Handbook of air conditioning and refrigeration / Shan K

REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.41
air passage and reduces the rate of heat transfer of the coil, it must be removed periodically. The
process of removing frost from the evaporator is called defrosting.
If air enters the coil at a temperature above 36°F (2.2°C), the ambient air flowing through the
DX coil can be used for defrosting. However, in low-temperature refrigeration systems, if the ambient
air temperature is also below 32°F (0°C), an electric heating element may be used as a simple
and effective way to defrost the coil. Hot-gas defrosting is effective, but must be planned during the
design stage to ensure an adequate quantity of hot gas at a pressure high enough to flow through
the coil. After defrosting, the high-pressure condensate should be returned to the liquid line, and the
pressure of the desuperheated gas should be reduced gradually and released to the suction line.
The defrosting cycle can be controlled by sensing the pressure or temperature difference of air
entering and leaving the DX coil over a fixed time interval.
Proper Refrigerant Charge. A proper amount of refrigerant charge is neccessary to ensure a
proper DX system performance. Too much refrigerant charge will reduce the condensing area and
therefore have a higher condensing pressure. An insufficient charge will decrease the required evaporation
in the DX coil and thus have less than the required cooling capacity. A new refrigeration
system should be charged with the exact amount of refrigerant specified by the manufacturer. For
an operating DX system, a practical and convenient way to ensure a proper refrigerant charge is to
check the discharge/suction pressure by means of pressure gauges and the degree of superheat at
the outlet of the DX coil through its outer surface temperaure measurement.
If the condensing temperature of an air-cooled DX reciprocating system at an outdoor air temperature
of 95°F (35°C) is 125°F (51.7°C), the discharge pressure is about 293�4 � 297 psia
TABLE 11.6 Electrically Operated Packaged Units and Condensing Units—Minimum Efficiency Requirements
Minimum Efficiency as of Test
Equipment type Size category Rating condition efficiency * 10/29/2001 * procedure
Packaged units, � 65,000 Btu/h † Split system 10.0 SEER 10.0 SEER ARI 210/240
air cooled Single package 9.7 SEER 9.7 SEER
� 65,000 Btu/h and Split system and 8.9 EER § 10.3 EER §
� 135,000 Btu/h single package
� 135,000 Btu/h and Split system and 8.5 EER § 9.7 EER § ARI 340/360
� 240,000 Btu/h single package
� 240,000 Btu/h and Split system and 8.5 EER § 9.5 EER §
� 760,000 Btu/h single package 7.5 IPLV § 9.7 IPLV §
� 760,000 Btu/h Split system and 8.2 EER § 9.2 EER §
single package 7.5 IPLV § 9.4 IPLV §
Packaged units water, � 65,000 Btu/h Split system and 9.3 EER 12.1 EER ARI 210/240
and evaporatively cooled single package
� 65,000 Btu/h and Split system and 10.5 EER § 11.5 EER §
� 135,000 Btu/h single package
� 135,000 Btu/h and Split system and 9.6 EER § 11.0 EER § ARI 340/360
� 240,000 Btu/h single package
� 240,000 Btu/h Split system and 9.6 EER § 11.0 EER §
single package 9.0 IPLV § 10.3 IPLV §
Condensing units, � 135,000 Btu/h 9.9 EER 10.1 EER ARI 365
air cooled 11.0 IPLV 11.2 IPLV
Condensing units, water, � 135,000 Btu/h 12.9 EER 13.1 EER
or evaporatively cooled 12.9 IPLV 13.1 IPLV
* IPLVs are only applicable to equipment with capacity modulation.
† Single-phase air-cooled air conditioners � 65,000 Btu/h are regulated by NAECA. SEER values are those set by NAECA.
§ Deduct 0.2 from the required EERs and IPLVs for units with a heating section other than electric resistance heat.
Source: ASHRAE/IESNA Standard 90.1-1999. Reprinted by permission.