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Hi ,
how do we work out the heat exchange capacity of the copper tubes used in an air-cooled condenser with 8 fpi, fins per inch ( aluminum ) ???
Or how do we decide the length of copper tube say 3/8 inch diam for a 30 kw air cooled condenser with 15 deg f td , ambient 40 deg c ???
I eed some input to work on condenser design ..
Designers use some software.
Basically its the cooling capacity+compressor input+20% or more, depends on the ambient.
CONDENSER SELECTION
Capacity for air-cooled condensers are based on Total Heat of Rejection (THR)
at the condenser. Total heat of rejection is equal to net refrigeration at the
evaporator (compressor capacity) plus the energy input into the refrigerant
by the compressor (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 is
recommended that you obtain the heat of compression value from the
compressor manufacturer.
If this is not available, the THR can be estimated using the following formula:
THR = (Compressor Capacity) * (Heat of Compression Factor, Tables 1 & 2)
Table 1 contains heat of compression factors for suction cooled compressors
and Table 2 contains factors for open drive compressors. For refrigeration
systems beyond the range of Tables 1 and 2, use the following equations to
estimate THR:
Open Compressors:
THR = Compressor Capacity (BTUH) + (2545) * (Break Horsepower, BHP)
Suction Cooled Compressors:
THR = Compressor Capacity (BTUH) + (3413 * KW)
The compressor capacity is affected 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: 270,000 BTUH
Evaporator temperature: +25° F
Condensing temperature: 110° F
Ambient temperature 95° F
Refrigerant: R-404A
Compressor type: Semi-hermetic, suction cooled
Condenser type: N RG
Condenser altitude: 1,000 feet
Step 1: Estimate Condenser THR
From Table 1 for suction cooled compressors, at +25° F suction and 115° F
condensing temperature, select a heat of compressor factor of 1.335.
THR = Compressor Capacity * Heat of Compression Factor
= 270,000 * 1.335
= 360,450
Step 2: Correct for Altitude
From Table 3 obtain an altitude correction factor of 1.02 for 1,000 feet.
THR = THR (from step 1) * Altitude Correction Factor (design)
= 360,450 * 1.02
= 367,659
Step 3: Calculate Design Condenser T.D.
Design Condenser T.D. = Condensing Temp — Ambient Temp
= 110°F - 95
= 15° T.D.
Step 4: Condenser Selection
Condenser capacitiesare located on page 6. These capacities are given in
MBH/°TD. Convert the THR calculated in step 2 to MBH/°TD by dividing
by 1,000 to get THR in MBH. Then divide the THR by the design TD to get
MBH/°TD.
THR (MBH) = 367,659 / 1,000 = 367.7
THR (MBH/°TD) = 367.7 / 15 = 24.5
Locate the capacity column and read down until you locate a value equal to or
just larger than 24.5. This value is 27.0. Read horizontally to the left to obtain a
condenser model of NRGD04A027
Step 5: Calculate Actual T.D. and Condensing Temperature
The actual condenser T.D. can be calculated by dividing the design THR by
the condenser rating.
Actual T.D. = THR (Design) / (Rating @ 1° T.D.)
= 367.7 / 27.0
= 13.6°F. T.D.
The actual condensing temperature is the actual T.D. plus the ambient
temperature.
Actual Condensing Temperature = (Actual T.D.) + (Ambient)
= 13.6 + 95
= 108.6°F.
Altitude Correction Factor
0 1
1,000 1.02
2,000 1.05
3,000 1.07
4,000 1.1
5,000 1.12
6,000 1.15
7,000 1.17
Table 1. Heat of Compression Factor for Suction Cooled Compressors.
Table 2. Heat of Compression Factor for Open Drive Compressors.
Table 3. Altitude Correction Factors.
Evaporator Condensing Temperature °F
Temp. °F 90° 100° 110° 120° 130° 140°
-30° 1.37 1.42 1.47 — — —
-20° 1.33 1.37 1.42 1.47 — —
-10° 1.28 1.32 1.37 1.42 1.47 —
0° 1.24 1.28 1.32 1.37 1.41 1.47
5° 1.23 1.26 1.3 1.35 1.39 1.45
10° 1.21 1.24 1.28 1.32 1.36 1.42
15° 1.19 1.22 1.26 1.3 1.34 1.4
20° 1.17 1.2 1.24 1.28 1.32 1.37
25° 1.16 1.19 1.22 1.26 1.3 1.35
30° 1.14 1.17 1.2 1.24 1.27 1.32
40° 1.12 1.15 1.17 1.2 1.23 1.28
50° 1.09 1.12 1.14 1.17 1.2 1.24
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 1.7 1.8
-20° 1.43 1.49 1.55 1.62 1.7
-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° 1.2 1.24 1.28 1.33 1.39
40° 1.17 1.2 1.24 1.28 1.33
50° 1.13 1.16 1.2 1.24 1.28
Note: These units are optimized for operation at or below 15° T.D.
Operation above 15° T.D. may result in excessive pressure drop.
Table 4. Correction Factors.
Correction Factor Capactiy Refrigerant Charge
R-404A 1.00 1.00
R-22 1.02 1.09
R-410A 1.02 1.07
R-507 1.00 1.00
50 Hz Power Supply 1.00 --
Hassanfrag,
Good copy from some unidentified publication but does not answer the question posed by DD Koranne.
All condenser manufacturers will specify performance versus highest ambient dry bulb condition, and I take that with a grain of salt, I prefer evap/condensers and take advantage of the wet bulb effect. all dependent on refrigerant applied and max discharge temps applicable
How professional it is!
The question is about air cooled condenser DESIGN, not selection.
I will have to look through some of my notes to list the exact process, I would suggest it is easier to approach a coil manufacturing company who do this stuff all the time. They will usually use a program to work out the required coil configuration and airflow, it can be done by hand but at the end of the day you still need to order it from a coil company so you may as well use their expertise in design as well.
http://www.coolit.co.za/coilsim/index.htm
There are some excellent software packages on this site. I use them for industrial & RHVAC design. The license prices are very reasonable. Send my regards to Bruce.
Hi brian ,
yes that is right . I have software which calculates the total heat rejection in condenser . But when it comes to buy a condenser , manufacturers expect me to specify the tube diameter for cond coil ,size of face area , total tube length , and number of rows etc which constitutes the manufacturing data.
They can fabricate one based on the info but cannot design a cond based on the purely load data of the condenser ......... All in all i need to understand how to work out the tube length and diam etc and feed the cond manufacturer .............
And the same applies to water cooled condenser