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AIR CONDITIONING

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Introduction

Heating, Ventilating and Air Conditioning (HVAC), generally means the provision of an acceptable thermal environment within buildings. It includes heating, cooling, humidifying, dehumidifying, filtering, and distribution of air at suitable conditions for the maintenance of human comfort or for the undertaking of a particular process.

In order to design an air conditioning system the appropriate heating, cooling, and other environmental loads must first be calculated and a variety of other factors such as initial and running costs, plant room and distribution space, control requirements etc. must be assessed. In practice, there are a wide variety of air conditioning systems available and selection is often dictated by factors other than the air conditioning loads.

Need for air conditioning

The need for air conditioning is dictated by the required conditions within the space and the incident loads. In the case of process conditioning the requirements are often quite critical where even a small deviation in the temperature or humidity might result in catastrophic damage to the process. In fact, many products of today could not be produced at all were the environment not controlled within narrow limits.

On the other hand, conditioning for human comfort can be less critical and acceptance of diurnal swings in temperature and humidity can result in considerable savings in plant costs and in energy during operation. Careful design of thermal mass of the building fabric and of natural ventilation can often reduce or even obviate the need for air conditioning altogether.

Until recently, air conditioning was generally considered to be a prestigious component of a building, with "fully air-conditioned" buildings commanding far higher rents than their "heated and ventilated" counterparts. However, in climates such as the UK there has been a swing in demand towards low energy naturally ventilated buildings. This together with the changes in work practices away from the use of large, centralized, prestigious headquarter buildings has led to less significance being attached to air conditioning in the eyes of the building owners and occupiers. However, there will always be a need for air conditioning in harsher climates, but even in the UK when building on restricted, noisy, polluted center city sites that require deep plan, high rise buildings. Under these circumstances daylight does not penetrate the center of the building and the continuous use of artificial lighting produces high cooling loads. In addition, natural ventilation is restricted as traffic noise and external pollution often preclude the use of openable windows. Air conditioning applications and their general criteria and design considerations are discussed in detail in the ASHRAE Handbook, HVAC applications volume (1991).

Thermal comfort and indoor air quality

The provision of thermal comfort to a building occupant depends on the correct balance of air temperature, mean radiant temperature, air velocity and vapor pressure to suit the level of activity and clothing worn by the occupants. Much research has been undertaken to identify the ranges and combinations of these six factors that result in comfortable environments. Fanger (1972) described an extensive study of thermal comfort and presented a method of calculating the Predicted Mean Vote (PMV) of building occupants with respect to their thermal comfort on a scale of -3 to +3 with zero being the comfortable equilibrium. From this figure the Predicted Percentage Dissatisfied (PPD) can be determined. An instrument has been developed to measure the appropriate factors, integrate their impact on comfort of the occupants and produce readings of PMV and PPD.

Other factors that are controlled by the HVAC system involve the maintenance of clean, healthy, and odor-free indoor environments. These factors are often what are intended by the term indoor air quality or IAQ. Maintaining good indoor air quality involves keeping gaseous and particulate contaminants below some acceptable level. A description of common contaminants and methods of controlling their levels is given in McQuinston (1994).

The Chartered Institution of Building Services Engineers (CIBSE) in the United Kingdom and the American Society of Heating, Refrigeration and Air- Conditioning Engineers (ASHRAE) in the United States of America each publish recommendations for achieving comfort in buildings. CIBSE (1979) and ASHRAE (1993) Chapter 8.

Building loads

Air conditioning systems are required to overcome the heat loads on the building to maintain acceptable comfort conditions within the space. The systems should provide heating to overcome the heat losses and cooling to overcome the heat gains.

Heat losses

Heat losses from the building are usually calculated on a steady state basis using an internal design temperature defined by the comfort requirements of the occupants and an external design temperature appropriate for the geographical location.

The total steady state heat loss of a building is the sum of the Fabric and Ventilation heat losses (see Buildings and Heat Transfer). The fabric loss is the heat loss through the walls, floor and roof of the building and is dependent upon the overall thermal transmittance or “U” value of each component and their respective surface areas. The ventilation heat loss is dependent upon the fresh air ventilation rate into the space and the need to raise it to room temperature (see Overall Heat Transfer Coefficient).

These calculations are documented in CIBSE (1980), ASHRAE (1993) Chapter 8, and McQuinston (1994). They produce a calculated heat load in kW, which can be used to select the appropriate boiler plant and heat exchangers for the heating cycle.

