HVAC&R Control Systems

Heating, Ventilation, Air Conditioning & Refrigeration (HVAC&R) is the technology of indoor and vehicular environmental comfort. Its goal is to provide  and acceptable indoor air quality. In present day of industrialization and modern living style, we are always in possession of any one of Heating, Ventilation, Air-conditioning or Refrigeration systems in our homes or in industry.  HVAC& R is an important part of residential structures such as single family homes, apartment buildings, hotels and senior living facilities, medium to large industrial and office buildings such as skyscrapers and hospitals, onboard vessels, and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. HVAC&R system design is a sub discipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. The central functions of heating, ventilation, air conditioning and cooling are interrelated, especially with the need to provide thermal comfort and acceptable indoor air quality within reasonable installation, operation, and maintenance costs. Basic principles of equipment‘s working and performance along with its design  and installation in a place (building), its control systems further play a great role in its working with respect to its efficiency and long life. Since last few decades, manufacturers of HVAC&R equipment have been making an effort to make the systems more efficient. This was originally driven by rising energy costs, and has more recently been driven by increased awareness of environmental issues. Additionally, improvements to the HVAC system efficiency can also help increase occupant health and productivity.

Basics of HVAC&R Control system

  To keep customers satisfied, there is a need to supply robust machines that are energy-efficient and reliable all at a reduced cost and with short lead-times. Customers also expect the best service, anytime and anywhere in the world. Choice of control solutions is the key to distinguish at every stage of the process, from design and development, to implementation and machine maintenance. Electrical control system is an integral and important part of HVAC&R system. Any HVAC&R equipment needs a control system to regulate the operation of a heating and/or air conditioning system. Usually, a sensing device is used to compare the actual state (e.g. temperature) with a target state. Then the control system draws a conclusion what action has to be taken. An HVAC&R control system has to conform with stringent health and safety laws, and react accordingly when a state of emergency occurs, such as fire, smoke or flood. It must also interface with other systems in the same building - boilers, chillers, etc. Tailor made electrical control panels available will provide the information needed at the required time and at the right place. Control panel information includes instrumentation and controls geared to individual requirements. This would include, but not be limited to:

• Motor and process control, environmental control (HVAC&R)

• Comprehensive local or remote, relay or PLC automatic controls

  With all the necessary cable entries and correct instrumentation to suit the application and systems will be designed to interface with fire alarm, boiler, and chiller systems as required. Other features or integrated options will include alarm systems and annunciators. Need is to design a cost-optimized HVAC&R control system with onboard energy-efficiency solutions. Improve machine performance with innovative automation technology and dedicated HVAC&R application functions, supplemented with advanced drive technology, to increase energy efficiency while reducing maintenance and improving reliability with energy-efficiency related functions such as energy management, floating high-pressure control, and compressor management. The benefits of HVAC&R Control Systems can be summarized as:

• Lower energy cost

• Lower operations cost

• Increase flexibility

• Ensure quality building environment

Elements of HVAC&R Control System

  HVAC control system, from the simplest room thermostat to the most complicated computerized control, has four basic elements: i) Sensors, ii) Controllers, iii) Controlled Devices and iv) Source of energy.

1. Sensors: Sensor measures actual value of controlled variable such as temperature, humidity or flow and provides information to the controller. 
Type of Sensors: Different types of sensors produce different types of signals as follows:
 
• Analog sensors are used to monitor continuously changing conditions. The analog sensor provides the controller with a varying signal such as 0 to 10V.
 
• Digital sensors are used to provide two position open or closed signal such as a pump that is on or off. The digital sensor provides the controller with a discrete signal such as open or closed contacts.

Classification of Sensors

  Typical sensors used in electronic control systems are:

Resistance sensors are ‘Resistance Temperature Devices (RTD’s)’ and are used in measuring temperature. Examples are BALCO elements, Copper, Platinum, 10K Thermistors, and 30K Thermistors.

Voltage sensors could be used for temperature, humidity and pressure. Typical voltage input ranges are 0 to 5 Vdc (Volts direct current), 1 to 11 Vdc, and 0 to 10 Vdc.

Current sensors could be used for temperature, humidity, and pressure. The typical current range is 4 to 20 mA (milliamps).

Temperature Sensors

• Bi-Metallic Strip

• Sealed Bellows

• Bulb & Capillary Sensors

Electronic Sensors

• Resistance Temperature Devices (RTD)

• Thermistors

• Thermocouples

Relative Humidity Sensors

• Resistance Relative Humidity Sensor

• Capacitance Relative Humidity Sensor

• Temperature Condensation

• Condensation & Wetting

• Quartz Crystal Relative Humidity Sensor

Pressure Sensors

• Variable Resistance

• Capacitance

Flow Sensors

• Orifice

• Venturi Tube

• Flow Nozzles

• Vortex Shedding Sensors

• Positive Displacement Flow Sensors

• Turbine Based Flow Sensors

• Magnetic Flow Sensors

• Ultrasonic Flow Sensors

Air Flow Measurements

• Hot Wire Anemometers

• Pitot – Static Tube

Liquid Level Measurements

• Hydrostatic Sensors

• Ultrasonic Sensors

• Capacitance Sensors

2. Controllers: Controller receives input from sensor, processes the input and then produces intelligent output signal for controlled device.

