HVAC DESIGN: SELECTING AIR CONDITIONING SYSTEMS

INTRODUCTION

The main objective of comfort air conditioning is to provide building occupants with a comfortable, safe and healthy indoor environment. The benchmark for comfort, safety, health and indoor air quality varies depending on the building use such as

·         Commercial: Office buildings, supermarkets, shopping malls, restaurants etc.

·         Institutional:  Recreation centers, theaters, indoor stadia, schools, museums etc

·         Residential: Hotels, private homes, low or high rise residential buildings

·         Health Care Facilities: Hospitals, nursing homes etc

Though the design criteria for the above spaces show slight variations, the basic design principle remains as follows:

·         Consideration of air conditioning fundamentals

·         Cooling load calculations

·         Consideration of thermal distribution requirements

·         Concepts of equipment selection

As shown in Figure 1, an air conditioning system comprises an air conditioning plant and a thermal distribution system. For space cooling as shown, heat energy in the form of sensible heat and or latent heat has to be extracted (transferred) from the conditioned space. The thermal distribution system serves as an energy transfer medium between the air conditioning plant and the conditioned space. The thermal distribution system also acts to improve the air quality in the conditioned space by regulating the introduction of a design quantity of fresh air into the conditioned space.

Generally the air conditioning system functions to:

• Provide the required cooling and heating energy
• Control and maintain the indoor environment parameters such as temperature, humidity, differential pressure between conditioned space and its surroundings, air movement, air quality and sound levels within specified limits. 
• Distribute the conditioned air to the conditioned space.

As facility developers, owners and users present varying requirements for the design, installation and operation of HVAC systems coupled with variations in building occupancies, location and outdoor conditions, HVAC systems now come in different types and configurations. A sound knowledge of the classification and the ability to distinguish one system from another is key on the part of the designer, to selecting an appropriate air conditioning system for the client.

 This post takes a look into the types of air conditioning systems and the criteria for selecting a suitable air conditioning system. To get a good grasp of the discussion here, the reader should be familiar with some basic HVAC terminologies and principles. These include methods of HVAC design criteria, load calculations (sensible and latent heat loads, infiltration load, equipment and appliances load, ventilation requirements, etc), building solar orientation and heat transfer mechanisms through the building envelope, etc.

 

TYPES OF AIR CONDITIONING SYSTEMS

In considering air conditioning for large buildings, the space is divided into thermal zones. The movement of heat from one place to the other has led to the development of HVAC system variations. The classification of air conditioning systems is dependent on various factors. Air conditioning systems can be classified into comfort air conditioning systems and process (industrial) air conditioning systems according to their applications. Similarly, air conditioning systems can be classified according to construction and operating characteristics as:

·         Individual room air conditioning systems

·         Evaporative-cooling air conditioning systems

·         Desiccant-based air conditioning systems

·         Thermal storage air conditioning systems

·         Clean-room air conditioning systems

·         Space conditioning air conditioning systems

Going by the heat transfer media (thermal distribution system media), HVAC systems are also classified into air, water and refrigerant systems. Design complexities have also led to a closer classification of these systems as:

·         All-air systems

·         Air and water systems

·         All water systems

·         Direct refrigerant systems

 

ALL-AIR SYSTEMS

In this system, air is the energy transfer medium from the conditioned space to the plant room and from the plant room back to the conditioned space. It should be noted that in general the all-air

system is adjudged best over other systems even though  air holds much less heat per unit volume than water.

This heat carrying limitation makes all-air systems utilize higher duct velocities (reducing duct size). This high velocity causes higher friction loses and noise levels which in turn results in higher energy utilization by the fans. Note that the use of higher velocities is only necessitated by extreme space limitations.

The terminologies used in this classification of all-air systems are as implied: in single duct systems there is only one duct therefore the system can cool only or heat only whereas in dual duct systems, heating and cooling can be done simultaneously.

For the Constant Volume Systems a constant volumetric flow  rate of air is supplied to the conditioned space while the zone damper is used to vary the volumetric air flow rate in Variable Air Volume (VAV) Systems. Some of the All-Air System configurations are shown in Figures 3 to 6.

