Trane PKGP-PRC003-EN manual Selection Procedures, Cooling Capacity, Heating Capacity

Selection Procedures

Cooling Capacity

Step 1

Calculate the building’s total and sensible cooling loads at design conditions. Use theTrane calculation methods or any other standard accepted method.

Factors used in unit selection:

A

Total Cooling Load: 59 MBh

B

Sensible Cooling Load: 40 MBh

C

Airflow: 2000 cfm

D

Electrical Characteristics: 460/60/3

E

Summer Design Conditions: Entering

Evaporator Coil: 80 DB, 67WB Outdoor

Ambient: 95

F

External Static Pressure: 0.36 in. wg

G

Downflow Configuration

H

High Efficiency

I

Economizer

Step 2

As a starting point, a rough determination must be made of the size of the unit.The final selection will be made after examining the performance at the given conditions. Divide the total cooling load by nominal BTUH per ton (12 MBh per ton); then round up to the nearest unit size.

59 MBh / 12 MBh = approx. 5 tons

Step 3

Table PD-3 shows that a WSC060A4 has a gross cooling capacity of 63.1 MBh and

47.5MBh sensible capacity at 2000 cfm and 95 DB outdoor ambient with 80 DB, 67 WB air entering the evaporator.

To Find Capacity at Intermediate Conditions Not in theTable

When the design conditions are between two numbers that are in the capacity table, interpolation is required to approximate the capacity. Note: Extrapolation outside of the table conditions is not recommended.

Step 4

In order to select the correct unit which meets the building’s requirements, the fan motor heat must be deducted from the gross cooling capacity.The amount of heat that the fan motor generates is dependent on the effort by the motor - cfm and static pressure.To determine the total unit static pressure:

External Static (duct system)

0.36 wg

Standard Filter 1 in.	0.09 wg
fromTable PD-23
Economizer	0.18 wg
(100% Outside Air)	fromTable
	PD-23
Electric Heater Size 6 kW	0.056 wg
fromTable PD-23

(reference “Heating Capacity” section on this page for determination of heater size)

Total Static Pressure 0.69 wg

Note:The Evaporator Fan Performance Table PD-12 has deducted the pressure drop for a 1 in. filter already in the unit (see note belowTable PD-12).Therefore, the actual total static pressure is 0.69 - 0.09 (fromTable PD-23) = 0.60 wg.

With 2000 cfm and 0.60 wg.

Table PD-46 shows .90 bhp for this unit. Note below the table gives a formula to calculate Fan Motor Heat,

2.829 x bhp + .4024 = MBH.

2.829 x .90 + .4024 = 2.95 MBH.

Now subtract the fan motor heat from the gross cooling capacity of the unit: NetTotal Cooling Capacity

= 63.1 MBH - 2.95 = 60.15 MBH.

Net Sensible Cooling Capacity = 47.5 MBH - 2.95 = 44.55 MBH.

Step 5

Compare results to original load requirements.

If the performance will not meet the building’s required total or sensible cooling load, try a selection at the next higher size unit.

Heating Capacity

Step 1

Calculate the building heating load using theTrane calculation form or other standard accepted method.

Step 2

Size the equipment usingTable PD-31 to match the heating loads at design conditions.

A

Total heating load: 50 MBH

B

Outdoor Ambient (Winter) 17 DB

C

Indoor ReturnTemperature: 70 DB

D

Airflow: 2000cfm

Use the integrated portion ofTable PD-31 for the WSC060A4 to determine capacity at winter design conditions.The mechanical heating portion of the heat pump will provide 32.7 MBh.

Step 3

Because 32.7 is less than the building’s required heating capacity at winter design conditions, a supplementary heater must be selected.

50 MBh-32.7 MBh = 17.3 MBh

The auxiliary eletric heat capacities are listed inTable PD-38. From the table, a 6 kW heater will deliver 20.48 MBH at 480 volts. In order to determine capacity at 460 volts, the heater voltage correction factor fromTable PD-39 must be used. Therefore, 20.48 MBH x .918 (voltage correction factor) = 18.80 MBH. A 6kW heater should be selected.

Air Delivery Selection

External static pressure drop through the air distribution system has been calculated to be 0.60 inches of water. EnterTable PD-12 for a WSC060A4 at 2000 cfm and 0.60 static pressure.The belt drive motor will give the desired airflow at a rated bhp of 0.90 and 998 rpm.

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