Selection Procedure
This section outlines a
•Summer outdoor design conditions — 95 DB/76 WB ambient temperature
•Summer room design conditions
•Total cooling load — 430 MBh (35.8 tons)
•Sensible cooling load — 345 MBh (28.8 tons)
•Outdoor air ventilation load — 66.9 MBh
•Return air temperature — 80 DB/65 WB
Winter Design:
•Winter outdoor design conditions
•Return air temperature — 70 F
•Total heating load — 475 MBh
•Winter outdoor air ventilation load — 133 MBh
Air Delivery Data:
•Supply fan cfm — 17,500 cfm
•External static pressure — 1.2 in wg
•Minimum outdoor air ventilation — 1,750 cfm
•Exhaust fan cfm — 12,000 cfm
•Return air duct negative static pressure
— 0.65 in wg
Electrical Characteristics:
•Voltage/cycle/phase — 460/60/3 Unit Accessories:
•Gas fired heat exchanger — high heat module
•Throwaway filters
•Economizer
•Modulating 100 percent exhaust/ return fan
COOLING CAPACITY SELECTION
Step 1 — Nominal Unit Size Selection
A summation of the peak cooling load and the outside air ventilation load shows: 430 MBh + 66.9 MBh = 496.9 MBh required unit capacity. From
Table
is selected.
Step 2 — Evaporator Coil Entering Conditions
Mixed air dry bulb temperature determination:
Using the minimum percent of OA (1,750 cfm ÷ 17,500 cfm = 10 percent), determine the mixture dry bulb to the evaporator.
RADB + % OA (OADB - RADB) = 80 + (0.10) (95 - 80) = 80 + 1.5 = 81.5 F
Approximate wet bulb mixture temperature:
RAWB + % OA (OAWB - RAWB) = 65 + (0.10) (76 - 65) = 65 + 1.1 = 66.1 F
Step 3 — Determine Supply Fan Motor Heat Gain
Having selected a nominal 50 ton unit, the supply fan bhp can be calculated.The supply fan motor heat gain must be considered in final determination of unit capacity.
Supply Air Fan
Determine unit total static pressure at design supply cfm:
External Static Pressure | 1.2 inches |
Evaporator Coil | 0.25 inches |
(Table |
|
Return Duct Negative | 0.65 inches |
Static Pressure |
|
Heat Exchanger | 0.31 inches |
(Table |
|
Throwaway Filter | 0.10 inches |
(Table |
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Economizer w/Exhaust Fan 0.12 inches
(Table |
|
Trane Roof Curb | 0.13 inches |
(Table |
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UnitTotal Static Pressure | 2.76 inches |
Using total of 17,500 cfm and total
static pressure of 2.76 inches, enterTable
From Chart
Step 4 — DetermineTotal Required Cooling Capacity
Required capacity =Total peak load + OA load + supply air fan motor heat.
Required capacity = 430 + 66.9 + 46.0 = 543 MBh (45.2 tons)
Step 5 — Determine Unit Capacity
FromTable
66.1WB entering the evaporator, 17,500 supply air cfm, 95 F outdoor ambient, is 561 MBh (45.8 tons) with 426 MBh sensible.
Step 6 — Determine Leaving Air Temperature
Unit sensible heat capacity corrected for supply air fan motor heat = 426 MBh - 46 MBh = 380 MBh.
Supply air dry bulb temperature difference =
Sensible Btu | = |
1.085 x Supply cfm |
|
380 MBh ÷ (1.085 x 17,500 cfm) = 20.0 F
Supply air dry bulb = 81.5 DB - 20.0 = 61.5 F
Unit enthalpy difference =
Total Btu = 4.5 x Supply cfm
561 MBh ÷ (4.5 x 17,500 cfm) =
7.12 Btu/lb
Leaving enthalpy = h(ent WB) - h(diff). From Table
Leaving enthalpy = 30.9 Btu/lb - 7.12 Btu/lb = 23.78 Btu/lb
Supply air wet bulb = 55.9
Leaving air temperature = 61.5
DB/55.9 WB
16 |