AIR-COOLED CONDENSERS How to Size and Select … · Air-cooled condensers are provided with either a single refrig-erant circuit or with two equally sized circuits as standard, but

B Y : C H A R L I E B A R T L E T T

How to Size and Select

T A B L E 1

Suction Temp. °F

Condensing Temperature °F

90° 100° 110° 120° 130°

-40° 1.56 1.63 1.72 1.81 1.94

-30° 1.49 1.55 1.62 1.70 1.80

-20° 1.43 1.49 1.55 1.62 1.70

-10 1.38 1.43 1.49 1.55 1.63

0° 1.34 1.38 1.43 1.49 1.56

5° 1.31 1.36 1.41 1.48 1.55

10° 1.29 1.34 1.39 1.44 1.52

15° 1.26 1.31 1.36 1.41 1.48

20° 1.24 1.28 1.33 1.38 1.44

25° 1.22 1.26 1.31 1.36 1.42

30° 1.20 1.24 1.28 1.33 1.39

40° 1.17 1.20 1.24 1.28 1.33

50° 1.13 1.16 1.20 1.24 1.28

A ir-cooled condensers, often referred to as remote condensers, became more com-mon in commercial refrigeration back in

the 1960s and 1970s. This is due to the fact that water-cooled systems, which dumped city water down the drain, became cost prohibitive or even banned in some areas. In many cases the remote condenser is a good solution for indoor mechanical rooms or basements with inadequate ventilation to exhaust the heat generated by the refrigeration systems.

Head Pressure Control Another factor that boosted their popularity was the advent of the self-contained head pressure regulator (HPR), most notably being Emerson’s Alco Controls HP Series Headmaster, which allowed relatively trouble-free winter operation with air-cooled condensers. Prior to the Headmaster-type HPR, condensers used various methods of control, including pressure-actuated or motorized dampers, which were prone to icing or jamming up.

The HPR is also known as a condenser flooding control because it causes liquid refrigerant to stack up in the condenser, quite literally flooding the coil. This action reduces the available condensing sur-face area of the condenser, which reduces its condensing capacity and in turn raises the condensing pressure to the setpoint of the valve.

Fan cycling is a factory-installed option configured on the number and arrangement of the fans, which is typically a single or double row wide and from one up to seven fans lengthwise. It’s generally recommended that the first fan (or pair of fans) closest to the inlet header run continuously and not cycle due to thermal expansion and contraction of the tubes.

Fan cycling can be controlled by either head pressure or ambi-ent temperature. Pressure controlled fan cycling is usually staged for more stable control.

Variable speed fan motors have become more popular in recent years due to the increased availability and reduced cost for the motors and variable frequency drives VFD or controllers. These are usually available factory-installed on the larger direct drive or belt-driven condensers.

Condenser splitting is often used

for reducing the amount of addi-tional refrigerant required for winter opera-

tion, which help minimize the system refrigerant charge. It’s field-installed and typically controlled by either a three-way valve (similar to a heat reclaim valve) or with two solenoid valves (one NO and one NC). Either method requires a pump-out bleed to the system suction to empty the unused portion of the condenser. Splitting is usually controlled by

ambient temperature, sometimes with a high-pressure override con-trol. Most often the split is a simple 50/50, however a 1/3 – 2/3 configuration allows for more steps of control.

REPLACEMENT AIR-COOLED CONDENSERS

w w w . r s e s j o u r n a l . c o m16 RSES Journal JULY 2019

A I R - C O O L E D C O N D E N S E R S

Condenser Selection ProceduresSelecting Condenser TD: Typically supermarkets specify low TD’s for their systems. For low temperature systems 10°F TD is the norm while medium temperature will require a 15°F TD. With the higher temperature systems like prep rooms and A/C, 20°F-25°F is common. If you are replacing an existing water-cooled or evaporative-cooled condenser with an air-cooled condenser, note that the original design saturated condensing temperature (SCT) for those systems typically would be 105°F, so select your TD accordingly.

Determining Condenser Capacity Requirement: The capacity of an air-cooled condenser is based on Total Heat of Rejection (THR) of the refrigeration system. For hermetic or semi-hermetic compressors, the THR is equal to com-pressor cooling capacity plus the heat equivalent of the

power input to the compressor motor (see Note 1). Values for the motor input power can be found in the compressor performance data published by the manufacturer and is typically expressed as Watts (W) or kilo-Watts (kW). The input power expressed as heat is also some-times referred to as the heat of compression.

Whenever possible, it should calculated by multiplying Watts x 3.413 Btuh/Watt to convert it to Btuh.

The heat of compression will vary depending on the com-pressor manufacturer, type of compressor and the operating conditions of the compressor. Table 1 contains heat of compression factors for hermetic and semi-hermetic compressors (see Note 2).

Compressor capacity is affected by altitude. If the condenser location is above sea level, an additional correction is required to the THR, as follows: THR (altitude) = THR x Altitude Correction Factor, Table 2. Circle Reader Service No. 89

T A B L E 2

DVT Model

Capacity (MBH / 1 °F)

R-404A / R-507 R-407C * R-407A /

R-407F R-448A * R-449A *

001 0.74 0.70 0.73 0.71 0.73

002 1.00 0.82 0.98 0.96 0.99

003 1.54 0.94 1.51 1.48 1.52

005 2.38 1.45 2.33 2.28 2.36

008 3.92 2.24 3.84 3.76 3.88

010 4.77 3.68 4.67 4.58 4.72

012 5.96 4.48 5.84 5.72 5.90

014 6.85 5.60 6.71 6.58 6.78

016 7.83 6.44 7.67 7.52 7.75

021 10.29 7.36 10.08 9.88 10.19

023 11.07 9.67 10.85 10.63 10.96

026 13.03 10.41 12.77 12.51 12.90

Altitude Correction Factor

0 1.00

1,000 1.02

2,000 1.05

3,000 1.07

4,000 1.10

5,000 1.12

6,000 1.15

7,000 1.17

REPLACEMENT AIR-COOLED CONDENSERS

T A B L E 3

w w w . r s e s j o u r n a l . c o m JULY 2019 RSES Journal 17w w w . r s e s j o u r n a l . c o m

