HEAT LOAD CALCULATIONS
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Calculations of Refrigeration Load
When determining the refrigeration load, all relevant loads must be assessed and summed. The operating time of the equipment is then estimated, and suitable products must be selected.
Key Factors in Determining Refrigeration Loads
Consider the following factors when calculating refrigeration loads:
- Wall, floor, and ceiling heat gain due to conduction
- Wall and ceiling heat gain from solar radiation (if the store has external surfaces)
- Air change load due to the ingress of outside air from infiltration and door openings
- Product load from incoming goods that need to be reduced to storage temperature, including freezing duties
- Heat of respiration from stored products
- Heat from operatives working in the area
- Lighting load
- Miscellaneous loads, e.g., forklifts, trucks, conveyors, etc.
- Cooler fan load
Wall, Floor, and Ceiling Conduction Heat Gains
Conduction heat gains refer to the heat entering the walls, ceilings, and floors of the building or storage room. This can be computed using the formula:
Conduction Gain = A x U x TD
Where:
- A is the total external surface area of the refrigerated room in square meters. This can be calculated as:
This accounts for the walls, ceiling, and flooring of the refrigerated area.
U is the overall rate of heat transfer for the wall panel in W/m²-C. If the ceiling has greater insulation thickness than the wall panels, separate calculations should be made.
TD is the temperature difference across the insulated panel, i.e., the difference between the internal temperature of the refrigerated compartment and the ambient temperature of the surrounding air.
For small rooms, the floor can be included in the total surface area. For larger cold stores, the floor heat gain should be calculated separately because the underfloor temperature will differ from the ambient air temperature.
Calculations for heat losses at edges and corners can be included for greater accuracy. Refer to any heat transfer textbook for the formula.
Solar Heat Gain
Solar heat gain can be calculated using the formula:
Solar Heat Gain = A x U x TD
Where:
A is the external surface on which solar radiation can fall. Remember that the sun can only shine on two walls and the roof at any one time.
U is the overall rate of heat transfer for the insulated panels, which will be the same value used for calculating the conducted heat gain.
TD is the additional temperature difference above the conduction heat gain temperature difference caused by the effect of solar radiation.
Air Infiltration Load
Air Change Load Calculation
The air change load can be determined using the following formula:
Air Change Load (w) = (room volume x heat to be removed x number of air changes per day) / 86,400
Key Variables
- Room Volume (m³): The internal volume of the refrigerated space, calculated by multiplying the internal length, width, and height of the room.
- Heat to be Removed (J): This value, relevant for the infiltration air, is provided in Table 8.3, given in kJ per m³ of infiltrated air based on the appropriate internal and ambient temperatures.
- Number of Air Changes Per Day: Obtained from Table 8.2, this depends on the temperature (above or below a certain threshold) and room volume. Adjust for heavy or very light service using the appropriate correction factor.
Product Load Calculations
Product loads are categorized into two main types: cooling load and respiration load. Respiration load will be addressed separately. Product cooling is handled in three distinct sections, as per Tables 11.1 to 11.5:
- Reduction in Temperature Above Freezing (Sensible heat above freezing)
- Freezing of the Product (Latent heat)
- Reduction in Temperature Below Freezing (Sensible heat below freezing)
Cooling Load Equation
Agricultural products emit heat when stored in a cool place. The heat gain from the product can be generally expressed as:
QT = Q1 + Q2 + Q3
Where:
- QT = Cooling load (kJ)
- Q1 = m * Cpa * (Te - Tf) = Heat to cool from entering temperature to freezing temperature (kJ)
- Q2 = m * hL = Heat to freeze (kJ)
- Q3 = m * Cpb * (Tf - Ts) = Heat to cool from freezing to final storage temperature (kJ)
Definitions of Variables:
- m = Mass of the product (kg)
- Cpa = Specific heat above freezing (kJ/kg °C)
- Cpb = Specific heat below freezing (kJ/kg °C)
- Te = Entering temperature (°C)
- Tf = Freezing temperature (°C)
- Ts = Final storage temperature (°C)
- hL = Latent heat of freezing of the product (kJ/kg)
Product Load for Cooling Above Freezing
Product Load (w) = (weight of the product x specific heat x temperature change) / 86,400
- Weight of the Product: Measured in kg loaded per day (over 24 hours).
- Specific Heat of the Product: Measured in J/kg-°C (see Tables 11.1 to 11.5).
- Temperature Change: Difference between entering product temperature and storage temperature (°C).
For multiple products, either conduct separate calculations for each or take an average specific heat. For bulk inputs of different types, use the largest possible loading, which may necessitate multiple calculations to identify the largest load.
Product Load for Freezing
Product Load (w) = (weight of the product x latent heat) / 86,400
Factors in this equation include:
- Weight of the Product: Total weight loaded per day.
- Latent Heat of Freezing: Found in J/kg (in Tables 11.1 to 11.5).
Freezing begins at the outer surface of the product; once it has frozen, a barrier forms, prolonging the freezing time.
Product Load for Cooling Below Freezing
Product Load (w) = (weight of the product x specific heat x temperature change) / 86,400
This follows the same parameters as cooling above freezing.
Respiration Load
Product Load (w) = (weight of the product x heat of respiration) / 86,400
- Weight of the Product: Total weight in storage (not limited to daily input).
- Heat of Respiration: Measured in J/kg-day (as seen in Tables 11.1 to 11.5).
The respiration load results from metabolic activities as the product ages/ripens during storage. This is particularly significant for fruits and vegetables. As storage temperature decreases, respiration rates also decline, with variations illustrated in Table 11.1 to 11.5.
Note that fruits and vegetables can emit carbon dioxide and ethylene, necessitating proper gas control for the safety of cold store operatives and the product’s longevity.
Heat Equivalency from Occupancy
Personnel in cold storage areas generate heat from even minimal activities. When prolonged occupancy occurs, it is vital to compute and include this heat load:
Heat Load = number of people x (hours of occupancy / 24) x heat equivalent per person
Alternatively, use the equations for radiation and convection.
- Hours of Occupancy: Average hours per person per day.
- Heat Equivalent: Rate of heat dissipation per person.
Lighting Load
All electrical lighting installed within a cold store generates heat equivalent to its light rating (W).
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