 

 AIR-AND-WATER SYSTEMS

In this system, two fluids (air and water) are used to convey energy between the conditioned space and the air condition plant room. A typical representation of this system is shown below.

The basic components of this system include a central plant for producing secondary water, a central plant for producing primary air, a room terminal (a fan coil unit, an induction unit or radiant panel), water pipes, pumps, air ducts and controls.

The controlled delivery of air and water to the room unit is dependent on the design ventilation requirement and the required sensible cooling capacity at maximum space load. In this system, cooling and dehumidification of the air takes place in the central plant and the air parameters are designed to offset the entire space latent load. While the air takes care of the latent heat load, chilled water is supplied to the room unit to offset the sensible cooling load of the conditioned space. With the sensible heat load taken care of by the room unit, condensation of room air inside the conditioned space is avoided thereby relieving the engineer of the problems of condensate drainage and related issues. As can be seen from the diagram, the primary air being supplied at a much higher pressure than the conditioned space pressure induces flow of secondary air from the conditioned space. The water lines from the plant can be 2- or 4-coil depending on the system design for cooling (or heating) only or for both cooling and heating.

ALL-WATER SYSTEM

The all-water system uses water as fluid for the thermal distribution system. Since water is the only fluid transported between the air conditioning plant and the conditioned space, separate provision must be made for supplying the required amount of treated outdoor air to meet ventilation requirements. As in all systems that use water as energy transfer fluid, the all-water systems can be classified into 2-pipe and 4-pipe systems. A 2-pipe system can be used for either cooling only or heating only application. The 2-pipe system cannot be used simultaneously for cooling and heating. Instead the 4-pipe system is used. 

As shown in Fig. 8, a flow control valve, controlled by a zone thermostat, is used to control the flow of hot or cold water to the conditioned space. A pressure relief valve is installed in the water line to maintain a balanced flow rate. The heat transfer between cold and hot water in the conditioned space takes place through convection, radiation, conduction or a combination of these in the room unit (fan coil unit, convectors or radiators etc.)

THE DIRECT REFRIGERANT SYSTEMS

The direct refrigerant systems consist of air conditioning units with individual refrigeration systems. These systems are factory assembled and are available in the form of packaged units of varying capacities (cooling equipment capacity is often rated in tons of refrigeration, TR). The units come complete with capacities ranging from 0.75 TR to 100 TR. The package unit consists of cooling and dehumidification coils, compressor(s), expansion device, condenser coil, fans, filters, controls, etc. In these systems cooling is controlled by switching the compressor off-and-on. The fan speed can also be regulated to provide modular cooling capacity control or enable the unit to be used for air circulation only (switching off the refrigeration system completely).

BASIC APPROACH AND CRITERIA FOR SELECTING AIR CONDITIONING SYSTEMS

As mentioned earlier, a sound knowledge of the classification, configuration, system application criteria, advantages and disadvantages of the various systems and configurations are essential in the selection of a suitable air conditioning system for a facility. Basically, selecting an air conditioning system will require a careful consideration of the following:

• Architectural constraints (building application, occupancy, space constraints, building form, flow, orientation, etc)
• Cost constraints (initial and operating costs)
• Ease of accessibility and maintenance
• Capacity and performance requirements (indoor air quality, zoning and thermal control, energy considerations, noise levels, fire safety and smoke control etc.)
• Required system reliability and flexibility

It is worth mentioning that the weight of the above factors in the decision making process varies according to project peculiarities (ownership, construction expertise, etc). Usually, the engineer considers all the systems, and narrows his focus to about two or three from which he selects the most appropriate according to the above criteria. Also to help in the selection, my advice is to consider the air conditioning system as comprising subsystems, main components and support equipment or accessories.