C O I L C O AT I N G SA I R - C O O L E D C O N D E N S E R S

Compressor

Head PressureControl Valve

Condenser

Receiver

D

RC

Condenser Multi-Circuiting Worksheet Circuit Loads

Circuit # Base THR Corrected THR 3 TD Specific Circuit Loading

Circuit 1 _____________Btuh _______________Btuh_____________

Deg F________________

Btuh/1°F TD


Deg F________________

Btuh/1°F TD


Deg F________________

Btuh/1°F TD


Deg F________________

Btuh/1°F TD

TOTAL Specific Loading _________Btuh/1°F TD

Condenser Selection for Multi-CircuitingSelect a condenser model from the performance tables that is the closest match for the calculated

specific loading.

Model ___________ Capacity @ 1°F TD _____________Btuh/1°F TD Available Feed_________

Dividing Capacity by # of feeds available: ________________Btuh/1°F TD >>> Capacity per feed

Condenser Multi-Circuiting Worksheet Circuit LoadsCircuit # Specific Circuit Loading / Capacity per feed = # Feeds Required

Circuit 1 _________Btuh/1°F TD / _________Btuh/1°F TD = _____________Feeds Req’d




Calculate Actual TDCircuit # Circuit THR / # Feeds Req’d / Capacity per feed = Actual TD

Circuit 1 ___________Btuh / ______Feeds Req’d / ___________Btuh = ___________Deg F




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-ft. Step 1: Estimate Condenser THRFrom Table 1 for suction cooled compressors, at +25°F suction and 110°F condensing, find the heat of compression factor as 1.31.

Step 2: Correction Factors for AltitudeFrom Table 2 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) x Altitude Correction Factor Selection THR = 58,950 x 1.02 = 60,129 Btuh. Step 3: Calculate Design Condenser TDDesign Condenser TD = Condensing Temp - Ambient Temp = 110 - 95 = 20°F TD Step 4: Condenser SelectionCondenser capacities (for 60 Hz) are located in Table 3. 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 refrig-erant 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 TemperatureThe actual condenser TD can be calcu-lated 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 TD.

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

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

Multi-Circuited Air-cooled CondensersAir-cooled condensers are provided with either a single refrig-erant circuit or with two equally sized circuits as standard, but can be ordered with more at no charge, a featured known as multi-circuiting.

Multi-circuiting allow the system designer to use one remote air-cooled condenser to serve many smaller units.

Each condenser model specifications include how many indi-vidual circuits are available for that model and may give circuit capacity information expressed in Btuh/1°F TD. If only the number of circuits is listed, then it’s a simple matter of dividing the total condenser capacity by the number of available circuits. A typical multi-circuiting worksheet is provided below.

Head Pressure Control With Multiple CircuitsIt is generally recommended to utilize a condenser flooding

w w w . r s e s j o u r n a l . c o mw w w . r s e s j o u r n a l . c o m


18 RSES Journal JULY 2019

type control with multi-circuited con-densers. The condenser fans may also be controlled with ambient thermostats for added energy efficiency.

Charlie Bartlett spent 45 years in the HVACR industry after graduating from Wentworth Institute of Technology in Boston in 1970 and attending Northeastern University, studying mechanical engineering. He worked for his dad’s refrigeration contracting firm; in sales and service for a Hussmann distributor; for Dunham-Bush in West Hartford, CT, as a sales engineer; as a refrigeration engineer for First National Supermarkets in Windsor Locks, CT, and then as the head of his own refrigeration-contracting firm in Florida. He’s been active in the forum HVAC-Talk since 2000, chairing its educational committee. His website is Icemeister’s Back Room (https://icemeister.net/backroom/) and he can be reached at [email protected].

Notes:(1) For open-drive compressors the input

power is usually expressed in brake horsepower (Bhp) which can be converted to Btuh by multiplying the Bhp x 2,545.

(2) For refrigeration systems beyond the range of Table 1, use the following equa-tions to estimate THR = Compressor Capacity (Btuh) + (3,413 x kW)

(3) The corrected THR would include corrections factors for altitude and type of refrigerant. Most manufacturer capacity data is based on using R-404A at sea level. Use the appropriate factors provided in their tables. Using the cor-rection factors won’t result in a different number of feeds required but will only change the calculated actual TD.

References:Heatcraft/Bohn - Catalog BN-RACCTB-Sept 2016 Model DVT Technical Guide (p. 3-5/ Selection) https://www.heatcraftrpd.com/PDF/Bohn%20Tech%20Bulletins%20Folder/BN-TB-C-AIRCOOLED-1-26.pdf

Heatcraft/Bohn - Catalog BN-ACCTB-Oct 2016 Air-Cooled Condensers Guide (p. 7-8 /Multi-circuiting) https://www.heatcraftrpd.com/PDF/Bohn%20Tech%20Bulletins%20

w w w . r s e s j o u r n a l . c o m JULY 2019 RSES Journal 19w w w . r s e s j o u r n a l . c o m


Circle Reader Service No. 90

Documents

AIR-COOLED CONDENSERS How to Size and Select … · Air-cooled condensers are provided with either a single refrig-erant circuit or with two equally sized circuits as standard, but