33
SELECTION PROCEDURE (WITH 48TC*A07 EXAMPLE)
1
I. Determine cooling and heating loads.
Given:
Mixed ai r dry bulb 80_F(27_C)
Mixed ai r we t bulb 67_F(19_C)
Ambient dry bulb 95_F(35_C)
TC
Load
72.0 MBH
SHC
Load
54.0 MBH
Ve rtical supply air 2100 CFM
Heati ng load 85.0 MBH
External static pressure 0.67 in. wg
Electrical characteristics 230--3 --60
II. Make an initial guess at cooling tons.
Refrig. tons = TC
Load
/ 12 MBH per ton
Refrig. tons = 72.0 / 12 = 6.0 tons
In this ca se, start by looking at the 48TC**07.
III. Look up the rooftop’ s TC and SHC.
Table 19 shows that, at the applicat ion’s supply air
CFM, mixed air and ambient temperatures, the
48TC*A07 supplies:
TC = 73.7 MBH
2
SHC = 54.4 MBH
2
IV. Calculate the building latent heat load.
LC
Load
=TC
Load
-- S H C
Load
LC
Load
= 72.0 MBH -- 54.0 MBH = 18.0 MBH
V. Calculate RTU latent heat capacity.
LC = TC -- SHC
LC = 73.7 MBH -- 54.4 MBH = 19.3 MBH
VI. Compare RTU capacities to loads.
3
Compare the rooftop’s SHC and LC to t he
building’s sensible and latent heat loads.
VII. Select factory options (FIOP)
Local code re quires an economizer for any unit with
TC greater than 65.0 MBH.
VIII. Calculate the total static pressure.
External static pressure 0.67 in. wg
Sum of FIOP / Accessory static +0.13 in.
wg
Total Static Pressure 0.80 in. wg
IX. Look up the indoor fan RPM & BHP.
Table 47 shows, at 2100 CFM & ESP= 0.8,
RPM = 1358 & BHP = 1.52
X. Convert BHP (Step VIII) into fan motor heat.
Fan motor heat = 2.546* BHP/Motor Eff.
4
Fan motor he at = 4.9 MBH
XI. Calculate RTU heating capacity.
Building heating load 85.0 MBH
Fan motor he at --4.9
MBH
Required heating capacity 80.1 MBH
XII. Select a gas heater.
Table 3 shows the heating capacities of the
48TCEA07 = 93.0 MBH. Select the 48TCEA07
XIII. Determine electrical requirements.
The MCA and MOCP tables show 48TC*A07
(without convenience outlet) as:
MCA = 30.5 amps & MOCP = 45.0 amps
Min. disconnect size: FLA = 30 & LRA = 157.
Legend
BHP — Brake horsepower
FLA — Full load amps
LC — Latent capacity
LRA — Lock rotor amp
MBH — (1,000) BTUH
MCA — Min. circuit ampacity
MOCP — Max. over--- current protection
RPM — Revolutions per minute
RTU — Rooftop unit
SHC — Sensible heat capacity
TC — Total capacity
NOTES:
1. Selection software by Carrier saves time by performing many
of the steps above. C ontact your Carrier sales representative
for assistance.
2. Unit ratings are gross capacities and do not include the effect
of evaporator fan motor heat. See Step X. for determining
amount of evaporator fan motor heat to subtract from total
and sensible capacities to obtain n et cooling and net sens-
ible capacities.
3. Selecting a unit with a SHC slightly lower than the SHC
Load
is
often better than oversizing. Slightly lower SHC’s will help
control indoor humidity, and prevent temperature swings.
4. Indoor fan motor efficiency is available in the Electrical Infor-
mation. Use the decimal form in the equation, eg. 80% = .8.
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