Johnson Controls
5513350-JTG-1018
12
Selection Procedure Examples
GIVEN:
Required Cooling Capacity
290,000 Btuh
Required Sensible Cooling
210,000 Btuh
Required Heating
180,000 Btuh
Entering Air on Evaporator
80.0°F dry bulb (DB)/
67.0°F wet bulb (WB)
Outside Design Temperature
95.0°F
Supply Fan CFM
12,000 CFM
External Static Pressure (ESP)
3.72 iwg
Electrical Supply Voltage
460-3-60
Economizer Required
2-inch Throwaway Filters
Variable Air Volume (VAV)
Calculating Cooling/Heating Capacity
1. Assume that the required cooling capacity and re-
quired sensible capacity include the space load re-
quirements as well as the ventilation load require
-
ments.
2. Calculate the supply fan motor heat Btuh addition.
a. See
Select Fan Speed and Horsepower Require-
ments for Supply Fan on page 13
to determine
the horsepower (HP) of the supply fan. The ex
-
ample is based on a 14.91 HP requirement.
b. Calculate sensible Btuh addition as a result of the
supply fan HP.
• Supply fan sensible Btuh addition = 14.91
HP x 2,750 (constant for motor heat calcula
-
tion)
• Supply fan sensible Btuh addition = 41,003
Btuh
c. Calculate the total capacity requirement.
• Total capacity = required cooling capacity
(Btuh) + supply fan motor HP (Btuh)
•
Total capacity = 290,000 Btuh + 41,003 Btuh
• Total capacity = 331,003 Btuh
d. Refer to
, and find the ca-
pacity of units that meet 331.0 total cooling ca-
pacity (TMBH) and 210 sensible cooling capacity
(SMBH) at 12,000 CFM, 95.0°F ambient, 80.0°F
entering dry bulb (EDB), and 67.0°F entering wet
bulb (EWB) onto the coil. A 30-ton unit can pro
-
duce 362 TMBH and 287 SMBH.
e. Calculate the leaving air temperature.
1.
Calculate sensible unit capacity with motor
heat included.
• Sensible Btuh = 287 MBH - 41.0 MBH
= 246.0 MBH
2. Calculate dry bulb supply air temperature
(SAT).
•
Sensible Btuh = CFM x 1.085 x ∆T
•
∆T = 246,000 Btuh / (12,000 CFM x
1.085)
•
∆T = 18.9°F
• Dry bulb SAT = entering air tempera-
ture – ∆T
• Dry bulb SAT = 80.0°F – 18.9°F = 61.1 °F
3. Calculate enthalpy delta.
•
Total capacity Btuh = CFM x 4.5 x ∆h
•
∆h = 363,000 Btuh / (12,000 CFM x 4.5)
•
∆h = 6.7 Btu/lb
4.
Calculate wet bulb SAT.
• Leaving enthalpy = enthalpy entering –
enthalpy delta (reference psychometric
chart to convert unit wet bulb tempera
-
ture to Btu/lb)
• Leaving enthalpy = 31.6 Btu/lb – 6.7
Btu/lb
• Leaving enthalpy = 24.9 Btu/lb
• Wet bulb SAT = 57.7°F
5. Leaving air temperature = 61.1°F / 57.7°F
The 30 ton unit will meet the cooling requirements.
From the nomenclature, digit 3 will be B for 30 ton
capacity. Digit 4 will be 1 for standard capacity, stan
-
dard efficiency as assumed.
