How to calculate the cooling load in a cold storage?
Cooling load in a cold storage indicates the amount of heat that must be removed from the refrigerator in a certain time to reach the desired temperature. In calculating the refrigeration load, the capacity, volume and dimensions of the cold room, the number of doors, the type of insulation, the amount of air exchange, etc., must be taken into account, which are fully explained in this article.
What is the purpose of calculating the cooling load in a cold storage ?
The purpose of calculating the cooling load in a cold storage is to obtain correct and accurate information to choose the best compressor, condenser, evaporator and expansion valve for our desired cold storage.
Accurate calculation of cooling load in a cold storage is very important because, as mentioned, the cold storage equipment is selected based on the cooling load required by the cold storage, and if the equipment is not selected correctly, there is a possibility of spoilage of medicine, food, etc. which ultimately leads to financial losses.
In addition to the fact that there are special formulas for calculating the cooling load in a cold storage, Palmore International experts and specialists use software designed for this purpose in order to speed things up and perform accurate calculations.
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The formula for calculating the cooling load in a cold storage
To calculate the cooling load in a cold storage, the total heat energy wasted from the following items must be added together:
1- Energy losses from the walls
2- Losses caused by opening and closing doors
3- The heat that is transferred to the cold store from the products that have just entered
4- Other heat losses related to employees, lighting, etc.
1- Calculation of energy losses from the walls
Usually, the walls, floor and ceiling of the cold storage are insulated with the help of sandwich panels of 10 to 15 cm and sometimes more. In some cases, injection foam is used for insulation. However, some of the outside heat still enters the cold room and affects the produced cold to some extent.
To calculate this amount, just use the following formula:
Q = A×U×∆T
A = area of the wall
U = heat transfer coefficient of wall type (available in relevant tables)
T∆ = temperature difference between outside and inside cold storage
2- Calculation of heat losses from opening and closing the door
Every time the door of the fruit refrigerator or any other refrigerator is opened and closed, a large amount of heat will enter the refrigerator and remove the resulting coldness. In many large and industrial cold stores, air curtains are used to prevent this heat exchange.
In general, the following formula is used to calculate the amount of energy lost due to the door being left open.
Q = N × H
In the above formula, N is the air entry rate and H is the amount of heat entered per liter of air, which values can be obtained by referring to the relevant tables.
3 - Estimating the amount of coldness absorbed by the products inside the cold room
Usually, the items that enter the cold room have a higher temperature than the temperature inside the cold room and absorb some of the coldness. But in some cases, to maintain the quality of the products, they are brought into the cold room after passing through the freezing tunnel, in which case they will have a lower temperature than the cold room temperature, and there is no need to calculate their cold load.
The following formula is used to calculate the heat obtained from newly imported products to the cold storage, which is explained below:
Q=mC1(T1-Tf) + ml + mC2(Tf-T2)
M = product weight
C1 = specific heat of product at above freezing temperature
C2 = specific heat of the product after freezing
T1 = product temperature when entering the cold storage
Tf = freezing temperature of the product
T2 = final product temperature
L = latent heat of freezing
After calculating the heat obtained from newly imported products to the cold storage based on the above formula, we must divide the amount obtained by the time required to freeze the product. For example, if the time required to freeze an apple is 18 hours, the cooling load should be divided by this time.
It should be noted that the heat related to product packaging should be calculated separately, for example, for wooden boxes it is calculated 2.5 kJ/kg and cardboard boxes 1.4 kJ/kg. For other boxes, you can refer to the relevant table.
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4- Calculating other heat losses
Other factors such as the entry and exit of labor, the heat from the work of the evaporator fan and the defrost system, the heat from the lamps, etc. will also be estimated according to the following formula.
Heat caused by workers:
Q = N×T×H
N = number of employees
T = number of hours of activity
H = heat generated per person
Heat caused by equipment:
Q = N×P×H
N= number of equipment
P = thermal power of each of them
T = duration of their operation
In this article, we explained the formula for calculating cooling load in a cold storage in a simple way. If you need help to buy or set up a cold storage and do related calculations, Palmore International experts are ready to give you advice for free. Palmore is a supplier and manufacturer of cold storage and cold room in Oman, which offers you all the services from consulting to setting up and installing the equipment you need in the field of cold storage and cold room.