ARCHITECTURAL CONSTRAINTS

Designing air conditioning for different building applications and occupancies requires a consideration of different design criteria, operating hours, and different system characteristics. Specific design criteria usually dictate the type of air conditioning system that should be selected. A good example is the design of an air conditioning system for a class 10 clean room for fabrication of semiconductor wafer. In this case a constant-volume central system is always the preferred option. When the clean room is in operation, adequate clean air must be provided to maintain unidirectional flow to prevent the contamination of semiconductor wafers by sub-micrometer-size particulates.  It should be noted that a constant-volume system with electric terminal reheat is always preferable to a VAV system in places requiring high-precision constant temperature to be maintained in the conditioned space.

For guest rooms in luxury hotels, a four-pipe fan-coil system is the most widely used air conditioning system as in addition to the systems ease of maintenance, the four-pipe fan-coil system provides individual temperature and fan speed controls as well as a positive supply of adequate outdoor ventilation air. When the room is not occupied, the fan coil unit (in the room) can be turned off conveniently.

Space limitations specified by the architect or facility owner also influence the selection of the air conditioning system. For example where the design for a high-rise building provides no rooftop space for AHUs and other mechanical equipment, or if there is not enough space for supply and return duct shafts, a floor-by-floor AHU central system may be the practical choice.

CAPACITY AND PERFORMANCE REQUIREMENTS

Another vital consideration for the selection of air conditioning system is the system capacity.  For a single-story small retail shop, a constant-volume packaged system is often chosen. If the conditioned space is a large indoor stadium with a seating capacity of  up to 70,000 spectators, a single-zone VAV central system is often selected. This system also guarantees the provision of minimum ventilation controls for required indoor air quality regulation.

MAINTENANCE CONSIDERATIONS

It is worth mentioning here that a central system with AHUs, a few water-cooled centrifugal chillers, and cooling towers needs less maintenance work than a packaged system with many rooftop air-cooled units. A VAV reheat central system needs less maintenance work in the fan and plant rooms than fan-coil system, which often requires much maintenance work in the ceiling space directly above the conditioned space.

COST CONSIDERATIONS

Initial cost and operating costs (mainly energy cost) are always primary factors that affect the selection of an air conditioning system. The initial cost of the air conditioning system in a building, expressed in $ /m2, depends on the building occupancies, system configurations, size of the building, and capabilities of specific systems. Generally, the more complex an air conditioning system becomes and the more features it has, the higher will be the initial cost.

TYPICAL APPLICATIONS OF AIR CONDITIONING SYSTEMS

ALL-AIR SYSTEMS

All-air systems have found extensive use in both comfort and industrial air conditioning applications where uniform loads are encountered and where precision control is required. Typical applications include operation theatres, office buildings, computer rooms, clean rooms, classrooms, research facilities, laboratories, hospitals, hotels, ships etc.

Advantages of all-air systems:

1. All-air systems offer the greatest potential for energy conservation by utilizing the outdoor air effectively.

2. By using high-quality controls it is possible to maintain high precision in the temperature and relative humidity of the conditioned space.

3. Using dual duct systems, it is possible to provide simultaneous cooling and heating. Changeover from cooling to heating and vice versa is relatively simple in all air systems.

4. It is possible to provide good room air distribution and ventilation under all conditions of load.

5. Building pressurization can be achieved easily.

6. The complete air conditioning plant including the supply and return air fans can be located away from the conditioned space. Due to this it is possible to use a wide variety of air filters and avoid noise in the conditioned space.

Disadvantages of all-air systems:

1. They occupy more space and thus reduce the available floor space in the buildings. It could be difficult to provide air conditioning in high-rise buildings with the plant on the ground floor or basement due to space constraints.

2. Balancing of air in large and particularly with variable air volume systems could be difficult.

AIR – WATER SYSTEMS

These systems are mainly used in exterior buildings with large sensible loads and where close control of humidity in the conditioned space is not required. These systems have also found extensive use in some office buildings, hospitals, schools, hotels, apartments etc.

Advantages of air-water systems:

1. Individual zone control is possible in an economic manner using room thermostats, which control either the secondary water flow rate or the secondary air (in fan coil units) or both.