Controller Types

• Temperature Controllers

• Relative Humidity Controllers

• Enthalpy Controllers

• Universal Controllers

Controlled devices: Controlled device acts to modify controlled variable as directed by controller.
 
Controlled Devices Types

• Control Valves

• Heating and Cooling Coils

• Dampers

• Actuators

3. Source of energy: Source of energy is needed to power the control system. Control systems use either a pneumatic or electric power supply.

Supervisory Control System

  The role of supervisory control is to control the scheduling and interaction of all the subsystems inside a building to meet building needs with appropriate operator input. Supervisory control systems have many names; each used for a particular emphasis. Among the names and their acronyms are the following:

BMS: Building management system

EMCS: Energy monitoring and control system

FMS: Facility management system

EMS: Energy management system

BAS: Building automation system (The most generic of these terms)

  BAS is where mechanical and electrical systems and equipment are joined with microprocessors that communicate with each other and possibly to a computer. This computer and controllers in the building automation system can be networked to the internet or serve as a standalone system for the local peer to peer controller network only. Additionally, the BAS controllers themselves do not need a computer to process the control functions as the controllers have their own internal processors.

Type of HVAC&R Control Systems

  There are five different types of HVAC&R Control Systems as follows:

  Direct Acting Systems: The simplest form of controller is direct-acting, comprising a sensing element which transmits power to a valve through a capillary, bellows and diaphragm. The measuring system derives its energy from the process under control without amplification by any auxiliary source of power which makes it simple and easy to use. The most common example is the thermostatic radiator valve which adjusts the valve by liquid expansion or vapor pressure.

  Electric / Electronic Systems: Electric controlled devices provide ON / OFF or two-position control. In residential and small commercial applications, low voltage electrical controls are most common. A transformer is used to reduce the 115 volt alternating current (AC) to a nominal 24 volts. This voltage signal is controlled by thermostats, and can open gas solenoid valves, energize oil burners or solenoid valves on the DX cooling, control electric heat, operate two position valves and damper or turn on-off fans and pumps. A relay or contactor is used to switch line voltage equipment with the low voltage control signal. An electronic control system can be enhanced with visual displays that show system status and operation.

  Pneumatic Systems: The most popular control system for large buildings historically has been pneumatics which can provide both On-Off and modulating control. Pneumatic actuators are described in terms of their spring range. Compressed air with an input pressure can be regulated by thermostats and humidistat. By varying the discharge air pressure from these devices, the signal can be used directly to open valves, close dampers, and energize other equipment. The copper or plastic tubing carry the control signals around the building, which is relatively inexpensive. The pneumatic system is very durable, is safe in hazardous areas where electrical sparks must be avoided, and most importantly, is capable of modulation, or operation at part load condition. While the 24-volt electrical control system could only energize a damper fully open or fully closed, a pneumatic control system can hold that damper at 25%, 40% or 80% open. This allows more accurate matching of the supply with the load. Pneumatic controls use clean, dry & oil free compressed air, both as the control signal medium and to drive the valve stem with the use of diaphragms.

  Microprocessor Systems: Direct Digital Control (DDC) is the most common deployed control system today. The sensors and output devices (e.g., actuators, relays) used for electronic control systems are usually the same ones used on microprocessor-based systems. The distinction between electronic control systems and microprocessor-based systems is in the handling of the input signals. In an electronic control system, the analog sensor signal is amplified, and then compared to a set point or override signal through voltage or current comparison and control circuits. In a microprocessor-based system, the sensor input is converted to a digital form, where discrete instructions (algorithms) perform the process of comparison and control. Most subsystems, from VAV boxes to boilers and chillers, now have an onboard DDC system to optimize the performance of that unit. A communication protocol known as BACNet is a standard protocol that allows control units from different manufacturers to pass data to each other.

  Mixed Systems: Combinations of controlled devices are possible. For example, electronic controllers can modulate a pneumatic actuator. Also, proportional electronic signals can be sent to a device called transducer, which converts these signals into proportional air pressure signals used by the pneumatic actuators. A sensor-transducer assembly is called a transmitter.

