Selection Procedure, Cooling Capacity Step

Selection Procedure

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:

Total Cooling Load: 59 MBh

Sensible Cooling Load: 40 MBh

Airflow: 2000 cfm

Electrical Characteristics: 460/60/3

Summer Design Conditions: Entering

Evaporator Coil: 80 DB, 67WB Outdoor

Ambient: 95

External Static Pressure: 0.45 in. wg

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.

60 MBh / 12 MBh = 5Tons

Step 3

Table PD-4 shows that aTSC060A4 has a gross cooling capacity of 63.1 MBh and

48.2MBh sensible capacity at 2000 cfm and 95 DB outdoor ambient with 80 DB, 67WB 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)

0.45 wg

Standard Filter 1 in.	0.15 wg
fromTable PD-59
Economizer	0.02 wg

(100% Return Air) fromTable PD-26

Electric Heater Size 6 kW	0.05 wg
fromTable PD-51
Total Static Pressure	0.67 wg

Note:The Evaporator Fan Performance Table PD-20 has deducted the pressure drop for a 1 in. filter already in the unit (see note belowTable PD-20).Therefore, the actual total static pressure is 0.67 - 0.15 (fromTable PD - 26) = 0.52 wg.

With 2000 cfm and 0.52 wg.

Table PD-20 shows .90 bhp for this unit. Note below the table gives a formula to calculate Fan Motor Heat,

4.39x bhp = MBH.

4.39x .90 = 3.95 MBH.

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

= 60 MBH - 3.95 = 56.05 MBH.

Net Sensible Cooling Capacity = 48.2 MBH - 3.95 = 44.25 MBH.

Step 5

If the performance will not meet the required load of the building’s 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 system heating capacity to match the calculated building heating load.The following are building heating requirements:

Total heating load of 15 MBH

2000 cfm

460 volt/3 phase Power Supply

The electric heat accessory capacities are listed inTable PD-62 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-60 must be used.Therefore, 20.48 MBH x .9118 (voltage correction factor) = 18.80 MBH.

Air Delivery Selection

External static pressure drop through the air distribution system has been calculated to be 0.45 inches of water. FromTable PD-59 static pressure drop through the economizer is 0.02 and the 6 kW heater is 0.05 inches of water (0.45

+0.02 + 0.05). EnterTable PD-20 for a TSC060A4 at 2000 cfm and 0.52 static pressure.The standard direct drive motor at high speed will give the desired airflow at a rated bhp of 0.90.

RT-PRC005-EN