Selection Example - Johnson Controls RC Series Engineering Manual

Selection Example

To achieve optimal performance, proper selection of each heat
pump is essential. A building load program should be used to
determine the heating and cooling load of each zone. A computer
software selection program can then be used to develop an
accurate and complete heat pump schedule. Software can be
obtained from your local representative.
While a software program is the easiest and most accurate
method to size and select equipment, however, selection can still
be accomplished manually using this manual and the following
selection procedure. Sizing so that the actual sensible capacity of
the equipment will satisfy the sensible capacity of the zone is the
recommended method for best results.
Boiler/Tower Application
Typical boiler/tower application will result in entering water
temperatures of 60-90°F with 70°F for heating and 90°F for
cooling. Water to refrigerant insulation option would not be
required. Flow rates are 2.5 to 3 gpm per ton with 2.5 gpm per ton
often representing an economical design point.
Geothermal Application
Typical geothermal application can result in a wide entering
water temperature range of 30-100°F. Typically minimum heating
entering water temperatures can range from 30 to 50°F depending
upon loop type and geographical location. Cooling performance
should be calculated using a maximum loop temperature of 100°F
in most loop applications. Water flow is typically 2.5 to 3 gpm per
ton with 3 gpm per ton recommended with the more extreme loop
temperatures. PLEASE NOTE THAT WATER COIL INSULATION
OPTION SHOULD BE SELECTED WHEN ENTERING WATER
TEMPERATURES ARE EXPECTED TO BE BELOW 45-50°F.
Geothermal Selection Example
Step 1: Determine the actual heating and cooling loads at the
desired dry bulb and wet bulb conditions.
Step 2: Obtain the following de sign 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 and wet bulb.
Corrected Total Cooling = tabulated total cooling x wet bulb
correction.
Corrected Sensible Cooling = tabulated sensible cooling x wet/dry
bulb 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 complete, 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 - Cooling
1. Load Determination:
Assume we have determined that the appropriate cooling load
at the desired dry bulb 75°F and wet bulb 60°F conditions is as
follows:
Total Cooling ............................................................ 14,800 BTUH
Sensible Cooling.......................................................11,200 BTUH
Entering Air Temp ..........................75°F Dry Bulb / 60°F Wet Bulb
2. Design Conditions:
Similarly, we have also obtained the following design parameters:
Entering Water Temp ............................................................100°F
Water Flow (Based upon 10°F rise in temp.) 5.5 GPM
Air Flow Required ........................................................... 450 CFM
3, 4 & 5. HP Selection:
After making our preliminary selection (RC18), we enter the tables
at design water flow and water temperature and read
Total Cooling, Sens. Cooling and Heat of Rej. capacities:
Total Cooling ............................................................ 16,600 BTUH
Sensible Cooling...................................................... 12,600 BTUH
Heat of Rejection ..................................................... 21,400 BTUH
6 & 7. Entering Air and Airflow Corrections:
Next, we determine our correction factors. (Refer to Correction
Factor Tables - Air Flow and Entering Air correction tables — using
450 cfm. or 450÷500 nom. = 90%).
Corrected Total Cooling = 16,600 x 0.982 x 0.897 = 14,622
Corrected Sens Cooling = 12,600 x 0.933 x 0.995 = 11,697
Corrected Heat of Reject = 21,400 x 0.980 x 0.895 = 18,770
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RC SERIES ENGINEERING GUIDE