Heat gains

The heat gains to the building are rather more complex and must be calculated on a dynamic basis considering variations in external conditions and time lags of heat flow through the building fabric. The instantaneous heat gain into a conditioned space, is quite variable with time, because of the strong transient effect created by the hourly variation in solar radiation. There may be an appreciable difference between the heat gain of the structure and the heat removed by the cooling equipment at a particular time. This difference is caused by the storage and subsequent transfer of energy from the structure and contents to the circulated air. Ignoring these factors can lead to gross oversizing of equipment.

In addition to the external heat gains from solar radiation and ventilation, internal gains caused by occupants, lighting and machinery must be included (see Physiology and Heat Transfer). The internal gains can be determined from the density of occupation and activity levels together with the expected lighting and equipment levels for the conditioned space. It is, however, necessary to predict future demands especially for equipment which has increased rapidly in modem office buildings.

Solar radiation is dependent upon the intensity of incident solar radiation, its angle of incidence and the properties of the building envelope (see Solar Energy). Solar gains can be broadly subdivided into direct and diffuse components. The direct component is directly transmitted through the earth's atmosphere and as such has a defined angle of incidence on the surface of the building. The diffuse component is scattered by the earth's atmosphere and has no discernible angle of incidence. Nevertheless, it is absorbed at the building surface and can be conducted through to the inside. In addition, it serves to warm the external temperature, which adds to the heat gain through the intake of ventilation air.

Direct solar radiation received at the outside surface of the building will be reflected, absorbed, and directly transmitted in proportions dependent upon the surface materials and the angle of incidence. The component directly transmitted through glass is of greatest importance to the air conditioning cooling load.

The cooling loads for different configurations of single and double glazing with different types of glass, clear, absorbing, reflecting etc. are tabulated in the CIBSE guide CIBSE (1979a).

In addition to the sensible heat gains described above there is a need to evaluate the latent heat gains based on the moisture generated from the occupants, ventilation and any process moisture that is generated within the space.

CIBSE (1986), CIBSE (1986a), and Chapter 26 of ASHRAE (1993) provide detailed calculation procedures for estimating heat gains. These procedures and others have been computerized so that most heat gain analysis is carried out by computer calculation or modeling. Simpler methods suitable for manual calculations are described in McQuinston (1994).

Estimation of energy consumption

The selection of air conditioning plant is carried out on the basis of design conditions, ensuring that the plant is of adequate capacity to maintain the required comfort conditions within the space under the normal range of climatic conditions for the region. The estimation of energy consumption on the other hand requires detailed information of the diurnal and seasonal variations in external temperature and incident solar radiation as well as the pattern of occupation of the building.

Energy consumption will depend upon the efficiency of the air conditioning plant and of the controls employed to maintain the comfort conditions.

Many computer codes are available that model building systems quite well, and they may be used with all types of structures. The building simulation is usually carried out for a whole year considering heating, cooling and other energy requirements. There are cases, however, where computer simulation cannot be justified. In these cases, reasonable results can be obtained using the simpler degree day or bin method described in McQuinston (1994). Energy estimating is discussed in detail in Chapter 28 of ASHRAE Handbook of Fundamentals ASHRAE (1993).

Air conditioning psychrometrics

Comfort conditions are maintained within a building by supplying conditioned air at an appropriate state to overcome the incident heating and cooling loads. The required state will have specified conditions of temperature and moisture content and the processes required to achieve that supply state can be evaluated by air conditioning psychrometrics. Tabulated data defining the psychrometric properties of moist air can be found in the data section of the CIBSE Guide [CIBSE (1975)].

The changes in psychrometric properties of a sample of air as it passes through an air conditioning system can best be visualized by use of a psychrometric chart which, for a given barometric pressure, presents the psychrometric properties of moist air over a range of conditions. The standard psychrometric chart used in the UK is that produced by the Chartered Institution of Building Services Engineers (CIBSE) and reproduced here as Figure 1. Any point on the chart is defined by two psychrometric properties and from the chart all other properties of the state point can be determined. A schematic representation of the various air conditioning processes is given in Figure 2.

Psychrometric chart.

Figure 1. Psychrometric chart.

Schematic representation of air conditioning processes.

Figure 2. Schematic representation of air conditioning processes.

