Heatcraft Refrigeration Products H-ENGM0806 manual Job Survey, Refrigeration Load Calculations

Job Survey

The person involved in a heat transfer calculation needs information in order to predict accurately the heat load on a refrigerated structure. The more complete the information, the better the calculation. Good calculations are the first step in assuring adequate refrigeration equipment is selected for the project.

The initial job survey should be as complete as possible and include the following:

Design Ambient Temperature

This is the ambient surrounding the box necessary for the load calculations. Another ambient to be considered on air cooled projects is the one surrounding the condensing unit which will affect equipment selection.

Storage Temperature and Humidity Requirements

Refrigeration equipment by its nature is a dehumidification process. We try to minimize or maximize the drying effect of the equipment by selecting the appropriate Temperature Difference (T.D.) between the saturated suction temperature of the evaporator and the room air. The T.D. selected approximates the desired relative humidity (see page 21).

Dimensions, Insulation, Type of Construction, Exposure

This criterion lends itself to well established, straight forward calculations, but the information while elementary, is often omitted from the initial job survey. Transmission load for 4” Styrofoam is double the transmission load for 4” formed in place urethane.

Infiltration or Air Changed Load

Heat, both sensible and latent, enters an enclosure through door openings whenever the air surrounding the enclosure is warmer than the box temperature. Knowing the location, size and number of the door openings and the temperature to which they are exposed will greatly aid in determining the heat load of the infiltration air.

Product

1.Type - storage requirements

2.Weight

3.Entering temperature

4.Pull down time

Miscellaneous Loads

1.Lights

2.Motors including fan motors, fork lifts, conveyers

3.People

4.Glass doors

Operations

1.Holding cooler or freezer

2.Blast cooling or freezing

3.Preparation, processing or cutting rooms

4.Distribution warehouses

5.Reach-in or walk-in boxes

Unusual Conditions

Electrical Service and Type of Equipment Desired

While not directly affecting refrigeration load calculations,

this is essential in the job survey to select the proper equipment.

Refrigeration Load Calculations

With the initial survey complete, the heat load calculation is separated into the following main sources of heat for a given 24 hour period:

1.Transmission load

2.Air change load

3.Miscellaneous load

4.Product load

Accuracy

Accuracy in calculation is the first step in having a satisfied customer. There are short cuts, based on averages, that may be taken and which must be used when the product load is indefinite or unknown (see Quick Selection Guide on page 41 and the Rapid Load Calculator on page 43). But when all the data necessary to calculate the four main sources of heat gain are available, the complete calculation should be made.

Quick Selection Chart for Small and Medium Coolers and Freezers

The Quick Selection Guide on page 41 may be used for a quick comparison of heat load figured on Bulletins Above32-05 or Below32-05 or to obtain approximate heat loads for small and medium sized boxes. The loads are shown for a 95ºF. outside temperature.

Rapid Load Calculator for Large Coolers and Freezers

The Rapid Load Calculator on page 43 may be used for quick approximations of the heat load in large boxes and for a reasonable comparison of heat loads figured on Bulletins Above32-05 or Below32-05. The Calculator graph on page 43 is based on the following average daily product loadings for coolers and freezers:

			Average Daily			Average Daily
Volume-			Product Loads (lbs.)			Product Loads (lbs.)
Cu. Ft.			for Coolers			for Freezers
500	-	3,000	6,200	-	8,000	1,600	-	2,000
3,000	-	4,600	8,000	-	11,000	2,000	-	2,500
4,600	-	8,100	11,000	-	17,000	2,500	-	4,000
8,100	-	12,800	17,000	-	26,000	4,000	-	6,200
12,800	-	16,000	26,000	-	33,000	6,200	-	7,500
16,000	-	20,000	33,000	-	40,000	7,500	-	9,500
20,000	-	28,000	40,000	-	56,000	9,500	-	13,000
28,000	-	40,000	56,000	-	66,000	13,000	-	17,000
40,000	-	60,000	66,000	-	110,000	17,000	-	25,000
60,000	-	80,000	110,000	-	150,000	25,000	-	34,000
80,000	-	up	150,000	-	up	34,000	-	up

1. Transmission Load

Methods of determining the amount of heat flow through walls, floor and ceiling are well established. This heat gain is directly proportional to the Temperature Difference (T.D.) between the two sides of the wall. The type and thickness of insulation used in the wall construction, the outside area of the wall and the T.D. between the two sides of the wall are the three factors that establish the wall load. Tables are provided to simplify the calculations (see Table 1, page 13). Some coolers for above freezing temperatures have been constructed with only a floor slab (no floor insulation). The factors shown in the wall heat gain (Table 1) are based on a concrete floor slab and the T.D. between the local ground temperature and the storage room temperature.

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