Carrier 48MA - Manual (Page 9)

The unit capacity available to offset room loads is.
Room TC = Unit TC — outdoor air TC
= 282,000 -46,500
= 235,500 Btuh
Room SHC = Unit SHC — outdoor air sensible heat
= 219,000 -21,800
= 197,200 Btuh
For comparison:
GTL
SL
RTL
RSL
Load
275.000 Btuh
215.000 Btuh
226.000 Btuh
197,200 Btuh
Unit Capacity
TC = 282,000 Btuh
SHC = 219,000 Btuh
RTC = 235,500 Btuh
RSHC = 197,200 Btuh
The 48MA/50ME size meets or exceeds the
total and zone load requirements at the specified
conditions. The excess RTC decreases space
average relative humidity slightly below the room
design of 50%. By increasing air quantity above
9000 cfm, this excess latent capacity can be
converted to additional sensible capacity if desired.
Since the modular multizone is a constant
volume machine, the selected supply cfm per zone
must be proportioned to satisfy each zone’s peak
load condition.
Room sensible capacities (RSC) are divided
equally among the modules if an equal cfm is going
to each. In this example, the 48MA/50ME028 has
10 modules and the nominal cfm is 900 cfm per
module.
The cfm to each zone can be varied (with
field-supplied manual dampers in zone ducts) to
match different zone requirements, but since the
original rating was based on 9000 cfm supply air,
all variations must total 9000 cfm. The effects of
changing cfm quantities on room TC and room
SHC in each module are shown in Fig. 7. When the
cfm is changed (by some percent) from the
nominal in a specific module, then the room
capacity multipliers in Fig. 7 are used to correct
room TC and room SHC. Capacity versus cfm
changes for the example is given in Table 2.
By analyzing each zone’s ratio of deviation
from equal sensible heat allocation, the proper cfm
change is determined. In the example, if building
room SHC is 197,200 Btuh and 10 zones are used,
each zone’s normal room SHC is 19,720 Btuh. But
if zone 3 has 22.505 Btuh room SHC, then by ratio
of 22,505 : 19,720 or 1.14, the cfm change is
+20% (see Fig. 7). Correspondingly, if zone 1 had
16,935 Btuh room SHC, the cfm change is -20%.
In applications where the zone selection is not
an increment of the number of unit modules (i.e.
one zone requiring 500 cfm in a 48MA/50ME028
with 10,000 cfm), refer to Module Cfm Limits,
page 10, for details on using cfm’s below
600 cfm/module.
Formulas required to use ratings are:
Outdoor Air Total Heat (OATH)
OATH = 4.5 (OA cfm) (hoa-hioom)
Outdoor Air Sensible Heat (OASH)
OASH = 1.09 (OA cfm) (toa ~ troom)
Room Total Capacity (RTC)
RTC = Unit TC-OATH
Room Sensible Heat Capacity (RSHC)
RSHC = Unit SHC - OASH
Room Sensible Heat Factor (RSHF)
portp _ RSHC
Leaving Air Temperature (LAT)
LAT = room temperature
RSHC
1.09 cfm
Determine Heating Capacity:
The specified requirement for electric heat
dictates the selection of a 50ME028 unit with a kw
option that meets or exceeds the heating load.
Table 9, page 50 indicates that the 028 unit has
heating capacity options of 66, 88 and 132 kw.
The 132 kw option is selected as it provides
adequate heat for this application. The kw/zone
and number of heat stages available are:
Zone No. Load
Zone Heating
Capacity
Stages
of Heat
1 10.0 kw 13.2 kw 3
2 12.9 kw 13.2 kw 3
3 12.9 kw 13.2 kw 3
4 32.5 kw 39.6 kw 9
5 12.3 kw 13 2 kw 3
6 12.9 kw 31.2 kw 3
7 23.7 kw 26 4 kw 6
Total 117.2 kw 132.0 kw 30
Stages of heat are controlled individually in the
small zones or collectively in large zones to provide
flexible and continuous control for each zone.
Table 2 — Capacity vs Cfm Changes
ZONE
NO.
NO. OF
MODULES
RSL/ZONE
PEAK LOAD
% DEVIATION
(RSL/NOM UNIT RSHC)
% CFM CHANGE
FROM NOMINAL
CFM
UNIT TOTAL
CAPACITY
MULTIPLIER*
X
UNIT
NOMINAL
RTC/ZONE
ADJUSTED
UNIT RTC
1 1 16,935 16,935/19,720 = 86 -20 720 9 X 23,550 21,195
2 1 22,505 22,505/19,720 =1.14 +20 1080 1 1 X 23,550 25,905
3 1 22,505 22,505/19,720 =1.14 +20 1080 1 1 X 23,550 25,905
4 3 59,160 59,160/3 X 19,720 = 1.00 0 2700 1 0 X 3 X 23,550 70,650
5 1 19,720 19,720/19,720 =1.00 0 900 1.0 X 23,550 23,550
6 1 22,505 22,505/19,720 =1.14 +20 1080 1.1 X 23,550 25,905
7 2 33,870 33,870/2 x19,720 = .86 -20 1440 .9 X 2 X 23,550 42,390
10 197,200 9000 235,500
*Unit total capacity multiplier is obtained from Fig. 7. Use % change from nominal and read multiplier from graph.
RSHC — Room Sensible Heat Capacity RSL — Room Sensible Load RTC — Room Total Capacity