A full description of the psychrometrics of various air conditioning plants is given in Jones (1985), Look and Sauer (1986), Eastop and McConkey (1993), and McQuinston (1994).

Supply state

The required supply state of conditioned air to maintain thermal comfort within a room will depend upon the sensible and latent energy gains.

There are an infinite number of combinations of supply temperature and moisture content and volume flow that will satisfy the heat load requirements. These conditions can be identified on the psychrometric chart by determining the Room Sensible Heat Factor (RSHF).

where

= sensible heat gain (kW),
= latent heat gain (kW),
= total head gain (kW).

The cosine of this factor gives the slope of a line on the psychrometric chart which, when drawn through the required room conditions, is known as the Room Line. Supply of air at any point on that line will provide the correct combination of temperature and moisture content to overcome the loads.

The volume flow rate can then be determined from the following equations [Jones (1985)]

or

where

= volume flow rate at temperature t (m3/s) ;
tr = room temperature (°C) ;
ts = supply temperature (°C);
T = temperature at which the flow rate volume flow rate is calculated (k);
gr = room moisture content (g/kg);
gs = supply moisture content (g/kg).

Air Conditioning Systems

Air conditioning systems are generally characterized by their thermofluid distribution medium, air or water, and by their means of controlling heating and cooling. They may also be classified as central or local depending on the plant employed.

Four major categories of air conditioning systems may therefore be identified

  1. Centralized all air

  2. Centralized air and water

  3. Centralized all water

  4. Localized

Centralized all air systems

These systems consist of a central plant room where incoming air is conditioned to a controlled supply state and then distributed throughout the building. They are classified [ASHRAE (1992)] into single duct, dual duct and multizone systems which are then subdivided into constant and variable air volume systems.

Constant volume systems meet changing air conditioning loads by varying the supply state of the air. Thus, the heat gain to the space rises so the supply temperature is reduced. On the other hand, variable air volume systems meet changing loads by keeping the same supply state but varying the volume supplied.

Centralized air and water

These systems again utilize a central plant room to generate chilled or hot water and conditioned air to distribute throughout the building. However, the volumes of air distributed are usually much less than for the all air systems because it is only the intake of fresh air for ventilation that is conditioned centrally. Additional room air is treated within the conditioned room by passing it through a terminal plant served by the chilled or heated water distributed from the central plant room.

The systems can be further categorized by the mode of operation of the terminal plant. There are two main types of terminal plants: fan coil units or induction units. Full description of these systems can be found in ASHRAE (1992).

Centralized all water systems

These systems operate on the distribution of chilled and heated water from a centralized plant room to terminal plant such as fan coil units where heating and cooling of the air takes place locally. The requirement for fresh air ventilation must be provided locally either by natural infiltration or by an opening through the wall.

It is not possible with these systems to control the humidity within the space as the terminal plant includes only sensible heat exchangers.

Localized air conditioning systems

As the name suggests, these systems are stand alone units that are located in or near the space to be conditioned. They are based upon the vapor compression cycle (see Refrigeration). In this case the evaporator is housed within the conditioned space, cooling the room air that is drawn across it. The condenser on the other hand is positioned outside the conditioned space dissipating the heat to atmosphere.

If the roles of the two heat exchangers, evaporator and condenser, are reversed the unit can be used as a heat pump heating the inside air from the external source.

Heat recovery

Wherever air conditioning systems employ return air ductwork there is a potential for heat recovery from the exhaust air before disposal to atmosphere.

This heat recovery is accomplished through the use of air-to-air heat exchangers (see Heat Exchangers). Under certain climate conditions, when the return air is nearer the supply conditions than the external air, maximum recirculation will reduce energy consumption. However, there will always be a need to exhaust some air as there is always a need for fresh air intake for ventilation and the well being of the occupants.

The viability of the introduction of heat recovery depends upon the expense of routing the ductwork, the capital and installation costs of the heat recovery plant and the usefulness of the recovered heat.

Control and instrumentation

Since the loads in various zones of a building vary with time, control systems are used to match the output of the HVAC system to the loads. A HVAC system is designed to meet the extremes in the demand, but most of the time it operates at part load conditions. A properly designed control system maintains good indoor air quality and comfort under all anticipated conditions with the lowest possible cost of operation.