Future of HVAC&R Control System

  The future of smart HVAC&R control systems is critical towards the comforts of life and the economy with the cost of natural resources, especially fossil fuels, undoubtedly set to rise over the coming years. Two futures are possible for HVAC&R controls. One is exciting; the other not so much so. In the exciting scenario, controls rapidly evolve so that, in just a few years, building controls have extensive self-commissioning, self-tuning, self-diagnostic and correction, and even self-configuring features. HVAC&R systems simply require components to be connected together with a short list of parameters set, and the system takes off from there—notifying the commissioning agent, contractor, engineer and/or operator if it is meeting its specified high-performance criteria, or, if it is not, what corrective steps are necessary. Multi-variable relational control can greatly improve performance, energy efficiency, and system stability. But relational control offers much more. This multivariable method of control provides an ideal platform for extension into a type of artificial intelligence called neural net control, which will begin a new era in building control. Relational control allows the software designer to select a wide variety of system variables that may influence the optimal operation of a system. The multivariable relationships may be very basic, such as fluid mixing laws, or much more complex, such overall energy optimization via the equal marginal performance principle. The logical next step in HVAC&R control software development is software modules that will automatically discover other variables (and/or combination of variables) that will assist further in tuning, optimization, self-configuration, self-setup, and fault detection with prescribed corrective actions. It’s becoming universally clear that such widespread implementation of advanced building control could cut total energy consumed by our buildings by about half, while at the same time improving occupant comfort.

Innovations That Will Change HVAC&R Forever

  Innovative technologies are taking the world by storm. As high-tech gadgets and the latest smart phone innovations continue to improve our lives, people have something else to look forward to in terms of revolutionary HVAC&R technologies that could change how we heat and cool our comfort zones to industry. Many of these HVAC technologies are still on the drawing board, but there are some we can take advantage of now to boost HVAC&R comfort levels. These technologies are:

• Movement-Activated Air Conditioning

• Thermally Driven Air Conditioning

• On-Demand Hot Water Recirculator

• Ice-Powered Air Conditioning

• Sensor-Enhanced Ventilation

• Dual-Fuel Heat Pumps

• Geothermal Heat Pumps

• Smart Homes

• Fully Automated Homes

• 3-D Printed Air Conditioners

• Harnessing Heat from a Computer

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AUTHORS CREDIT & PHOTOGRAPH

Dr S S Verma
Department of Physics, 
S.L.I.E.T., Longowal, 
Distt.-Sangrur, Punjab.

Source: http://coolingindia.in/blog/post/id/14492/hvacr-control-systems

Optimising Healthcare Environment Spaces

Designing for healthcare patient and critical environment spaces is strongly dictated by strict environmental and safety standards. However, possibly one of the most important components that must be taken into consideration is the one you can’t see. Effective Airflow Design (EAD) not only helps meet airflow change and industry standards, but is critical in limiting the contraction of airborne illnesses and can reap considerable cost savings for facilities. When designing for healthcare facilities, it is important to abide by airflow and air quality standards, in particular for three priority rooms: Hybrid operating rooms, Patient rooms and Isolation rooms.

Standards and Approaches 

   To determine what airflow plan is right for a space, engineers first meet with a designer and give them a general layout for the room, diffuser size and placement, requirements for airflow and other details. Although the designed layout has a big impact, the effectiveness of an airflow design boils down to the velocity of the air through the space and what direction it is flowing. In majority of the spaces within the Healthcare environment the primary objective is to ensure the cleanest air is supplied first to the patient then into the remainder of the room and that it’s filtered before it circulates back into the area. 
   
   As for requirements, most states (42) have adopted some version of the Facility Guidelines Institute (FGI) recommendations for healthcare facilities, but each administration has its own rules and regulations so it’s critical for those involved to be aware of what standard(s) they’re designing to. Though it would not be a drastic shift, this individualistic approach among states to regulations may change within the next two decades as results of research projects that are adopted into code. This research, commissioned by ASHRAE, FGI, and others, entails determining how much airflow is needed to prevent contamination in certain spaces based on evidence rather than conjecture, which has been the standard practice. The International Code Council (ICC) has formed an Ad Hoc Committee on Healthcare that is working to ensure standards and codes are not increasing the cost of construction and operation purely based on assumptions or outdated practices. This is a key area of focus, since 9% of the annual energy usage in the United States is dedicated to healthcare spaces; of that usage, HVAC is responsible for half.

   Finally, thermal comfort is addressed in ASHRAE Standard 55. Since most regulations are concerned with airflow and air quality, thermal comfort is not a priority. However, a room’s temperature and humidity is important because it can impact recovery time of patients as well as the performance of the facility’s staff – an overly cold or warm environment makes it difficult for surgical staff to perform at the highest level. ASHRAE Standard 170 also has stipulations as far as minimum and maximum humidity temperatures. While the scope of Standard 170 includes occupancy comfort, it should not be assumed that meeting the prescriptive design minimums will ensure compliance with ASHRAE Standard 55. Appropriate step must be taken to realise thermal comfort in the space for patients, as well as for visitors.

Hybrid Operating Rooms 

   Hybrid Operating Rooms (Hybrid ORs) are surgical areas equipped with advanced medical imaging devices – such as CT and MRI scanners. Incoming air should be HEPA-filtered to minimise the pathogens entering the space. Hybrid ORs have 30% more Air Changes per Hour (ACH) than catheterization labs. The increased airflow and type of procedures in the space dictate a different approach to EAD. Rather than conventional or radial flow diffusers, Hybrid ORs utilise unidirectional diffusers so air comes straight in one direction.

