COOLING LOAD CALCULATIONS CLASS NOTES FOR MECHANICAL ENGINEERING
COOLING LOAD
CALCULATIONS CLASS NOTES
FOR MECHANICAL
ENGINEERING
Cooling load calculations help to find the
cooling capacity of a refrigeration plant
for a certain application (cinemalibrary
). These cooling loads consist of sensible
and latent heat loads. Sensible heat loads
are due to temperature difference. Latent
heat loads are due moisture difference.
Some of cooling loads are building
transmission load, occupant’s loads,
infiltration load, lighting load,
ventilation load etc..
OUTSIDE AND INSIDE DESIGN CONDITIONS
Outside design conditions (ODC) differ from place to place. These are different for Delhi, Tamil Nadu or elsewhere. One such example is
ODC IN SUMMER= 35^{0}C DBT and 45 % RH
IDC IN SUMMER= 23^{0}C DBT and 60 % RH
ODC IN WINTER = 4^{0}C DBT and 50 % RH
IDC IN WINTER = 21^{0}C DBT and 50 % RH
Sensible heat
Sensible heat= Caused by Dry Bulb Temperature difference and no change in moisture content
Latent heat
Latent heat = Caused by change in moisture content but no change in Dry Bulb Temperature
Total heat = Sensible heat + latent heat
Fig. Cooling Load Calculations

ROOM SENSIBLE HEAT LOADS (RSHL)
RSHL =Room Sensible Heat Loads are due to
(a) Conduction through four walls, floor and roof
Q=U _{R}oof A_{Roof }ΔT_{Roof} + U_{1}A_{1}ΔT_{1 }(For wall 1) + U_{2}A_{2 }ΔT_{2 }(For wall 2) + U_{3}A_{3}ΔT_{3} (For wall 3) + U_{4}A_{4}ΔT_{4} (For wall 4) + U_{Floor}A_{Floor}ΔT_{Floor} kJ/h
where 1/U (for a wall) = 1/h_{i} +1/h_{o}+ x1/k1+ x2/k2 + x3/k3 + x4/k4
(When a wall consists of four layers)
(b). Solar heat gains through window glass
Q_{g} =A_{g} (U_{g }ΔT_{g} + SHGF x SC) kJ/h
A_{g}= net glass area =assume 80 % window size
SHGF=solar heat gain factor from tables, depending

On glass location west or east side

Time of the day

Date

Type of glass—ordinary or of some other type
Sc = shading coefficient found from tables depending upon color of the glass (light, medium, dark etc.)
(c) Sensible heat load from occupants (assume if not given as 160 kJ/h/person)
Q_{Occ}= n x160 kJ/h n is the number of persons
(d). Sensible heat gains from light loads

From incandescent light = wattage installed x use factor kJ/h

From fluorescent lights = 1.25 x Wattage installed x use factor kJ/h
(e) Sensible heat load from infiltrated air, it is due to the door openings in the AC room
Assume the size of door and frequency of door opening. Take volume flow rate of outside air due to door openings 4 times the volume of the room per hour.
=4 air changes/h
Q = V^{.}ρ C_{p} (t_{o}—t_{i}) kJ/h
Where V^{.} = volume flow rate due to air changes/h
(f) Sensible heat load from additional infiltrated air
It is due to the outside air entering the room due to
(i) clearance between the door and floor
(ii) Any crack or opening in the windows or ventilators
Assume air changes per hour due to additional infiltration
= 2 air changes/h
Q = V^{.}ρ C_{p} (t_{o}—t_{i}) kJ/h
Where V^{.} = volume flow rate due to air changes/h
(g) Sensible heat load from ventilated air
Assume air changes per hour because of fresh air requirements to keep carbon dioxide level < 2 %. It also keeps required oxygen in room for comfort of the persons.
Q = V^{.}ρ C_{p} (t_{o}—t_{i}) kJ/h
Where V^{.} = volume flow rate due to air changes/h
(h) Sensible heat load from miscellaneous sources like from computers, printers, tea, coffee, smoking etc.
Assume Q_{Misc} =10 % of sum of 1 to 7 SH loads kJ/h
Room Sensible Heat Load = sum of 1 to 6
(i) Sensible heat load from products
It will be applicable say for a fridge/cold storage/meat shop in a mall.
Q _{products}= m^{.} C_{p bf} (t—T_{Freezing}) + m^{.} C_{p aft }(T_{Freezing}—T_{Frozen state}) kJ/h
Suffix bf = before freezing and suffix aft = after freezing
Assume time of cooling and freezing to find the heat load per hour.
2. ROOM LATENT HEAT LOADS (RLHL)
RLHL = Room Latent Heat Loads are due to
(i) Latent heat load from occupants
(assume 240 kJ/h/person)
Q_{occ} = n x240 kJ/h
n is the number of persons
(ii) Latent heat load from infiltrated air, it is due to the door openings in the AC room
Assume the size of door and frequency of door opening. Take volume flow rate of outside air due to door openings 4 times the volume of the room per hour
=4 air changes/h
Q = V^{.}ρ C_{p} (w_{o}—w_{i}) LH of water kJ/h
where V^{.} = volume flow rate due to air changes/h
(iii) Latent heat load from additional infiltrated air. It is due to the outside air entering the room via clearance between the door and floor and through any crack or opening in the windows or ventilators.
Assume air changes/h due to additional infiltration= say 2 air changes/h
Q = V^{.}ρ C_{p} (w_{o}—w_{i}) LH of water kJ/h
where V^{.} = volume flow rate due to air changes/h
(iv) Latent heat load from ventilated air
Assume air changes/h because of fresh air requirements to keep carbon dioxide level <2 % and to keep more oxygen in room for comfort
Q = V^{.}ρ C_{p} (w_{o}—w_{i}) LH of water kJ/h
where V^{.} = volume flow rate due to air changes/h
(v) Latent heat load from miscellaneous sources like from tea, coffee, food etc.
Assume Q_{misc} =10 % of sum of (i) to (iv) LH loads
Room Latent Heat Load =Sum of LHL (i) to (v)
ROOM SENSIBLE HAET FACTOR (RSHF)
RSHF=RSHL/(RSHL+RLHL) =RSHL/TRHL
where RSHL is room sensible heat load
RLHL is room latent heat load
TRHL is total room heat load
Example – Room Sensible Heat Factor – RSHF
The sensible heat gain in a room is 40 kW while The latent heat gain is 16 kW.
The Room Sensible Heat Factor will be:
RSHF = (40 kW) / ((40 kW) + (16 kW)) = 0.71
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