Heat Controller, Inc. | HBH/V SERIES | Engineering Design Guide |
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Selection Procedure
Step 1 Determine the actual heating and cooling loads at the desired dry bulb and wet bulb conditions.
Step 2 Obtain the following design parameters: Entering water temperature, water flow rate in GPM, air flow in CFM, water flow pressure drop and design wet and dry bulb temperatures. Air flow CFM should be between 300 and 450 CFM per ton. Unit water pressure drop should be kept as close as possible to each other to make water balancing easier. Go to the appropriate tables and find the proper indicated water flow and water temperature.
Step 3 Select a unit based on total and sensible cooling conditions. Select a unit which is closest to, but no larger than, the actual cooling load.
Step 4 Enter tables at the design water flow and water temperature. Read the total and sensible cooling capacities (Note: interpolation is permissible, extrapolation is not).
Step 5 Read the heating capacity. If it exceeds the design criteria it is acceptable. It is quite normal for Water- Source Heat Pumps to be selected on cooling capacity only since the heating output is usually greater than the cooling capacity.
Step 6 Determine the correction factors associated with the variable factors of dry bulb, wet bulb and air flow.
Corrected Total Cooling =
tabulated total cooling x wet bulb correction x air flow correction
Corrected Sensible Cooling =
tabulated sensible cooling x dry bulb correction x air flow correction
Step 7 Compare the corrected capacities to the load requirements. Normally if the capacities are within 10% of the loads, the equipment is acceptable. It is better to undersize than oversize, as undersizing improves humidity control, reduces sound levels and extends the life of the equipment.
Step 8 When completed, calculate water temperature rise and assess the selection. If the units selected are not within 10% of the load calculations, then review what effect changing the GPM, water temperature and/or air flow and air temperature would have on the corrected capacities. If the desired capacity cannot be achieved, select the next larger or smaller unit and repeat the procedure. Remember, when in doubt, undersize slightly for best performance.
Example Equipment Selection For Cooling
Step 1 Load Determination:
Assume we have determined that the appropriate cooling load at the desired dry bulb 80°F and wet bulb 65°F conditions is as follows:
Total Cooling | 23,700 BTUH |
Sensible Cooling | 16,500 BTUH |
Entering Air Temp.... 80°F Dry Bulb / 65°F Wet Bulb
Step 2 Design Conditions:
Similarly, we have also obtained the following design parameters:
Entering Water Temp | 90°F |
Water Flow (Based upon 10°F rise in temp.) .6.0 GPM
Air Flow | 800 CFM |
Step 3, 4 & 5 HP Selection:
After making our preliminary selection (TC024), we enter the tables at design water flow and water temperature and read Total Cooling, Sens. Cooling and Heat of Rej. capacities:
Total Cooling | 23,400 BTUH |
Sensible Cooling | 17,500 BTUH |
Heat of Rejection | 30,200 BTUH |
Step 6 & 7 Entering Air and Airflow Corrections:
Next, we determine our correction factors.
Table | Ent Air Air Flow Corrected |
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Corrected Total Cooling = 23,400 | x 0.9681 x 1.0050 = 22,767 | |
Corrected Sens Cooling = 17,500 | x 1.1213 x 0.9820 = 19,270 | |
Corrected Heat of Reject = 30,200 x 0.9747 x 1.0434 = 30,713 | ||
Step 8 Water Temperature Rise Calculation & Assessment: | ||
Actual Temperature Rise | 10.2°F | |
When we compare the Corrected Total Cooling and Corrected Sensible Cooling figures with our load requirements stated in Step 1, we discover that our selection is within +/- 10% of our sensible load requirement. Furthermore, we see that our Corrected Total Cooling figure is within 1,000 Btuh the actual indicated load.
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