   These diffusers introduce highly filtered air into a space – right above where critical work is happening. This air then expands out and pushes the contaminants away. A body’s natural convection can also protect itself from unclean air, so it is a best practice for diffusers to have very low velocities that do not disrupt the wound’s convective plume. Recent studies have shown that in some surgery types there is not a thermal plume generated at the wound site. In these instances delivering clean air at very low velocity is critical to minimising entrainment of contaminants since the natural defence does not always occur.

   Design specifications for Hybrid ORs, call for diffusers to be located right over operating tables, and to satisfy ASHRAE Standard 170 the diffusers must cover at least one foot beyond the table and emit no more than 25-35 CFM/ FT2. ASHRAE Research Project 1397: EXPERIMENTAL INVESTIGATION OF HOSPITAL OPERATING ROOM (OR) AIR DISTRIBUTION results showed that the unidirectional airflow collapses in towards the table and accelerates into the operating room – as a result of buoyant and gravitational forces.

   The amount of collapse and acceleration is affected greatly by the temperature difference between supply air temperature and room air temperature. Titus recommends that the diffuser array extend two to three feet. Doing so will allow for a smaller temperature difference, limiting the collapse and acceleration – so patients, nurses, surgeons and all surgical instrumentation are covered by the sterile field. This practice helps reduce costly Surgical Side Infections (SSIs), which make up about 30% of all Healthcare Acquired Infections (HAIs)

Patient Rooms

   Like Hybrid ORs, patient rooms are critical spaces that require a high standard of air quality. Designers do not typically have major issues designing these rooms. However, when using chilled beams and displacement ventilation systems, intuitive designs can lead to airflow patterns that are less than ideal. This can be a concern as an inefficient airflow design fails to minimise the amount of potential particles and pathogens in the air being circulated or re-circulated through the room, translating to higher levels of airborne contaminants potentially leading to HAIs, and thereby raising costs. An EAD in these spaces means lower costs because patients recover more quickly and there is a higher turnover rate. Facilities also do not have to treat or retreat patients for something they acquired during their stay.

   Use of chilled beams can be a useful means of developing an EAD within patient room spaces. The most intuitive design is to place a 2-way active beam near the patient bed with the throw – introduced into the room perpendicular to the patient’s bed. This is typical for most active beam designs, placement over the occupant seeks to minimise air velocity and create a uniform temperature around the patient for thermal comfort. Recently, the result a CFD study (Comparative Analysis of Overhead Air Supply and Active Chilled Beam HVAC Systems for Patient Room) showed that placement of a 1-way beam over the head of patient could potentially create an airflow pattern that results a single pass system in regards to airborne particulate in the room. A single pass airflow pattern or reduced pass airflow patterns strive to minimise the airborne particulates in the space to reduce HAIs.

   Displacement ventilation design also presents a challenge in some cases. The size and floor level installation of these diffusers can lead to their installation in corners – where they can be easily blocked by furniture or belongings, significantly reducing their efficacy. Placement of diffusers on the wall adjacent to the foot of the bed results in the most effective airflow pattern. Placing the exhaust above the patient’s bed at a 15 degree angle away from the head of the bed and towards the foot will be most effective in removing aerosolized saliva containing potentially viable viruses and bacteria from the space. Additionally, it is critical to have the transfer grille to the toilet space installed at least 6 feet above the finished floor to prevent short circuiting. Since the toilet room is to be negatively pressurised and has a high air change rate, a low level transfer grille could lead to the low velocity air discharged from the displacement ventilation unit being exhausted – from the patient room without addressing the load in the space. So, why are more facilities implementing displacement ventilation and chilled beams for projects? Both systems are very effective at getting air into spaces at the right temperature, exhausting and/or recirculating it without bringing contaminants back into the occupied space – the primary goals of EAD. In addition, displacement ventilation systems are extremely effective in removing pathogens from patient’s bedside areas.

Isolation Rooms

   There are some specialised types of patient rooms that rely heavily on EAD to achieve their individual goals. These are Airborne Infection Isolation (AII) Rooms and Protective Environment (PE) Rooms. PE is specifically designed to prevent patients with suppressed immune systems (i.e., chemotherapy patients, bone marrow or other organ transplant recipients, AIDS patients). AII rooms are designed to minimise transmission of airborne infectious diseases from an infected patient to staff, visitors, and other patients.

   To prevent infections in isolation rooms, ASHRAE Standard 170-2013 stipulates requirements to help achieve EAD. These requirements include room pressurisation, filtration, air change rate, and use specific diffuser type and their location. To prevent migration of particles into the isolation rooms a minimum requirement is – the room must maintain differential pressure +/-0.01 in wc to the adjacent spaces. However, ASHRAE Research Project 1344: Cleanroom Pressurization Strategy Update – Quantification and Validation of Minimum Pressure Differentials for Basic Configurations and Applications has shown that even when maintaining a pressurisation of +/-0.01 in wc – particles can migrate into the room as people enter and exit the rooms. To minimise transmission of particles into or out of isolation rooms, differential pressurisation of at least +/- 0.04 in wc or use of a anteroom is recommended. All air supplied to PE rooms must be HEPA filtered. To further develop air distribution to reduce the chance of Healthcare Acquired Infections (HAIs) use of non-aspirating, unidirectional diffusers are to be installed directly over the patient with exhausts/returns grilles located near the door the patient room. This is to create an airflow pattern within the space where the cleanest air possible flows over the patient first before moving into the rest of the room.

