Refrigeration capacity is a critical factor when designing a cold room. It refers to the amount of heat a refrigeration system can remove from a space in a given time. It is typically expressed in kilowatts (kW) or British Thermal Units (BTU) per hour.
The refrigeration capacity needed depends on several factors, including the size of the cold room, the desired temperature, and the type of goods being stored.
In this article, we’ll explore how to calculate the refrigeration capacity needed for a cold room and the factors to consider.
Refrigeration Capacity Calculation Process (Step-by-Step)
To calculate the refrigeration capacity needed, you must take into account the following elements:
1. Room Size (Volume of the Cold Room)
The first step is to calculate the volume of the cold room in cubic meters (m³) or cubic feet (ft³). This is done by multiplying the length, width, and height of the cold room:
Volume (m³) = Length×Width×Height
Example: If the room is 5 meters long, 4 meters wide, and 3 meters high:
Volume = 5 m×4 m×3 m = 60 m³
2. Temperature Difference (ΔT)
The temperature difference is the difference between the desired cold room temperature and the ambient temperature (the temperature of the room surrounding the cold storage area). This is often referred to as the “temperature differential” (ΔT). The colder the desired room temperature, the higher the refrigeration capacity needed.
For example:
- If the ambient temperature is 25°C and the desired temperature inside the cold room is 5°C, then ΔT = 20°C.
3. Heat Load from the Products Stored
The amount of heat introduced into the cold room by the products depends on their type, volume, and how often they are added or removed. A product’s heat load also takes into account factors like the temperature at which products enter the cold room and their thermal mass.
You can calculate the heat load of products by considering their weight and temperature difference from the cold room temperature. This is usually calculated with the following formula:
Q = m×C×ΔT
Where:
- Q is the heat load (in kilojoules or BTUs per hour)
- m is the mass of the product (in kilograms or pounds)
- C is the specific heat capacity of the product (in kJ/kg·°C or BTU/lb·°F)
- ΔT is the temperature difference between the product and the cold room
For instance, storing 1000 kg of produce at 20°C entering a cold room at 5°C requires calculating the heat load based on the specific heat capacity of the produce.
4. Heat Load from Lighting and Equipment
Lighting and other electrical equipment in the cold room generate heat. The heat load from lights can be estimated as follows:
- Incandescent lights typically release about 0.5-1.5 watts of heat per watt of energy consumed.
- Fluorescent lights typically generate about 0.1-0.2 watts of heat per watt consumed.
For example, if you have 10 fluorescent lights, each consuming 40 watts, the total heat load from lighting would be:
Heat Load from Lights = 10×40×0.2 = 80 watts
5. Other Factors to Consider
In addition to the heat load from the products, lighting, and ambient air, other factors can impact the refrigeration capacity:
- Insulation Quality: Well-insulated rooms require less refrigeration capacity.
- Door Frequency and Size: Frequent door openings allow warm air into the room, increasing the cooling load.
- Ventilation: Proper ventilation systems prevent condensation buildup, which can affect the cold room’s performance.
- Humidity Levels: High humidity levels can also increase the refrigeration load due to the latent heat of moisture in the air.
6. Total Cooling Load Calculation
The total refrigeration capacity required is the sum of all the individual heat loads:
Total Cooling Load (W or BTU/h)=Heat Load from Products + Heat Load from Lighting + Heat Load from Ambient Temperature + Other Factors
To convert the total load to refrigeration capacity (usually measured in tons of refrigeration or kW), divide by the appropriate conversion factors:
- 1 ton of refrigeration = 3.517 kW
- 1 ton of refrigeration = 12,000 BTU/h
7. Refrigeration Capacity Selection
Once you have the total heat load in watts or BTUs, you can select the refrigeration system with the required capacity. Ensure that the system is designed for a slight overcapacity to account for unforeseen variables such as higher ambient temperatures or unexpected product loads.
Example:
For a cold room with a volume of 60 m³, a temperature difference of 20°C, and a heat load from stored goods, lighting, and air infiltration, the calculated refrigeration capacity might look like this:
- Heat load from stored goods: 1500 W
- Heat load from lighting: 80 W
- Heat load from air infiltration and other factors: 500 W
Total cooling load:
Total Cooling Load = 1500+80+500 = 2080 W
Converting to refrigeration capacity:
Refrigeration Capacity = 2080/1000 = 2.08 kW
This would be the required refrigeration capacity for the cold room in kilowatts. A 2.5 kW system would likely be selected to provide a buffer.