Heatcraft Refrigeration Products H-ENGM0806 manual Derating Factors, Type of Operation/Air Flow

4. Type of Operation/Air Flow

Two important considerations in the selection and location of the unit cooler are uniform air distribution and air velocities which are compatible with the particular application.

The direction of the air and air throw should be such that there is movement of air where there is a heat gain; this applies to the room walls and ceiling as well as the product. The unit cooler(s) should be arranged to direct its discharge air at any doors or openings, if it all possible. Avoid placing the unit cooler in a position close to a door where it may induce additional infiltration in to the room; this can cause fan icing and a condition known as hoar-frost. Also, avoid placing a unit in the air stream of another unit, because defrosting difficulties can result.

For general storage coolers and holding freezers, there are not criteria for air velocities within the room. The total supply of air is such that approximately 40 to 80 air changes occur each hour. This is an air conditioning term which is calculated as follows:

Air Changes = (total cfm*) x 60 internal room volume

* includes all unit coolers and auxiliary fans

This equation disregards the air motion which is induced by the discharge air from the unit cooler. For simplicity, the gross volume of the room is used unless the product and equipment occupy more than 10% of the volume. Specific applications such as cutting rooms and banana ripening rooms have desired limits. The table below indicates the minimum and maximum quantities of air for particular applications.

Recommended Air Changes/Hour

	Recommended Number
	of Air Changes
Type of Application		Minimum	Maximum
Holding freezer	40		80
Packaged Holding center	40		80
Cutting Room	20		30
Meat Chill Room	80		120
Boxed Banana Ripening	120		200
Vegetables and Fruit Storage	30		60
Blast Freezer	150		300
Work Areas	20		30
Unpackaged Meat Storage	30		60

Derating Factors

A.Ambient

B.Altitude

C.Saturated Suction Temperature (S.S.T.)

D.50 Cycle Power

In the selection of refrigeration equipment it should be noted that the manufacturer’s equipment has ratings based on certain criteria. Care should be taken to determine actual job conditions and the proper derating factors should be applied. These factors may vary by manufacturer but can be used here as rule of thumb approximation.

A. Ambient

Condensing unit ambient is of concern as most equipment is generally cataloged as 90º to 95ºF. ambient.

Decrease condensing unit capacity 6% for each 10ºF. increase in operating ambient.

Increase condensing unit capacity 6% for each 10ºF. decrease in operating ambient.

B. Altitude

Most manufacturers rate their equipment at sea level conditions. An increase in altitude results in a decrease in air density. While the fans on direct drive equipment will deliver a constant cubic feet per minute of air regardless of density, the thinness of the air will affect capacity performance. Belt drive equipment can be speeded up to compensate for the decrease in air density.

Effects of Altitude on Air Cooled Equipment

Altitude	Absolute	Pressure	Standard		Capacity
Feet			Air		Multipliers
Above			Density	Air	Direct Drive Fans
Sea			At 70ºF.	Dens.	Refrig.	Air Cooled
Level	In. Hg.	PSIA	lbs./Cu.Ft.	Ratio	Evap.	Cond. Unit
-1,000	31.02	15.27	.0778	1.04	1.03	1.005
-500	30.47	14.97	.0763	1.02	1.02	1.002
0	29.92	14.70	.0749	1.00	1.00	1.00
500	29.38	14.43	.0735	0.98	0.98	0.995
1,000	28.86	14.28	.0719	0.96	0.96	0.998
2,000	27.82	13.67	.0697	0.93	0.93	0.985
3,000	26.81	13.27	.0671	0.90	0.90	0.98
4,000	25.84	12.70	.0647	0.86	0.875	0.975
5,000	24.89	12.23	.0623	0.83	0.85	0.969
6,000	23.98	11.78	.0600	0.80	0.82	0.960
7,000	23.09	11.34	.0578	0.77	0.79	0.955
8,000	22.22	10.92	.0556	0.74	0.76	0.946
9,000	21.38	10.50	.0535	0.71	0.73	0.939
10,000	20.58	10.11	.0515	0.69	0.71	0.93
12,000	19.03	9.35	.0477	0.64	0.66	0.91
14,000	17.57	8.63	.0439	0.59	0.61	0.88

C. Suction Temperature

Care should be taken in the selection of unit coolers, especially freezer models. There is no set rating standard adopted by the industry for the ratings criteria. The model number of a low temperature unit cooler can be rated at -30º SST, -20º SST, -10º SST, 0º SST, or even +10º SST. The capacity difference between the -30º SST and the +10º SST can be as much as 15% higher for the lower rated unit cooler. Most manufacturers provide a suction temperature correction factor for their unit coolers and this should be noted in equipment selections.

D. 50 Cycle Power

Since we live in a “global village,” the opportunity to quote refrigeration equipment for export markets is one not to be ignored. Motors that are sized for 60 cycle operation run at 83% (50/60) speed on 50 cycles operation. Compressors produce only 5/6 of their capacity. However, while fans are only running 83% speed, there is also a decrease in static pressure through the condenser or unit cooler coil and performance does not suffer the full 17% penalty. If it has been verified by the manufacturer that their equipment can be run on 50 cycle power then the following derating factors can be applied:

A.Unit coolers and air-cooled condensers (Capacity x 0.92)

B.Air-cooled condensing units (capacity x .85)

System capacity (unit cooler and air-cooled condensing unit) can be derated by 0.88

To select refrigeration equipment after the load has been determined, divide the BTUH required by (0.88):

=Conversion to select 60 cycle

0.88equipment for 50 cycle loadBTUH

This provides for larger equipment necessary to compensate for 50 cycle derating factor.

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