   However, to achieve effective airflow design in PE room thermal comfort of the patient must also be considered. Patients are going to have very low clo (clothing) levels and met (metabolism) rates, so additional diffusers must be used to keep the volume and velocity of the air flow out of the non-aspirating diffusers to a comfortable level. 
Displacement ventilation would complement the non-aspirating diffusers best in this space – as it would not disrupt the airflow pattern that is to be developed by the non-aspirating diffuser. In aII rooms, the goal is to prevent transmission of infections from the patient to staff, visitors, or other patients. As such, the location of the exhaust is to be directly over the patient's bed or in the wall at the head of the bed, and all air must be exhausted out of the building. To establish effective air distribution in aII rooms, supply diffusers should be installed near the entrance to the room with throw patterns directed towards the patient.

   Combination AII/PE isolation rooms are allowed by ASHRAE Standard 170-2013. Combined Isolation rooms must have an anteroom and must be pressurised to both the corridor and the isolation room itself. The differential pressure must be at least 0.01 in wc, and can be either positive or negative. In combined isolation rooms, air distribution must follow the same guidelines as PE rooms with diffusers located over the patient and exhaust by the anteroom door. And, as with the aII rooms – all of the air must be directly exhausted out of the building.

Conclusion

   Appropriate use of chilled beams, displacement ventilation and non-aspirating diffusers play a pivotal role in establishing Effective Airflow Design across many different critical and non-critical spaces. 

   Designing a system that utilises each piece in the best way possible not only creates an environment that is safer and more comfortable, but is also good for a facility’s bottom line. 

   Lowering re-admission rates and reducing the number of Healthcare Acquired Infections are goals – for which all healthcare buildings should strive for; EAD helps make that happen. Be sure to consult a designer before embarking on your next project – to determine which layout makes the most sense for your spaces.

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Written by: Matthew McLaurin, Product Manager, Titus HVAC

Source: http://coolingindia.in/blog/post/id/7260/optimising-healthcare-environment-spaces

Getting The Most Out Of Your Heat Exchangers

Buildings require to be cooled in summers and in some cases, heating of the building space is also required (cold climates as experienced in northern regions of India). Transfer of heat from or to a building environment is undertaken by the use of heat exchanger. A Heat Exchanger (HE) is thus essentially a mechanical system to transfer heat from one medium to another. Heat exchangers are thus a critical component of a building HVAC system as their design is fundamental to the operation of the whole plant. 

     Heat exchangers are simple in theory but complex in design as well as operation. A minor change in design parameters can alter a HE’s operating parameters, and in turn, the plant’s performance characteristics. Heat exchanger design is a continuously evolving field, with the main focus being on improvement in HE exchange. The HE is a static device, and hence leads to the assumption that not much maintenance would be needed for such a component of a HVAC system, the truth is, in fact, very different. HE as critical to the efficient operation of the HVAC plant and require as much – if not more – attention in operations as well as maintenance to ensure that the overall plant runs as per design. 

Overview of heat exchangers

     There are many types of HEs in the market depending on the type of application and design requirement. Heat exchange typically occurs between two fluids and usually across a medium. The most common classification methods of HEs are as follows:

Nature of heat transfer: This classification is based on the mechanism of heat transfer between the two fluids:

Direct type – where the two liquids physically mix and heat is transferred. An example of such a heat transfer is between water and air in HVAC system cooling towers.

Recuperators – Where the hot and cold fluids flow simultaneously across a separating medium and heat is transferred across this medium. The HE in the HVAC system – condensers and evaporators are of this type.

Type of flow: The direction of flow is a commonly used classification approach. Counter flow HE have the hot and cold fluids flowing opposite to each other, while parallel flow HE have fluids moving in the same direction.

Number of passes: Another common classification method is the number of passes the hot and cold fluids make over the passage of the HE. Single pass systems are not seen these days as HE design has evolved to allow multiple passes to increase heat transfer. 

HE components: The two HE in an HVAC system are the condenser and evaporator. Both of these are of the Shell and Tube type, where one of the fluids is in the shell and the other passes through the shell through tubes. The tubes are held together by the HE covers at either end and inlet and outlet points are provide for the hot and cold fluids. The other components of a HE are the instruments such as pressure gauges and temperature gauges, as well as any instrumentation that is fitted onto the condenser or evaporator. Figure 1 shows a typical heat exchanger.

Key HE terminologies: For the plant operator, detailed knowledge of HE design terminologies such as thermal conductivity of the material, expansion coefficients, etc. are not needed. What is essential is to understand the purpose of the HE, and accordingly observe the parameters so that any deviation are spotted immediately and corrective action can be taken Thus, for routine operations, the key parameters that the operator and maintainer are required to know are the temperature difference between the fluids entering and leaving the HE and the pressure at different points in the unit.



