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 (cinema-library

). 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= 35⁰C DBT and 45 % RH

IDC IN SUMMER=   23⁰C DBT and 60 % RH

ODC IN WINTER = 4⁰C DBT and 50 % RH

IDC IN WINTER = 21⁰C 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

1. ROOM SENSIBLE HEAT LOADS (RSHL)

RSHL =Room Sensible Heat Loads are due to                                                                    

 (a) Conduction through four walls, floor and roof

Q=U _Roof A_Roof ΔT_Roof + U₁A₁ΔT₁ (For wall 1) + U₂A₂ ΔT₂ (For wall 2) + U₃A₃ΔT₃ (For wall 3) + U₄A₄ΔT₄ (For wall 4) + U_FloorA_FloorΔT_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

 

1. On glass location west or east side

2. Time of the day

3. Date

4. 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

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

2. 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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