Air-Cooled Condenser Sizing & Selection

Air-cooled condensers, often referred to as “remote condensers”, became more commonly used back in the 1960’s and 1970’s as the use of water-cooled systems which dumped city water down the drain became cost prohibitive or banned.

Another factor which I think boosted their popularity was the advent of the Alco Headmaster head pressure control valve, which allowed relatively trouble-free winter operation with air-cooled condensers.  Prior the the headmaster, condensers used various methods of controls, including head-pressure actuated dampers which were prone to icing up or jamming up.

The capacity of an air-cooled condenser are based on Total Heat of Rejection (THR) of the refrigeration system. The THR is equal to compressor cooling capacity plus the heat equivalent of the power input to the compressor motor, usually found in the compressor ‘s performance data sheets published by the manufacturer, and expressed as Watts (W) or kilowatts (kw).  This heat energy is sometimes called the heat of compression.

The heat of compression will vary depending on the compressor manufacturer, type of compressor and the operating conditions of the compressor. Whenever possible, it should calculated by multiplying the Watts x 3.413 Btuh/watt to covert it to Btu/hr.  If the performance data values aren’t available, the THR can be estimated using the following formula:

THR = (Compressor Capacity) x (Heat of Compression Factor, Table 1)

Table 1 contains heat of compression factors for hermetic and semihermetic compressors.

For refrigeration systems beyond the range of Tables 1, use the following equations to estimate THR = Compressor Capacity (BTUH) + (3413 x KW)

The compressor capacity is effected by its altitude. If the condenser location is above sea level, an additional correction is required to the THR, as follows: THR (altitude) = THR * Altitude Correction Factor, Table 3

 

 

 


Selection Example:

Compressor capacity: 45,000
Evaporator temperature: +25° F
Condensing temperature: 110° F
Ambient temperature 95° F
Refrigerant: R-404A
Condenser altitude: 1,000 feet

Step 1: Estimate Condenser THR

From Table 1 for suction cooled compressors, at +25° F suction and 110°F condensing, find the heat of compression factor as 1.31.

THR = Compressor Capacity x Heat of Compression Factor
THR = 45,000 x 1.31 = 58,950 Btuh

Step 2: Correct for Altitude

From Table 3 obtain an altitude correction factor of 1.02 for 1,000 feet. This will give us a corrected value of the “Actual THR” which we’ll call the “Selection THR”

Selection THR = Actual THR (from step 1) * Altitude Correction Factor
Selection THR = 58,950 x 1.02 =  60,129 Btuh.

 

 

 


Step 3: Calculate Design Condenser TD

Design Condenser TD = Condensing Temp — Ambient Temp = 110 – 95 = 20° TD

Step 4: Condenser Selection

Condenser capacities (for 60 Hz) are located in Table 4.

 

These capacities are given in MBH/°TD.

Convert the THR calculated in step 2 to MBH/°F TD by dividing by 1,000 to get THR in MBH.

Then divide the THR by the design TD to get MBH/°F TD.

THR (MBH) = 60,129 / 1,000 = 60.13 THR (MBH/°F TD)
THR (MBH) = 60.13 / 15 = 4.01 MBH/°F TD

Locate the column for R-404A refrigerant and read down until you locate a value equal to or just larger than 4.01. This closest value larger than 4.01 is 4.77. Read horizontally to the left to get the condenser model,  DVT010.

Step 5: Calculate Actual TD and Condensing Temperature

The actual condenser TD can be calculated by dividing the design THR by the condenser rating:

Actual TD = Selection THR(MBH)  / (Rating @ 1°F TD)
Actual TD = 60.13 / 4.77 = 12.6°F. T.D.

The actual condensing temperature is the actual TD plus the ambient temperature:

Actual Condensing Temperature = (Actual T.D.) + (Ambient) = 18.4 + 95 = 113.4°F.