Refrigeration and Air-conditioning Unit-5.pptx

Introduction to cooling load calculations.
Types of load.
Design of space cooling load .
Heat transmission through building.
Solar radiation.
Infiltration.
Internal heat sources (sensible and latent).
Outside air and fresh air load.
Estimation of total load.
Domestic, commercial and industrial systems.
Central air conditioning systems.

1. UNIT - 5 Cooling Load Calculations

2. Introduction The primary requirement of cooling or heating equipment is that these must be able to remove or add heat at the rate at which it is produced, or removed and maintain the given comfort conditions in the room. The cost of the equipment will be quite high if it is designed for maximum heating or cooling loads. The Estimation of load involves the following variables: Magnitude and direction of wind velocity. Outside humidity and temperature. Nature of construction, materials used. Orientation of openings, windows and doors. Periods of occupancy and the number of persons in the room, activities of the persons etc. For a practising engineer it is not possible to make detailed calculations and therefore he mu evaluate consult latest editions of 'Heating, ventilation and air conditioning guide' and make quick easy estimates.

3. COOLING-LOAD ESTIMATE classification of cooling-load: Room load-which falls on the room directly. Total load-which falls on the air-conditioning apparatus. Room Load: Room sensible heat (RSH): Solar and transmission heat gain through walls, roof, etc. Solar and transmission heat gain through glass. Transmission gain through partition walls, ceiling, floor etc. Infiltration. Internal heat gain from people, power, lights, appliances etc. Additional heat gain not accounted above, safety factor etc. Supply duct heat gain, supply duct leakage loss and fan power.

4. b . Room Latent Heat (RLH): Infiltration. Internal heat from people, steam, appliances etc. Vapour transmission. Additional heat gain not accounted above, safety factor etc. Supply duct leakage loss . Grand total load (on air-conditioning apparatus) Sensible heat: Effective room sensible heat. Sensible heat of the outside air that is not by-passed. Return duct heat gain, return duct leakage gain, dehumidifier pump power and dehumidifier and piping losses . (b) Latent heat: Effective room latent heat. Latent heat of outside air which is not by-passed. Return duct leakage gain.

5. Grand total heat (GTH) = Total sensible heat (TSH) + Total latent heat (TLH ) Heating-load Estimate: Heating-load estimate is prepared on the basis of 'maximum probable heat loss of the room or space to be heated . Transmission heat loss. The transmission heat loss from walls, roof, etc., is calculated on the basis of just the outside and inside temperature difference . Solar radiation: Normally there is no solar radiation present and hence no solar heat gain at the time of the peak load which normally occurs in the early morning hours. 3. Internal heat gains. The heating requirement is reduced due to internal heat gains from occupants , lights, motors and machinery etc . Solar Radiation: The solar radiation intensity normal to the sun's rays incident upon a plane surface situated in the limits of the earth's atmosphere, varies with the time of the year as the distance of the earth from the sun changes. Its value when the earth is at its mean distance from the sun is called the 'solar constant. The normal value of the solar constant is assumed as 5045 kJ/m²-h.

6. Beam or direct radiation: The part of the sun's radiation which travels through the atmosphere and reaches the earth's surface directly is called Beam or direct radiation. It is maximum when the surface is normal to the sun's rays . Diffuser sky radiation . A large part of the sun's radiation is scattered, reflected back into space and absorbed by the earth's atmosphere. A part of this radiation is re-radiated and reaches the earth's surface uniformly from all directions. It is called diffuse or sky radiation . The total solar radiation reaching a surface is equal to the sum of the direct and diffuse radiations. Solar Heat Gain Through Glass : Glass which is major material of most buildings, provides the most direct route for entry of solar radiation. Heat transmitted through a glass surface depends on the wavelength of radiation and physical and chemical characteristics of the glass. Part of the radiation is absorbed, part is reflected and the rest is transmitted. Glass is opaque to the radiant energy emitted from sources below 200°C Thus glass has high transmitivity for short wavelength and low transmitivity for long wave length radiation .

7. Direct solar heat gain can be reduced by using different types of glass, glass construction and shades: Double pan glass reduces the solar heat by 10% to 20%. Special heat absorbing glass reduces the solar heat by 25 %. Stained glass can reduce it up to 65% depending upon its colour . Shading devices installed on the outside of windows reduce sun load up to 15 % Ventilation blinds and curtain shades reduce it by 30% to 35%. HEAT THROUGH BUILDING STRUCTURES (THERMAL BARRIERS )