Factor Affecting Air Conditioning Plant Capacity

Air conditioning is the simultaneous control of the temperature, humidity and pollution level within a limit of tolerance in a confined space. Such inside conditions may require to be maintained either for human comfort or for maintenance of equipment. The air conditioning load as such can be defined as that amount of heat which is instantaneously added or removed by the equipment. Heat flows from a hot medium to a colder medium and as such it is imperative that a comprehensive survey be made to insure accurate evaluation of components of the heat gain in the cooled space. The various factors affecting the heat gain and thereby contributing to the AC plant capacity are as follows:

  1. Orientation and location of building.
  2. General construction materials used.
  3. Types of windows and doors.
  4. Ceiling height.
  5. Outdoor and indoor design temperature and RH.
  6. Utility of space.
  7. Physical dimensions of space.
  8. Lighting.
  9. Occupancy.
  10. Appliances and equipment.
  11. Ventilation and infiltration.
  12. Leakages and heat loss in ducts.
  13. Thermal storage.
  14. Plant operation - Continuous or intermittent.
  15. Temperature swing and diversity factor.

    Orientation of the building has a direct bearing on the quantum of heat load. On account of sun's trajectory the Southern and Western sides transmit more solar heat compared to Eastern and Northern sides.

    The location of building is another important factor influencing the heat load. The solar heat gain through glass surface depends on the location of the building on earth's surface i.e, the latitude . The solar heat gain and transmission through walls depend also on the surroundings like nearby structure causing shading effect on the building.


    The type of materials used for construction of walls, roof, ceiling, floors, partition and their thickness play an important role on the heat load of the space to be air-conditioned. Painted or double glass, use of shading devices like venation blinds reduce the heat load. Provision of thicker walls, insulation and air space between double wall considerably reduce the heat transmission through the wall / roof surfaces.


    The solar heat on the outer edge of earth's atmosphere is estimated to be about 445 (Btu/hr)/ft² on December 21, when the sun is closest to earth and about 415 (Btu/hr)/ft² on June 21, when it is farthest away. The amount of solar heat outside the earth's atmosphere varies between these limits throughout the year. The solar heat reaching the earth surface is reduced considerably because large part of it is scattered, reflected back and absorbed by atmosphere. The solar heat by radiation and transmission through glass windows and transmission of heat through outer doors are considerable.


    The heat load through the walls is proportional to the surface area of the walls. More the ceiling height, more heat transmission will take place.


    The amount of heat to be removed also depends on the design temperature, RH and outside temperature RH. More the temperature difference of inside and outside conditions, higher the capacity of plant required.


    The area to be air-conditioned may be for the Office, Hospital, Auditorium, Factory or any specific application requiring different inside conditions.


    More the area of space to be air-conditioned, higher the plant capacity. The length, Width and height of the space determines the area exposed to outdoor / non-air-conditioned space.


    Lights convert the electrical power into light and heat. Heat gain from light will be 3.4 (Btu/h)/W in case of incandescent lights and 1.25 × 3.4 (Btu/h)/W in case of fluorescent light.


    The heat is generated within the human body by oxidation, commonly called metabolic rate. The metabolic rate varies with individual and the activity rate. The normal body processes are performed most efficiently at deep tissue temperature of about 98.6 °F. The human body is capable largely by conserving or dissipating the heat generated within it. The heat is dissipated by radiation, convection and evaporation of moisture from body surface and in the respiratory system. The amount of heat dissipated by radiations and convection is determined by the difference in temperature between the body surface and its surroundings. For normal office work this figure is taken as 450 Btu/h per person (250 Btu/hr sensible and 200 Btu/h latent heat).


    The total HP of various electrical motors, other electrical appliances in the space to be air-conditioned is to be assessed. Electrical motors contribute sensible heat to space by converting the electrical power input to heat. Some of this power is dissipated as heat in the motor frame. The location of equipment like motor, drive machine with respect to the conditioned space should be taken account while evaluating the heat gain from electrical motor at 3.4 (Btu/h)/W.


    Infiltration of air and particularly moisture into the conditioned space is frequently a source of sizeable heat gain or loss. The quantity of infiltration of air varies according to tightness of doors and windows, porosity of the building shell, height of building direction and velocity of wind, the amount of ventilation and exhaust air. Many of these cannot be accurately evaluated and must be based on the judgment of the estimator. Generally wind velocity or stack effect or both causes the infiltration.

    The introduction of outside air for ventilation of conditioned space is necessary to dilute the odors given, the amount of ventilation requirements varies primarily with the total number of people, the ceiling height and the number of people smoking. People gives of body odors which require a minimum of 5 CFM per person for satisfactory dilution and 7.5 CFM per person is recommended, based on a population density of 50 to 75 square feet per person and typical ceiling height of 8 feet. With greater population densities, the ventilation quantity shall be increased. When people smoke, a minimum of 15 to 25 CFM per person is found necessary. In special gathering rooms with heavy smoking, 30 to 50 CFM per person is recommended.

    The ventilation air requirement based on occupancy as well as any specific code requirements shall be evaluated and higher value should be used.


    This is mainly on account of supply air duct heat gain and leakages, AHU fan and humidifier pump heat gain. As per the normal design practice 17.5 % of total solar heat load, ventilation air and internal load is considered as heat gain through ducts.


    The actual cooling load is lesser than the instantaneous peak heat gain from sun, light, people, transmission through wall, roof and glass and other load, A large portion of this heat gain is radiant heat which does not become an instantaneous load on the equipment. The heat first strikes the solid surface and gets absorbed by the surface before becoming a load on equipment. When the radiant heat strikes the solid surface, it is absorbed, raising the temperature of the material and the air adjacent to the surface. This temperature difference causes the heat flow into the material by conduction and into the air by convection. Thus, the heat conducted away from the surface is stored and the heat convected from the surface becomes the instantaneous cooling load. The portion of radiant heat stored depends on the ratio of resistance to the heat flow and the resistance of most of the building material is much lower than the air resistance. Hence, most of the radiant heat is stored in building materials. However, with this process of absorbing radiant heat continues, the material becomes warmer and less capable of storing more heat. Highly varying and relatively sharp peak of the instantaneous solar heat gain results in a larger part of it being stored at the time of peak solar heat gain. This results in lower peak cooling load. Similar is the case with lighting load in a building.

    The storage heat factors are determined based on series of tests conducted in various types of buildings. The magnitude of the storage effect is determined largely by the thermal capacity of heat holding capacity of the materials surrounding the space. The specific heat of most of the building materials is 0.20 Btu/lb.°F and the thermal capacity is directly proportional to the weight of the material.


    The operating condition of air-conditioning plant is another factor which significantly affect the storage of heat. If the plant is not operated for 14 hours, some of the stored heat will remain in the building construction. This heat will be the extra-load to be removed and will appear as pull down when the plant is started the next day. Short periods of operation increase the pull down load.


    The permissible space temperature swing and the diversity factor on large air-conditioning system of multiple type of load also enable the selection of smaller plant capacity.

The proper selection of air-conditioning plant capacity requires the detailed consideration of all the above factors.


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