Liebert 60 HZ Selection Procedure, Example, Determining actual Btuh and MBH, Altitude correction

SELECTION PROCEDURE

Table 4 shows the performance specifications for Liebert drycoolers using a 40% by volume ethylene glycol solution at an average fluid temperature of 115°F with flow rates from 1.5 to 3 GPM/circuit and at standard air (.075 lbs/ft3). Figure 2 and Figure 3 offer correction factors to Table 4 for average fluid temperatures and glycol percentages other than 115°F and 40%. Table 3 correction factors may be used for performance at altitudes above sea level. For cooling applications other than shown, contact the Liebert Heat Transfer Division.

To select a drycooler from the tables in this bulletin, the following information must be known:

1.Fluid Flow Rate (GPM).

2.% Ethylene Glycol (% EG).

3.Design Air Temperature at the Drycooler (EAT).

4.Entering and Leaving Fluid Temperatures (EFT, LFT) or Total Heat Rejection (BTU/HR) and one of the Fluid Temperatures.

5.ITD (Initial Temperature Difference) = EFT -

EAT

From the known data, calculate the following:

•Average Fluid Temperature (AFT) = (EFT + LFT)/2.

•Heat Rejection (BTUH) = Fluid Temp. Diff. x GPM x BTU/GPM Factor (Table 2).

•Leaving Fluid Temperature =

EFT - BTUH

(GPM) (BTU/GPM)

•Other useful information: Leaving Air Temp. = EAT +

Drycooler BTUH

(1.08) (Drycooler CFM)

(Leaving Air Temperature should be lower than 145°F for proper motor operation.)

Using Table 4 to select a drycooler

Calculate required MBH/ITD with corrections for glycol % and average fluid temperature.

1.Required MBH/ITD =

BTUH

(EFT - EAT) (1000) (Fig. 1 Factor)

2.Locate Model No. in Table 4 having a GPM range within the required flow rate and an MBH equal to or greater than required. This gives an approximate size.

3.Divide the given GPM by the “No. of circuits” of the drycooler selected. The result is “GPM/CIR” and should be in the range of 1-1/2 to 3.

4.In Table 4, look up the model selected above and under “GPM/CIR” find the actual MBH. You may interpolate between columns.

The MBH found should be equal to or greater than the “required MBH /ITD.”

If the MBH is less than required, repeat from Step 2 with a larger model. You may wish to repeat from Step 2 with a smaller model for the most economical selection meeting the required MBH/ITD.

Pressure Drop - After selecting a model, look up the unit pressure drop following Step 3 and 4 above. Multiply the pressure drop found by the Figure 3 correction factor. If the product is higher than your system design, go back to Step 2 and select a model with more circuits. This may be the same, or larger, unit.

Example

Cool 40 GPM 20% ethylene glycol and water solution from 125(F) to 115(F). Design EAT = 95(F).

Calculate:

BTUH = (125 - 115) x (40 GPM) x (480 BTU/ GPM)

BTUH = 192,000

AFT = (125 + 115)/2 = 120(F)

From Figure 2, corr. factor for 120 AFT and 20% EG = 1.04

1.Required MBH/ITD = 192,000 BTUH

(125-95) (1000) (1.04)

2.Locate model in Table 4. Models 092 through 139 fall into the GPM range but do not have the MBH capacity. Model 174 with 16 circuits is the smallest model meeting both the GPM range and MBH requirements.

3.GPM/CIR = 40 GPM/16 CIR = 2.5 GPM/CIR.

4.In Table 4, Model 174 with 16 circuits at the 2.5 GPM/CIR column provides 6.9 MBH/ITD, which exceeds the required MBH/ ITD of 6.15.

Pressure Drop = 10.5 ft. (from Table 4) x 0.93 (from Figure 3) = 9.8 ft. H2O.

Table 2 Determining actual BTUH and MBH

% Glycol	0%	10%	20%	30%	40%	50%
Solution
BTUH/GPM	500	490	480	470	450	433

Table 3	Altitude correction
Alt. (Ft.)		0	1000	2000	5000	8000	12000	15000
Corr. Fact.		1	.979	.96	.9	.841	.762	.703

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