Cooling load is defined as, “the total heat required to be removed from the space in order to bring it to the desired temperature by air conditioning and refrigeration equipment”.
Purpose of cooling load estimation: To determine the size or capacity of air conditioning and refrigeration equipment, which is required to maintain desired inside conditions during the period of high temperatures. The design load is based on inside and outside design conditions. It is responsibility of ‘Air conditioning and Refrigeration Equipment Capacity’ to maintain the desired inside conditions.
Calculation of Cooling Load
The two main components of a cooling load are, (l) Sensible heat load and (2) Latent heat load.
Sensible heat load:
Different factors/sources of sensible heat load are :
Heat flow due to conduction, through exterior walls, floors, ceiling, doors, and windows due to temperature difference in the two sides:
U = Overall heat transfer coefficient,
A = Area of wall,
δT = Temperature difference between inside and outside
The heat flow is calculated as,
Q = Total heat transfer = U × A × δT
Heat flow due to solar radiation:
It consists of,
Heat transmitted through glass of Windows and doors
When a sheet of glass is subjected to solar radiation, a part of solar radiations incident on glass surface is reflected and the remaining part is transmitted directly into the interior of building.
Q = Total heat transfer = U × A × δT
Heat absorbed by walls and roofs exposed to solar radiation
It is calculated as,
Q = Total heat transfer = U × A × δT
Heat Gain to the space or room to be conditioned due to ventilation
This heat load is also called as “ventilation load”. The ventilation (i.e. supply of outside air) is provided to the conditioned space in order to minimise odour, concentration of smoke, carbon dioxide and other undesirable gases, so that, freshness of air could be maintained. The quantity of outside air used for ventilation should provide at least one-half air change per hour in buildings having normal ceiling heights. Also, if the infiltration air quantity is larger than the ventilation quantity, then the quantity of ventilation should be increased to at east equal to the quantity of infiltration air. This outside air adds sensible as well as latent heat.
Heat brought through leakage and infiltration OR Infiltration load:
The infiltrated air is the air, Which enters a conditioned space through Window cracks and opening of doors. This is caused due to the pressure difference between the room and the ambient pressure. Infiltration is also a function of wind velocity and density variation of air due to temperature difference between inside and outside air. There are two methods of estimating the infiltrated air.
(i) Crack length method (ii) Air change method.
In most of the cases, air change method is used. According to it, Quantity of infiltrated air through Windows and wall = (L × W × H × A)/60 in m3/min
Lightening load / Appliances load:
The appliances frequently used in air conditioned space may be electrical, gas fired or steam heated. The light generates heat due to conversion of electrical power into heat. If Wattage is known, then heat gain can be calculated as,
Q = Total Wattage × use Factor × Allowance Factor
Power equipment:
The power equipments such as fan, motor or any other equipment of this type adds heat in the air conditioned space. Also, the power consumed by the air conditioning fan is converted into heat energy, which is imparted to the air. If the fan is located before the air conditioner, the heat energy must be added to the tota load. If the fan is located after the air conditioner, the heat energy is added to the room sensible heat load. The electric motors used to operate the conditioning equipment within the conditioned space also add heat. The power consumed is converted into heat. For example, for electric motor, the heat added is given by,
Q = (Power ratinq of motor in (kW) / Motor efficiency) × Load factor in kW
Where, load factor is the fraction of the total load, at which, the motor is working.
Heat liberated by occupants
Heat gain from occupants is based on average number of people, who are expected to be present in the air conditioned space. Occupants load is very Important factor in cooling load calculations. On an average, human body liberates heat equivalent to heat given away by 200 W bulb. A healthy person in sleep also liberates heat equivalent to 60 W bulb. The heat loss to the surrounding is of main two types:
Sensible heat: This sensible heat loss to the surrounding is due to conduction and radiation, which depends upon difference between body temperature and surrounding temperature.
Latent heat: This latent heat loss to the surrounding is due to evaporation, which depends upon vapour pressure difference between body and surrounding air.
Thus, heat is thrown by all ways, i.e. conduction, convection and radiation. Evaporation of sweat also contributes to the cool-ng load. The factors affecting occupants load are,
- Age,
- Eating habits,
- Body structure,
- Activities done by human being.
Latent heat load
The latent heat gain to the space or room to be conditioned may occur due to any or all of the following sources.
- Heat gain due to moisture in the outside air entering by infiltration through window cracks and opening doors.
- Heat gain due to condensation of moisture from occupants.
- Heat gain due to condensation of moisture from any processes such as cooking foods, hot baths or any vaporization carried out in room.
- Latent heat gain from products or materials brought into the conditioned space.
The total heat load to be removed by air conditioning and refrigeration will be equal to addition of all the above heat gains i.e. sum of sensible and atent heat loads. Hence, cooling load of various enclosed infrastructures, such as, Cinema hall, Auditorium, Restaurant, Computer lab, MQC lab, MNC lab etc. can be calculated. Generally, split air condtioners are used for a restaurant, MQC lab or MNC lab. The number and size of spit air conditioners to be used depends upon the volume of space to be air conditioned.
Safety Factor Used in Cooling Load Calculation
For designing an air conditioned space, cool-ng load calculations are done for conduction, radiation from wall, occupant’s load, infiltration load, equipment load etc. All these calculations are done on a design day.
A design day is defined as,
- A day, when DST and WBT peak simultaneously.
- A day, when there is clear sky and there is no haze in air to reduce the solar radiation reaching earth surface.
- Internal heat loads being generated within the space are normal.
The calculation of all the heat loads requires many observations and standard tables and formulae to get total heat load. There are always chances of errors in observations and calculations. Therefore, to decide the design load, the total load is multiplied by safety factor. The safety factor usually ranges from 1.05 to 1.1.
Mathematically,
Design load = Safety factor × Total load
Safety factor is also called as factor of safety.