Refrigeration & Air-Conditioning

Module 8. Cooling load calculations and cold storage design

Lesson 30
COOLING LOAD CALCULATIONS

30.1 Introduction

It is necessary to select the capacity of refrigeration plant for the cold storage to maintain required storage temperature in the cold storage. Under capacity plant, may load to higher temperature of cold storage than required while over capacity may lead to higher initial cost of the refrigeration system. The capacity of the refrigeration plant should be such that it can take care of all the heat load of the cold storage. It is also necessary to reduce cold storage load in order to reduce the energy cost for the operation of refrigeration plant. The various factors contributing the total load of the cold storage are discussed below.

30.2 Wall Gain Load

The heat flow rate by conduction through the walls ceiling & floor of the cold storage from outside to inside is called wall gain load.

The value of U depends on the materials used in construction and insulation used in the construction of wall as well as on the thickness of these materials. If either U or 12∆T'> are different for different walls, then it is necessary to calculate Q_w of each wall/ceiling/floor separately taking corresponding values of U and 12∆T'>.

The overall heat transfer co-efficient is given by

Where, h_o = Convection heat transfer Co-efficient on the outer surface

h_i = Convection heat transfer Co-efficient on the inner surface

x_1, x₂…..= thickness of different layers of wall including insulation

k₁,k₂,….= conductivities of different layers of wall including insulation

30.3 Air Change Load

This is the amount of heat carried by the air when cold storage door is opened and part of cold air is replaced by outside warmer air. The air change load depends on the number of air changes occurring in the cold storage, enthalpy of outside air and inside air. The measurement of amount of air changed due to door opening is difficult and hence air change factor is used to estimate the amount of air changed.

Air change load, Q_a = m (h_o-h_i)

Where, m= mass of air entering, kg d. a. /h

h_o = Enthalpy of outside air, kJ/kg dry air

h_i = Enthalpy of inside air, kJ/kg dry air

Mass of air can be estimated by multiplying volume of cold storage with air change factor. The volume of the air is converted into amount of dry air in the volume taking specific volume of the outside air.

Air change factors (air changes/h) for different size of cold storage are given in table 30.1.

30.4 Product Load

It is necessary to cool the product from initial temperature to the storage temperature. The amount of heat to be removed from the product to lower the temperature of the product from initial temperature to storage temperature is called product load. It is also necessary to estimate the heat load for cooling of the packaging material along with the product as specific heat of product and material is different. For example, plastic crates are to be cooled from room temperature to the storage temperature.

Product load, Q_p = m_p × C₁ × (t₁-t₂)

Where m_p = Mass flow rate of the product in the cold storage, kg/h

C₁ = Specific heat of the product kJ/kg K

T₁ = Initial temperature of the cold storage

T₂ = Final storage temperature of the product.

Similarly, heat load of packaging materials transferred in the cold store along with the product is estimated as above taking the mass of packaging material, its specific heat and temperature difference. This load is added in the actual product load.

For frozen foods

Q_p = m_p x C₁ (t₁ + t_f) + m_p h_fg+ m_p x C₂ (t_f - t₂)

Where t_f = Freezing temperature

h_fg = Latent heat of freezing

Heat produced due to respiration of the fruits and vegetables are required to be considered for such types of cold storages.

Q_r = m_p (kg/h) x Respirate rate (kJ/kg)

30.5 Miscellaneous Load

The miscellaneous load consists of primarily of heat given off by light and electric motors present in the cold storage.

Cooling load for electric appliances in terms of kJ is given by

Q_c = kW × 3600 kJ/h

Heat Load from occupants is calculated based on the data available for heat loss from human body. It is necessary to refer standard data if heat loss from human body under different temperature conditions. For example, a person at rest at 20 ^oC, total heat loss from the body is about 400 kJ/h (Q_l= 160 kJ/h and Qs = 240 kJ/h)

Table 30.1 Air changes per hour for cold storage due to infiltration & door openings

Volume of Cold Storage, m2	Air Changes/h (Air Change Factor)
10	1.23
20	1.95
30	0.65
40	0.57
50	0.50
60	0.45
70	0.42
80	0.37
100	0.35
150	0.27
200	0.23
250	0.21
500	0.14
1000	0.10

Total heat load

Q_t = Q_w + Qa + Q_p + Q_m

It is common practice to add 10-15% of total load as safety factor. After adding safety factor, the cooling load is multiplied by 24 hours and divided by the desired operating time in hours to find capacity of the plant required for the cold storage.