varied. The precise control of temperature, reduced compressor wear can be achieved in this system. This system consumes less power to run the system fans and makes less noise compared to constant volume system.

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Source: http://coolingindia.in/blog/post/id/14652/air-handling--air-distribution-in-hvac-system