REFRIGERATION LOAD CALCULATION
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| Cold Storage |
The load calculation is generally, made for a 24 hrs period and covers the following:
1. Transmission Load,
2. Air changes load,
3. Product load,
4. Electrical lighting & motor load
5. Load due to person.
The method of calculating the various loads is as follow:
1. Transmission Load:
Transmission load is denoted by Qt. The transmission heat gain gain through roof, floor & walls can be detemined by the following equation :
It S.I unit in KWh
Qt = Area x (U factor/1000) x Δ T x 24 hrs
Where,
Qt - Transmission load in KWh
U Factor - Heat gain for wall, roof & ceiling in (W/m2°C)
Δ T - Temp. difference between the outside temp. & inside temp. plus the effect of solar gain on wall, roof & floors.
The solar effect may be considerd as follows:
Walls East South West Roof
Medium colour 3 2 3 8
Light colour 2 1 2 5
2. Air Changes Load :
The fresh air enters the cold store due to the door opening and the refrigeration load. The average air changes per day can be considered as follow:
Room Volume Average air changes Average air changes
upto m3 / 24 hrs at 0 °C / 24 hrs at -20 °C
Room Temp Room Temp
10 32 24
100 9 7
500 2.7 2.3
1000 2.5 2
2000 1.7 1.3
The air changes can be increased by 100% in case of heavy usage causing frequents door openings. if strip curtains are provided on cold room doors the air changes can be reduced by around 50%
The air changes load can be calculated by the following equation :
Qa = Air qty.kg/day x (H1-H2)
Where,
Qa - Air Changes load in KWh
H1 - enthalpy of air at ambient condition in kJ/kg
H2 - enthalpy of air at inside temp. in kJ/kg
3. Product Load :
The product load in case of normal cold store can be calculated as follow:
Qp = Product x specific heat x (Ti-Tf)
Where,
Qp - Product Load in Kwh
Product - Product (kg/day)
Specific Heat - For product in kj/kg/24hrs
Ti - Initial temp of product in °C
Tf - Final temp of product in °C
If the product is pre-frozen the additional cooling laod can be calculated as per above formula. However, if the product is not frozen the additional laod for freezing & sub cooling have to be considered apart from the product cooloing/freezing load. The heat generated due to respiration has to be considered. However, these values are very samll and can be ignored for pratical purposes.
4. Electrical Lighting @ motor load :
The lighting level vary between 2.5 watts/m2 to 10 watts/m2 in cold stores. The fan motors generate the heat as per the energy inputs to the motor. The cooling load can be calculated as follows:
For Lighting :
Ql (kwh) = (Total watts / 1000) x no. of hrs/day x 1
The following rule can be used for the heat generated by size of motors
For motor from 0.25 to 0.37 kw = 1.43
For larger motors kw = 1.14
Additional load may be considered for fork lift trucks etc. if operated in the cold store.
For Motors :
Qm (kwh) = Total kw x heat generated per kw x No. of hrs/day
5. Load due to person :
The person working in the cold store dissipate heat at a rate depending on the temperature of the room but, usually for a sort period during the day. The typical amounts of heat generated by a person can be taken as follows:
°C watts/person
5 240
0 270
-20 390
Thus the heat generated can be calculated as
Qw(kwh) = no. of person x no. of hrs day x heat generated/person
Safety Factor
The safety factor of 5 to 10 % can be considered on the refrigeration load to obtain the total refrigeration load calculated. Generally the running time for the plant is considered as 16 to 20 hrs/day and the hourly load can be calculated on the basis of plant operation.
Note:
- Generally the product cooling is assumed to be achieved within 24 hrs. However if faster cooling is required the product load can be calculated separately on hourly basis and added to the total of other loads reduced to hourly basis.
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