2. It is possible to provide simultaneous cooling and heating using primary air and secondary water.

3. Space requirement is reduced, as the amount of primary supplied is less than that of an all air systems.

4. Positive ventilation can be ensured under all conditions.

5. Since no latent heat transfer is required in the cooling coil kept in the conditioned space, the coil operates dry and its life thereby increases and problems related to odors or fungal growth in conditioned space is avoided.

6. The conditioned space can sometimes be heated with the help of the heating coil and secondary air, thus avoiding supply of primary air during winter.

7. Service of indoor units is relatively simpler compared to all water systems.

Disadvantages of air-water systems:

1. Operation and control are complicated due to the need for handling and controlling both primary air and secondary water.

2. In general these systems are limited to perimeter zones.

3. The secondary water coils in the conditioned space can become dirty if the quality of filters used in the room units is not good.

4. Since a constant amount of primary air is supplied to conditioned space, and room control is only through the control of room cooling/heating coils, shutting down the supply of primary air to unoccupied spaces is not possible.

5. If there is abnormally high latent load on the building, then condensation may take place on the cooling coil of secondary water.

6. Initial cost could be high compared to all air systems.

ALL-WATER SYSTEMS

All water systems using fan coil units are most suitable in buildings requiring individual room control, such as hotels, apartment buildings and office buildings.

Advantages of all - water systems:

1. The thermal distribution system requires very less space compared to all air systems. Thus there is no penalty in terms of conditioned floor space. Also the plant size will be small due to the absence of large supply air fans.

2. Individual room control is possible, and at the same time the system offers all the benefits of a large central system.

3. Since the temperature of hot water required for space heating is small, it is possible to use solar or waste heat for winter heating.

4. Simultaneous cooling and heating is possible with 4-pipe systems.

Disadvantages of all - water systems:

1. Requires higher maintenance compared to all air systems, particularly in the conditioned space.

2. Draining of condensate water can be messy and may also create health problems if water stagnates in the drain tray. This problem can be eliminated, if dehumidification is provided by a central ventilation system, and the cooling coil is used only for sensible cooling of room air.

3. If ventilation is provided by opening windows or wall apertures, then, it is difficult to ensure positive ventilation under all circumstances, as this depends on wind and stack effects.

4. Control of humidity, particularly during summer is difficult using chilled water control valves.

DIRECT REFRIGERANT SYSTEMS

Unitary refrigerant based systems are used where stringent control of conditioned space temperature and humidity is not required and where the initial cost should be low with a small lead time. These systems can be used for air conditioning individual rooms to large office buildings, classrooms, hotels, shopping centers, nursing homes etc. These systems are especially suited for existing building with a limitation on available floor space for air conditioning systems.

Advantages of Direct Refrigerant Systems:

1. Individual room control is simple and inexpensive.

2. Each conditioned space has individual air distribution with simple adjustment by the occupants.

3. Performance of the system is guaranteed by the manufacturer.

4. System installation is simple and takes very less time.

5. Operation of the system is simple and there is no need for a trained operator.

6. Initial cost is normally low compared to central systems.

7. Retrofitting is easy as the required floor space is small.

Disadvantages of Direct Refrigerant Systems:

1. As the components are selected and matched by the manufacturer, the system is less flexible in terms of air flow rate, condenser and evaporator sizes.

2. Power consumption per TR could be higher compared to central systems.

3. Close control of space humidity is generally difficult.

4. Noise level in the conditioned space could be higher.

5. Limited ventilation capabilities.

6. Systems are generally designed to meet the appliance standards, rather than the building standards.

7. May not be appealing aesthetically.

8. The space temperature may experience a swing if on-off control is used as in room air conditioners.

9. Limited options for controlling room air distribution.

10. Equipment life is relatively short.

FURTHER READING:

1.       Walter T. Grondzik et al, Mechanical and Electrical Equipment for Buildings, 11^th Ed., John Wiley & Sons Inc.

2.       HVAC Design Manual For New Hospitals, Published by the Dept. of Veterans Affairs, Feb., 2008

3.       Roger W. Maines, HVAC Systems Design Handbook, 3^rd Ed.

4.       Refrigeration and Air Conditioning, Kharagpur, India, 2008.