Condensers: As the name suggests, the function of a condenser is to condense the hot gases coming out from the chiller compressor and reduce the pressure and temperature. In water cooled system, water is circulated across the condenser to condense the refrigerant gas. Water takes away heat form the hot gases, which results in condensing of the gas (lowering of temperature, and hence, pressure). The water is then passed through a cooling tower where it gives up its heat to the atmospheric air in another heat exchange process and again pumped into the condenser for the next cycle of heat removal. In air cooled systems, air is passed over the condenser coils and air takes away the heat from the refrigerant. 

Evaporators: In the evaporator, the cold refrigerant liquid and water is the medium that gets cooled. Essentially, the cold refrigerant takes away the heat from the water that is circulating in the chilled water lines of a HVAC system. After evaporation, the heated refrigerant flows into the compressor, where it is heated and then condensed to repeat the vapour compression cycle of the HVAC plant. 

Heat exchanger operations:
     As mentioned earlier in the article, a HE is designed to operate within a narrow range of system parameters. Thus, it is essential to operate the plant as per the design recommendations to ensure that the correct heat transfer processes occur and the system operates to its best efficiency. In case the cooling water in a condenser fails, the hot gases will damage the condenser tubes. Similarly, if the flow of water stops in the evaporator, the cold refrigerant will cause the water in the tubes to freeze, resulting in the tubes cracking as ice occupies a larger volume than water. 

     While most modern HVAC systems have automatic control of the HE system, the operator should know how the control is affected and the implication of an out of design situation. The key parameter that an operator needs to monitor during 

HE operations is:

     Inlet / outlet temperatures: The temperature difference between the inlet and outlet temperatures of the hot and cold fluid as well as between the inlet of hot fluid and outlet of cold fluid are the basic parameters that should be observed. Any variation of these from design is an indication of a problem in the HE operation.

     Pressure drop across the HE: Since the diameter of the tubes is small to allow for multiple passes and a greater amount of fluid, the flow of water encounters resistance from the walls of the tubes. This is seen as pressure drop across the inlet and outlet of the tube headers. An increase in the pressure drop is an indication of a malfunctioning HE. 
Flow rates: While no instruments are usually installed to measure flow rates on HE in the HVAC systems in buildings, the operator should check that the pumps of the HVAC system feeding the condensers are operating at the correct rpm and pressures. Evaporators are fitted with anti-freeze sensors that trip the plant in case the cold fluid temperature reaches a certain temperature, usually 4-5 degrees Celsius. Modern systems also have flow sensors in the piping of the condensers and evaporators. 

Measuring HE performance:

     The performance of a heat exchanger deteriorates over a period of time on account of many factors such a quality of water and operating methods. Water is the most common HE fluid in HVAC systems as it is easily available, cheap and has good heat transfer characteristics. However, water quality is always suspect in the building services environment and these impacts the HE operations. The main problems that are encountered in HE operations are given below.

Fouling: This is the build of contaminants on the surface of the tubes leading to a reduction of heat transfer between the hot and cold fluids. In addition, since the contaminants reduce the pipe flow areas, the pressure drop increases, which leads to higher pumping power and consequently higher energy costs. Fouling is caused by biological contaminants in the water or mineral deposits, which attach themselves to the tube surfaces. Fouled tubes are usually cleared by mechanical means or by caustic leaning processes. 

Scaling: This occurs when certain salts in the water precipitate and form a film around the walls of the tube. Since the entire surface of the tube is covered, the heat transfer is adversely affected, impacting HE efficiency. The salts form the film at elevated temperatures and cannot be removed by mechanical means. Scaling is countered by using acidic solutions that dissolve the scales and is a time intensive process. 

Measuring HE performance:

     The effect of fouling and scaling is to reduce the heat exchange between the two fluids. There is no direct way to measure the drop in HE efficiency, although a lowering of the temperature difference between the inlet and outlet is an indication of a defective HE. The recommended way to assess the performance of the HE is to carry out a simple heat exchange calculation. 

     The overall heat transfer co-efficient ‘U’ is used to measure the performance of a HE. The formula used is shown in 
figure 2


In this equation: U is the overall heat transfer coefficient. Q is the heat duty measured for sensible and latent heat separately. A is the area of heat exchange and LMTD is the Logarithmic Mean Temperature Difference, which is a function if the temperature difference between the fluids. W is the mass of the fluid entering the system. 

The steps involved in finding out U are shown in figure 3. From data obtained through the calculations, the performance of the HE can be assessed by comparing the values obtained with the design values of the HE. The key inferences that can be obtained from the performance test are:

• Pressure drop across the HE: If this is more, it could be an indication of fouling. If it is less, it could be due to increased average bulk temperature of the HE due to lower performance.
• Temperature gradient: This gives an indication of the effectiveness of the heat transfer. 
• Heat Transfer Co-efficient: This is the overall parameter, which gives an indication of the condition of the HE. A lower U value is on account of fouling of the HE tubes. 

