w w w . r s e s j o u r n a l . c o m10 RSES Journal MAY 2013

Why specify a condensing unit? Why not simply spec-ify the components you would like to have for your refrigeration system and have it assembled? The an-

swer to these questions is, of course, to make it simple.Designing a refrigeration unit is not simply a matter of

specifying the proper compressor, condenser coil, expansion device, evaporator coil and temperature control(s). Designing a refrigeration system from scratch is complicated. The techni-cian or design engineer would also need to consider selecting some, if not all, of the following: receiver; suction accumula-tor; fan switches; fan motor(s); fan blades(s); fan guard(s); fan shroud(s); service valve(s); pressure switches or a dual pressure control; liquid-line solenoid valve; liquid-line filter-drier; de-frost timer; relay/capacitor box; control panel; and contactor.

A base plate is also needed as a platform for mounting and connecting the components together. Finally, the con-

densing unit should have as small a footprint as possible, but yet provide enough free space to allow for serviceability.

A detailed discussion on how to properly select each of these components and then some ideas on how to assemble and connect the components in an organized manner on a base plate is warranted. But, as a textbook author might say when the subject matter becomes too challenging for the task at hand, “It is beyond the scope of this article to discuss the proper selection of each of these components.”

SelectionSo how does one properly select a condensing unit? Condens-ing unit performance data is established using ambient and evaporator temperatures. ANSI/AHRI Standard 520-2004 lists the following rating conditions for indoor, outdoor and water-cooled condensing units.

Type ApplicationEvaporator

Temperature (°F)

Ambient Temperature

(°F)

Water Temperature In/Out (°F)

Return-gas Temperature

(°F)

IndoorHigh Temperature 45

90 -65

Commercial Temperature 20 40Low Temperature -10 40

OutdoorHigh Temperature 45

95 -65

Commercial Temperature 20 40Low Temperature -10 40

Water-cooledHigh Temperature 45

- 85/9565

Commercial Temperature 20 40Low Temperature -10 40

B y A n d y S c h o e n , c M

How to keep the specification process simple when selecting a condensing

unit for a refrigeration system.

All images courtesy of Tecumseh Products Co.

Proper Condensing Unit

selection

MAY 2013 RSES Journal 11w w w . r s e s j o u r n a l . c o m

It is at these conditions that the condensing unit is assigned its “standard rating” and “horsepower” classification. The con-densing unit’s horsepower is generally calculated as follows:

hp =std rating in Btuh

X

Where X = 12,000 (high temperature); 8,000 (commercial temperature); and 4,000 (low temperature). Note: A condensing unit’s “horsepower” rating is merely a classifi-cation. It should not be used for sizing purposes.

Manufacturers of condensing units provide extended rating tables that allow the system designer to accurately determine a condensing unit’s capacity at the system’s designed operating conditions. The extended rating table for a Tecumseh AWA-9518ZXNHN commercial-temperature R-404A outdoor con-densing unit is shown above. For example, a Tecumseh AWA9518ZXNHN condensing unit has an AHRI rating of 17,300 Btuh at a 20°F evapora-tor and a 95°F ambient temperature. However, if the system’s designed operating conditions are at a 25°F evaporator and a

EvaporatorAmbient Temperatures

80°F 90°F 95°F 100°F

°F psig Btuh Watts Cond T Btuh Watts Cond T Btuh Watts Cond T Btuh Watts Cond T-10 23.2 10,000 1,980 92 8,700 1,970 100 8,000 1,950 105 7,300 1,930 110

-5 27.5 11,400 2,120 93 10,000 2,120 102 9,200 2,110 106 8,500 2,090 111

0 32.1 12,900 2,250 94 11,400 2,260 103 10,600 2,260 108 9,800 2,250 112

5 37.0 14,600 2,390 96 13,000 2,410 105 12,100 2,410 109 11,300 2,410 114

10 42.4 16,500 2,530 98 14,600 2,550 107 13,700 2,560 111 12,800 2,570 115

15 48.2 18,500 2,670 100 16,500 2,700 108 15,500 2,710 113 14,500 2,730 117

20 54.5 20,700 2,800 102 18,400 2,840 110 17,300 2,860 114 16,200 2,880 119

25 61.2 23,000 2,940 104 20,500 2,990 112 19,300 3,010 116 18,100 3,040 121

30 68.4 25,400 3,080 106 22,700 3,140 114 21,400 3,160 118 20,100 3,200 123

35 76.1 28,100 3,220 109 25,000 3,280 117 23,600 3,310 120 22,200 3,350 125

40 84.4 30,800 3,360 111 27,500 3,430 119 25,900 3,460 123 24,400 3,510 127

45 93.2 33,700 3,500 114 30,100 3,570 121 28,400 3,610 125 26,700 3,660 129

Performance at 60 Hz, Return-gas Temperature 40°F, Subcooling 5°F.

100°F ambient temperature, the condensing unit’s capacity is 18,100 Btuh (based on the rating table). Once the system designer has selected the condensing unit with the proper capacity, the next challenge is to size a match-ing evaporator coil. To do this, the system designer needs to un-derstand the principle of temperature difference (TD). Air-cooling evaporators are sized on the basis of TD (i.e., entering air temperature minus the evaporator temperature).

“Once the system designer has selected the condensing

unit with the proper capacity, the next challenge is to size a

matching evaporator coil.

w w w . r s e s j o u r n a l . c o m12 RSES Journal MAY 2013

In the figure above, air is entering the evaporator at 35°F and the evaporator temperature is 25°F, giving a 35°F – 25°F = 10°F TD. Do not confuse TD with the temperature drop of the air across the coil, typically known as “Delta T” (in the example shown in Figure 1, 35°F – 30°F = 5°F Delta T). Evaporator capacity varies proportionally to TD. For ex-ample, if the TD were to double, the coil capacity would approximately double. Below is a rating table for an 18,000 Btuh commercial-temperature coil rated at 15°F TD.

TD (°F) Evaporator Rating (Btuh)

10 12,000

15 18,000

20 24,000

30 36,000

Application TD (°F)

Low Temperature 8 to 12

Commercial & High Temperature 10 to 15

Air-conditioning Temperatures 20 to 30

Selecting a matching evaporator for a condensing unit involves, in part, making sure the evaporator operates at an appropriate TD. Typical evaporator TDs by application are listed below:

Entering Air Temperature - Evaporator Saturation Temperature

Refrigerant Temperature 25°FRefrigerant Temperature 25°F

Discharge Air

Temperature 30°F

Entering Air

Temperature

35°F

Evaporator TD = 10°F

Figure 1

“Do not confuse TD with the temperature drop of the

air across the coil, typically known as “Delta T.”

After a matched evaporator has been selected for the condensing unit, the system designer has accomplished much of what needs to be done to put together the refriger-ation system.

Andy Schoen, CM, is Technical Services Manager for Tecum-seh Products Co. He has 33 years of HVACR industry experi-ence, and serves on the Technical Review Committee and the MSAC for RSES Journal. He can be reached at 734-585-9561 or via e-mail at andy.schoen@tecumseh.com. For more infor-mation, visit www.tecumseh.com.

Circle Reader Service No. 38