Controls may be pneumatic, electric, electronic or they may even be self contained, where no external power is required. Developments in both analogue and digital electronics and in computers, have allowed control systems to become much more sophisticated. These systems also offer additional monitoring capability allowing efficient energy management. Control systems are described in detail in ASHRAE (1991) and Haines (1983).

REFERENCES

ASHRAE (1993) ASHRAE Handbook of Fundamentals. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.

ASHRAE (1992) ASHRAE Handbook of HVAC Systems and Equipment. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.

ASHRAE (1991) ASHRAE Handbook HVAC Applications. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.

CIBSE (1986) CIBSE Guide Section A7 Internal Heat Gains. Chartered Institute of Building Services Engineers, London.

CIBSE (1986a) CIBSE Guide Section A8 Summertime Temperatures in Buildings. Chartered Institute of Building Services Engineers, London.

CIBSE(1982) CIBSE Guide Section A2 Weather and Solar Data. Chartered Institute of Building Services Engineers, London.

CIBSE (1980) CIBSE Guide Section A3 Thermal Properties of Building Structures. Chartered Institute of Building Services Engineers, London.

CIBSE (1979) CIBSE Guide Section A1 Environmental Criteria for Design. Chartered Institute of Building Services Engineers, London.

CIBSE (1979a) CIBSE Guide Section A9 Estimation of Plant Capacity. Chartered Institute of Building Services Engineers, London.

CIBSE (1975) CIBSE GUIDE Section C1 Properties of Humid Air, Chartered Institute of Building Services Engineers, London.

Eastop, T. D. and McConkey, A. (1993) Applied Thermodynamics, Longman Scientific and Technical, Harlow.

Fanger, P. O. (1972) Thermal Comfort. McGraw Hill, New York.

Jones, W. P. (1985) Air Conditioning Engineering. Edward Arnold.

Look, D. L. and Sauer H. J. (1988) Engineering Thermodynamics. Van Nostrand Reinhold (International), Wokingham.

McQuinston, F. C. and Parker, J. D. (1994) Heating, Ventilating and Air conditioning. Fourth Edition, New York: John Wiley & Sons, Inc.

Roger, W. H. (1983) Control Systems for Heating, Ventilating and Air Conditioning. 3rd Edition, New York: Van Nostrand Reinhold.

References

  1. ASHRAE (1993) ASHRAE Handbook of Fundamentals. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.
  2. ASHRAE (1992) ASHRAE Handbook of HVAC Systems and Equipment. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.
  3. ASHRAE (1991) ASHRAE Handbook HVAC Applications. American Society of Refrigeration and Air-Conditioning Engineers Inc. Atlanta, GA.
  4. CIBSE (1986) CIBSE Guide Section A7 Internal Heat Gains. Chartered Institute of Building Services Engineers, London.
  5. CIBSE (1986a) CIBSE Guide Section A8 Summertime Temperatures in Buildings. Chartered Institute of Building Services Engineers, London.
  6. CIBSE(1982) CIBSE Guide Section A2 Weather and Solar Data. Chartered Institute of Building Services Engineers, London.
  7. CIBSE (1980) CIBSE Guide Section A3 Thermal Properties of Building Structures. Chartered Institute of Building Services Engineers, London.
  8. CIBSE (1979) CIBSE Guide Section A1 Environmental Criteria for Design. Chartered Institute of Building Services Engineers, London.
  9. CIBSE (1979a) CIBSE Guide Section A9 Estimation of Plant Capacity. Chartered Institute of Building Services Engineers, London.
  10. CIBSE (1975) CIBSE GUIDE Section C1 Properties of Humid Air, Chartered Institute of Building Services Engineers, London.
  11. Eastop, T. D. and McConkey, A. (1993) Applied Thermodynamics, Longman Scientific and Technical, Harlow.
  12. Fanger, P. O. (1972) Thermal Comfort. McGraw Hill, New York.
  13. Jones, W. P. (1985) Air Conditioning Engineering. Edward Arnold.
  14. Look, D. L. and Sauer H. J. (1988) Engineering Thermodynamics. Van Nostrand Reinhold (International), Wokingham.
  15. McQuinston, F. C. and Parker, J. D. (1994) Heating, Ventilating and Air conditioning. Fourth Edition, New York: John Wiley & Sons, Inc.
  16. Roger, W. H. (1983) Control Systems for Heating, Ventilating and Air Conditioning. 3rd Edition, New York: Van Nostrand Reinhold.

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