Heat exchanger maintenance:
     As heat exchangers are in continuous operation, and there is usually not standby HE for a particular plant, maintenance of HE should be undertaken as per the OEM guidelines at a minimum. As there are no moving parts, preventive approach to maintenance is sufficient to keep the HE in a good condition. The key maintenance activities that need to be undertaken for the HE in a chiller plant are:

• Visual observation: Daily checks on the pressure and temperature parameters will help identify early on if any fouling has started.
• Maintaining water quality: This is a very important maintenance activity as poor quality of water will result in sludge formation, fouling and scaling. 
• Avoid water stagnation: In case the HE is not in use for an extended period of time, the water in the system should be circulated at regular intervals to avoid rust formation in the tubes.
• Use of Anti Fouling/Scaling Additives: Where possible, the water should be dosed with anti-fouling/scaling additives that help in inhibiting the formation of scales. 
• Annual tube cleaning: Minor amount of fouling and scaling will occur during operation. This is a progressive activity and if the scale layer is not removed in time, the impact on heat transfers increases substantially. Thus, the condensers should be cleaned at least once a year by shutting down the plant. 

Summary
     Heat exchangers are critical for the efficient operation of the chiller plant. Any deterioration in the condenser heat transfer will directly impact the efficiency of the chiller and lower the cooling capacity. A fouling factor of 0.001 (a measure of thickness of scale) can lead to an energy loss of 10 to 111 %. For a 500 TR plant, this translates to an annual increase in operating cost of approx. Rs 6,40,000. Thus, not only is regular HE maintenance a good practice, it also saves energy for the owners. The reality is however different, and HE maintenance is often the last priority in most maintenance activities, either due to ignorance or due to lack of priority when the budgets are drawn up for the maintenance activity. The best way to get the maximum out of the heat exchangers in the system is thus to have an effective maintenance program and regular measurement of the performance HE.

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

Sheetal Sutra

 Air conditioning energy costs are fast rising to unsustainable levels. Electricity demand for ACR averages 30% to 50% of the total energy budget of air-conditioned buildings. Skyrocketing energy cost has made air conditioning unaffordable for domestic users who cannot write it off as business expense. It has increased overhead costs of commercial users and production costs of industrial users to a level that makes them lose their competitive edge while facing global challenges. This article proposes a new formula that can reduce those energy bills substantially by reducing or eliminating air conditioning altogether.

Lowering The Energy Bills

  Much has been written about increasing the energy efficiency of the HVAC system that pumps out the solar and internal heat from the building. This article is about draining it out. Draining requires no energy, pumping uses that.

  The method described is rooted in our Indian heritage, but it does not involve khus screens or desert coolers. As a matter of fact the first known desert cooler in India was called the “Thermantidote.” A British engineer built it for the Jaipur royal family. It is still in the City Palace museum, albeit in the storeroom.
  Before talking about the solutions, let us understand the problem.

The Problem

  The problem is: thermal comfort in India is equated with air conditioning as its only stand alone solution. So far, every HVAC professional mechanically calculates the various heat gains into the building, either manually or by computers. A system is selected, tendered and installed, that will adequately meet the load.

  The main villain is that the designers, comprising the consultants, the sales engineers and the premises officers, fail to realise that we are applying, mindlessly, an energy hungry cooling technology born in a country where the buildings are insulated, the summers are mild and energy is cheap. Here the houses are bare, summers are sweltering and energy is not only very expensive, but also unreliable.

  So it is case of a right formula applied to a wrong problem. It is like walking down a Delhi street in a woolen suit, overcoat and a Bowler hat! It is proper in London but not here.

A Whole New Ball Game

  For example, a couple sleeping in a bedroom at night will generate only about 300 watts of metabolic heat. The heat load form will not show any other significant load. A one- ton air conditioner removes more than 3000 watts of energy from the room. Thus, in a 10- hour session, the compressor should work for a total of one hour only, and the monthly energy use should be less than 50 kWh. We know that it is much more. So, where is the extra load? Of course it is the stored solar gain.

  This example also establishes that all direct solar heat reaches the interior through, and only through, the structure. Even the sunlight coming through a window falls on the floor or the wall and is absorbed there.

  Also that all heat gains other than people, lights, fresh air and equipment are solar in origin and that solar loads are very heavy in India as compared to USA and Europe. Thus, providing thermal comfort in India is a whole new ball game.

  Herein lies one solution to our problem. If we could, somehow, keep the structure cool without using energy, then we would achieve a major reduction in our energy bill. The answer is in our heritage.

Our Heritage Has The Answer

  Our master builders of yore had learnt from nature to develop zero energy techniques that used mass to store heat, and flowing water or air to drain it out, thus keep masterpieces like the Gol Gumbaz and the Taj Mahal cool throughout the long hot Indian summers.

