For freezers it becomes necessary to provide heat in the base slab to avoid freezing of the ground water and heaving of the floor. Minimum slab temperature should be at least 40ºF. Normally, 55ºF. should be used for freezer applications.
2. Air Change Load
(a)Average Air Change- when the door to a refrigerated room is opened, warm outside air will enter the room. This air must be cooled to the refrigerated room temperature, resulting in an appreciable source of heat gain. This load is sometimes called the infiltration load. The probable number of air changes per day and the heat that must be removed from each cubic foot of the infiltrated air, are given in tables based on experience (see Table 4, 5 & 6, page 14). For heavy usage, the infiltration may be doubled or more.
(b)Infiltration Through a Fixed Opening- As an alternate to the average air change method using the Psychrometric Chart (page 37), the following formulas may be used to calculate the infiltration resulting from natural ventilation (no wind) through external door openings.
[ (4.88) ( door height) (area/2) (minutes open) ( temp. diff. ºF.)
(enthalpy incoming air – enthaply warehouse air) ] [
Where X = % of heat transmission blocked by thermal barrier.
The air change load can be substantial and every means should be taken to reduce the amount of infiltration entering the box. Some effective means of minimizing this load are:
•Automatic closing refrigerator doors
•Vestibules or refrigerated anterooms
•Plastic strip curtains
•Air Curtains
•Inflated bumpers on outside loading doors.
3.Miscellaneous Loads
Although most of the heat load in a refrigerated room or freezer is caused by wall heat leakage, air changes and product cooling or freezing, there are three other heat sources that should not be overlooked prior to the selection of the refrigeration equipment. Since the equipment has to maintain temperature under design conditions, these loads are generally averaged to a 24 hour period to provide for capacity during these times.
(a)Lights- typically storage requirements are 1 to
daily figure.
(b)Motors- smaller motors are usually less efficient and tend to generate more heat per horsepower as compared to larger motors. For this reason Table 11, on page 18, is broken down in to H.P. groups. Also, motors inside the refrigerated area will reject all of their heat losses as shown in Table 11. However, motors that are located outside but do the work inside, like a conveyor, will reject less heat into the refrigerated space. If powered material handling equipment is used, such as forklift trucks, this must be included under Motor Heat Loads. Generally only battery operated lift trucks are used in refrigerated rooms, which represent a heat gain of 8,000 to 15,000 BTU/hr. or more over the period of operation. If motor or loading conditions are not known, then calculate one motor horsepower for each 16,000 cubic foot box in a storage
cooler and one HP for each 12,500 C.F. in a storage freezer which allows for fan motors and some forklift operations. These figures can be higher in a heavily used area, i.e. loading dock or distribution warehouse.
(c)Occupancy- People working in the refrigerated storage area dissipate heat at a rate depending on the room temperature (Table 12, page 18). Multiple occupancies for short periods should be averaged over a 24 hour period. If occupancy load is not known, allow one person per 24 hour for each 25,000 cubic foot space.
4. Product Load
Whenever a product having a higher temperature is placed in a refrigerator or freezer room, the product will lose its heat until it reaches the storage temperature. This heat load consists of three separate components: (see Table 7, page 15- 16).
(a)Specific Heat- The amount of heat that must be removed from one pound of product to reduce the temperature of this pound by 1ºF., is called its specific heat. It has two values: one applies when the product is above freezing; the second is applicable after the product has reached its freezing point.
(b)Latent Heat- The amount of heat that must be removed from one pound of product to freeze this pound is called the latent heat of fusion.
Most products have a freezing point in the range of 26ºF. to 31ºF. If the exact temperature is unknown, it may be assumed to be 28ºF.
There is a definite relationship between the latent heat of fusion and the water content of the product and its specific and latent heats.
Estimating specific and latent heats:
Sp. Ht. above freezing = 0.20 + (0.008 X % water) Sp. Ht. below freezing = 0.20 + (0.008 X % water) Latent Heat = 143.3 X % water
(c)Respiration- Fresh fruits and vegetables are alive. Even in refrigerated storage they generate heat which is called the heat of respiration. They continually undergo a change in which energy is released in the form of heat, which varies with the type and temperature of the product. Tabulated values are usually in BTU/lb./24 hours (Table 8, page 17), and are applied to the total weight of product being stored and not just the daily turnover.
(d)Pull down Time- When a product load is to be calculated at other than a 24 hour pull down, a correction factor must be multiplied to the product load.
24hours
Pull down Time
Note: While product pull down can be calculated, no guarantee should be made regarding final product temperature due to many uncontrollable factors (i.e., type of packaging, position in the box, method of stacking, etc.)
5. Safety Factor
When all four of the main sources of heat are calculated, a safety factor of 10% is normally added to the total refrigeration load to allow for minor omissions and inaccuracies (additional safety or reserve may be available from the compressor running time and average loading).
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