Heat Load Calculations Made Easy by Popular Demands
Many readers have been requesting for the actual method of heat load calculations using which they can carry out the heat load calculations very easily without going into more details. As such the heat load calculations is a long and complicated process. To make the life easier for the readers and those who want to do heat load calculations in professional manner, I have discussed the method using the heat load calculation form. This is a very easy method that can be used to perform the calculations manually or you can put the data simply in the Excel sheet and get the heat load calculations done immediately.
In the attached form herewith you will see various sources of heat and some factors associated to them. First of all let us try to understand the various sources of heat mentioned in the “Item" column of the form. You can print this form and use it for all the heat residential load calculations. There is a separate form for the heat calculations of the commercial building, which will be shared in the next series of articles. By the way, the form attached here has all the terms in BTU, I will soon try to get the one in SI units as well, however the description given below remains the same for both.
Heat Load Calculations Form
Sources of Heat inside the Residential Room
The application of air conditioning is required because heat is generated inside the room from various sources. The main purpose of the air conditioning systems is to remove this heat and create comfort conditions. The temperature and humidity required for the human comfort are 25 degree C and 50% respectively. Due to various heat sources the temperature inside the room becomes very high so all the heat generated inside the room has to be removed.
Before actually starting the heat load calculations let us try to see the various sources of heat inside the room mentioned in the items list of the heat load calculations form (please refer the attached form above).
Solar Heat Gain by the Windows
1) Solar Heat Gain from the Windows
The solar heat is one of the prominent sources of heat inside the room. It enters the rooms via windows, walls and the roof. The solar heat entering the room via windows produces the heating effect immediately. The amount of heat entering the room via windows depends on following factors:
a) The size of the window: Larger the size of the window more is the heat gained from it.
b) Orientation of the window: This is the direction of the window in the room. As per the various positions of the sun throughout the day, it has been found that in the morning the maximum amount of heat is absorbed by the windows in the eastern direction (see the images below). This means the windows in east direction absorb maximum heat in the morning when sun rises. In the afternoon the sun reaches overhead position so the windows in south absorb maximum heat in the afternoon. But this intensity of heat is lesser than that absorbed from east and west. In the late afternoon sun reaches western side and its temperature becomes maximum around 4pm. Thus the windows in west absorb maximum heat in the late afternoon. The amount of heat absorbed by the windows in east and west directions is maximum, it’s lesser for the windows in south direction and least for the windows in north direction since sun does not move to north.
c) The glass used for the windows: The double glass used for the windows helps reducing the solar heat gained from the windows. Similarly, if the glass is covered with black or other color shades the amount of solar heat absorbed by it reduces.
d) The awnings used for the windows: The awnings built outside the windows help reduce the amount of direct heat absorbed by the window.
e) The curtains used for the windows: The curtains also play important role in absorption of heat by the windows. The curtains can be thick or thin, inside the room or outside the room or on both sides, the color of the curtains can be dark or light. All the curtains help reduce the amount of heat absorbed by the windows. The dark shades, thick curtains and curtains on both the sides are more effective.
2) Solar Heat Gained by the Walls
Just like the windows, the walls also gain solar heat by conduction and radiation heat transfer methods. The heat gained by the wall is not released inside the room immediately, rather the heat gained by the wall is stored inside it and it is released in the room in late night. This heat creates uncomfortable conditions and it has to be removed from the room. The amount of heat absorbed by the wall depends on following factors:
a) Size of the wall
b) Orientation of the wall
c) Thickness of the wall
d) Material of construction of the wall
e) Insulation on the wall, if any and it is of one inch or two inch
3) Heat Gained by the Partitions:
There can be partitions inside the room with air conditioned room or non-air conditioned room. The amount of heat absorbed by the partitions with non-air conditioned rooms is higher.
4) Solar Heat Gained by the Roof of the Room
Just like the windows and walls the solar heat is also absorbed by the roof of the room. Like the walls, the solar heat absorbed by the roof also reaches the room slowly. The roof exposed to the sun absorbs the heat continuously throughout the day, so it absorbs the maximum heat of all the factors. The amount of heat absorbed by the roof depends on:
a) The size of the roof
b) Thickness of the roof
c) Material of construction for roof
d) Insulation material stuck to the roof and its thickness
5) Heat Gained from the Ceiling:
The ceiling of the room is not exposed to the sun directly. Above the ceiling of the room there could be air conditioned room or non-air conditioned room. The heat is absorbed by the ceiling above which there is non-air conditioned room.
6) Heat Absorbed by the Floor:
If the floor of the room is exposed to some source of heat is absorbs the heat and it should be taken into account.
7) Heat Gained from Outside air or Infiltrated Air:
The outside air is at higher temperature than the room air. When it comes inside the room it raises the room temperature. The outside air can come inside the room due to opening of the door or it can leak inside through various openings in doors and windows, this air is also called as infiltrated air.
8) Heat Gained from the People
The people inside the room release lots of latent heat and the sensible heat. More the number of people inside the room more is the heat released inside it.
Once all the sources of heat described in the items list of the heat load calculations have been understood, we are now ready to perform the heat load calculations for the residential room. The first and the foremost step in heat calculations is carryout the survey of the room or building. In the next article we shall see various factors of the building to be surveyed.
Images and Heat Load Calculations Form Courtesy
Principles of Air Conditioning by V. Paul Lang Published in India by D. B. Taraporevala Sons & Co. Private Limited.