  The former, having 10- foot thick walls and measuring 100 feet square topped by a massive dome, depends on its enormous mass to absorb the solar load while maintaining its 18,000 odd square feet of its interior quite cool. Its exterior finish contains the mineral barite that is plentiful in that area, and has an emissivity of 0.95 in the infrared region. This allows re-radiation to the sky mostly during the night.

  The Taj Mahal is a massive building that sits on an equally massive podium measuring 325- feet square, 15 feet above ground and perhaps the same below.

  Their combined mass is tens of thousands of tons and can absorb an enormous amount of heat before its temperature rises by just one degree.

  The Yamuna River flowing next to it has near zero degree water all winter long. During that time, the whole massive podium cools down by a majestic heat transfer process and it turns into a heat sink for the superstructure, which gets additional cooling by rain water during the monsoon and by cold air during the winter.

  The heat absorbing capacity thus created is so large that by the time the building warms up, the summer has gone. This is the Sheetal Sutra or the Natural Cooling Principle.

Sheetal Sutra Runs Throughout Our History

  We find a continuum in the use of the same concept in lake palaces, temple/mosque complexes, royal residences etc. while searching our cultural history through ancient India. We could go past Mahabharata, where Duryodhan had fallen into a pool inside the Pandava palace, all the way back to the caves.

  Here we Homo Sapiens-Sapiens have lived for over forty thousand years. The caves are cooled by mountain streams, while the soil layer, with its trees, reduces solar heating. The mass of the mountain stabilises the inside temperature.

  Thus, conditions inside a cave are identified as a primary natural standard for human comfort. Radiant cooling of the human body by the cave walls is established as an important and necessary element for providing thermal comfort.

  Emulating the caves, our ancestors put massive buildings on massive bases, and provided a thermal path to a water body.

  Those of us who have visited a heritage building, or even the ancestral family home, know that this technique provides thermal comfort at zero energy cost.

  The reason why these buildings are comfortable is that the entire structure cools down to a temperature that is several degrees below that of the human skin.

  The reason why they consume little or no energy is because the heat is drained out to a low temperature sink in the form of an open water body.

  Anyone who has drunk water out of an earthenware jar knows how cool it is, particularly early in the morning.

  By contrast, air conditioning has to pump the heat out of the air to higher temperature ambient air. This requires oodles of energy.

  If used in an air-conditioned building, the heritage technique would reduce solar load by a good amount and would also shave the peak. It would mean a smaller plant and lower energy consumption. The question was how it could be implemented in modern times.

Modern Technology:

  Things change. There are more people in Mumbai today than there were in the entire known world 2000 years ago. Space constraints and economics have forced us to live in crowded localities where massive buildings and water bodies are no longer practical. While this has spurred the growth of the AC&R industry, we no longer have cheap, abundant power to feed this energy hungry technology. An inescapable conclusion that emerges is that air conditioning is a right technology used in a wrong way and on a wrong day. It must be used like a cake- as dessert after a meal, not the meal itself. By using new technologies not known to our ancestors, the following idea adapts our heritage technique to modern times:

Modern Sheetal Sutra

  Use barriers to reduce solar gains; absorb and drain out heat from the structure; then use air conditioning to pump out the balance load, if any, by cooling the air.

THE BASIC SYSTEM

  EASIER SAID THAN DONE: Many innovative solutions were tried, The first trial was a coir mat laid on the roof and wetted by a sprinkler. But dust, mosquitoes and water leaks ended that. Then welded iron pipes with water under vacuum were tried, It needed skilled welders at site and the pipes were prone to rust.

The cooling system was a bit more complex than the basic model...

Predicted temperature profile using a simulation software...

  THE GAME CHANGER: The final solution was a game changer. A plastic tube was laid on a cured concrete slab and was covered by screed. Water flowing through it picked up the solar heat and was sent to a radiator, where most of the heat was dumped into the air. Lukewarm water returned to the tank, this was recycled through the radiator during the night and was cooled to the morning ambient temperature and the cycle began again. There was no loss of water, and the little energy needed for the pump and the fan was supplied by a PV panel. That is all there is to it!

Conclusion
  Our ancestors did not have the technologies of plastics, pumping or thermodynamics except perhaps in rudiments. Of course they did not have electricity. By infusing these into their technique of structure cooling, We were able to keep the occupants comfortable without air conditioning, air cooling or even a mechanical ventilation system.

  Therefore, providing thermal comfort in India is a Whole New Ball Game! We are not promoting rocket science. It is very simple and is easily replicable by anyone with minimal technical training, using Indian materials and non patented published data.

  The Game Changer building was a result of close cooperation between the Clients, the Architects and the Contractors.

  It is hoped that many more such teams, the green NGOs, Architecture and Engineering Colleges and the Government will take notice, wake up and start a mass movement that will strike a big blow for the environment.

  This is not an isolated example. There are many more installations around us at many parts of the country.

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AUTHORS CREDIT & PHOTOGRAPH

Surendra Himatlal Shah
BE Mechanical Engg, 
Clemson University, USA
Founder & Owner of 
Panasia Engineers Pvt Ltd

Source: http://coolingindia.in/blog/post/id/12862/sheetal-sutra