Carrier ZONE-MIZER 50ME - Manual (Page 36)

Application data
(cont)
correction factors and formulas in the Capacity Correction
Factor table.
CAPACITY CORRECTION FACTOR (CCF)
LOAD
TC (Unit)
SHC (Unit)
DIVERSITY FACTOR
1.0
1 0
i 0
90
97
94
80
94
89
RTC (witli diversity)
RSHC (witii diversity)
[TC (CCF) - OATH]
Diversity Factor
[SHC (CCF) - OASH]
Diversity Factor
This is accomplished by rating the unit assuming that no
more than 9 or 10 zones would be on at one time, 90%
diversity The same logic applies to other diversity factors
on an average basis, such as 85 or 95%.
A rating with a diversity factor results in a lower room
SHF, therefore, a reselection at a higher total unit cfm is
advisable to take full advantage of the building diversity.
Limitations
Module cfm limits and fan performance — The cfm
limits per zone are 1200 cfm maximum and 600 cfm mini­
mum. The outboard zones in the 8-, 10- and 12-module
units are limited to a maximum of 1000 cfm.
The limita­
tions are necessary to prevent blow-off to the heat ex­
changers and into the ductwork. The minimum limit pre­
vents burner cycling on limit switches and prevents electric
heater cycling. At reduced cfm’s, zone evaporator coils
overfeed refrigerant, but there is no liquid flood-back to the
compressor as it is protected by a suction line accumulator.
For applications below 600 cfm, modify heating controls
as follows:
Gas fired (300 to 599 cfm) — Derate burners as shown
on page 33.
Electric Resistance (450 to 599 cfm) — Use first- and
second-stage heat on 3-stage units.
Electric Resistance (300 to 449 cfm) — Use first-stage
heat on 2- or 3-stage heat units.
Design the duct system so that differences between ad­
jacent zones is kept to a minimum. THs reduces internal
air leakage between zones in the evaporator section.
Optimum performance is delivered in the 800 to 1000 cfm
range. Extremely low cfm requirements reduce unit cooling
capacity. For low zone cfm applications, size the zone for
a higher cfm (to increase unit efficiency) and divert the
extra air into the return air system or a larger interior space.
Do not divert extra air into spaces with different perimeter
wall orientations.
Fan performance data are based on 15% outdoor air.
When the outdoor air dampers are closed and there is no
outdoor ventilation air into the unit, unit cfm is reduced by
2% to 6%. This reduction is due to the static pressure drops
existing in the separate airflows thru the unit. This reduction
is significant in special applications where little or no
ventilation is required and cfm requirements are critically
designed.
Maximum ventilation limits — Under normal mechanical
cooling, the amount of ventilation air that can be introduced
is a function of the outdoor air damper setting and negative
static pressure at the return air intake of the unit. The
Ventilation Air Charts show ventilation air versus negative
static pressure at various settings of the outdoor air
damper. A 5.5 setting of the ventilation control dial is the
maximum opening of the dampers. The ventilation dial can
be set in any position from 0 to 5.5 to obtain the desired
cfm of outdoor air. The ventilation dial is located on the
control panel adjacent to the heating section. NOTE: Out­
door air at other unit cfm values is proportional.
Dehumidification applications
A space with a high latent load and a very low sensible load
may require tempering capability for dehumidification.
Typical spaces of this type are conference rooms or visual
aids rooms where people congregate with the lights out.
Dehumidification control is achieved on the 48MA/50ME
unit by wiring a humidistat in parallel with the cooling ther­
mostat on any zone requiring dehumidification. This may be
done on one module or all modules. When using dehumidifi­
cation control on electric resistance heat units, use extreme
care with power wiring as heating and cooling can operate
simultaneously in each module.
When the zone’s humidity level reaches the setpoint of
the humidistat, mechanical refrigeration is activated for
that zone module and the air is dehumidified and then
tempered on room thermostat demand before being dis­
charged to the zoned space.
The 48MA/50ME economizer
The 48MA/50ME units can be equipped with an economizer
control. The control functions as follows, with ambient tem­
peratures above the economizer changeover point, the
outdoor air damper is set at the ventilation position,
cooling is accomplished by the compressors when the room
thermostat calls for cooling. If fhe zone is not calling for
cooling, the mixed air is circulated thru the space. When
the ambient temperature drops below the economizer
changeover point, the compressors are locked out and the
damper motor is under control of a mixed air thermostat to
maintain a mixed air temperature low enough to provide
cooling when the room thermostat demands it. NOTE: If a
non-crifical zone opens the unit economizer when most
other zones are in the heating mode, to save energy, dis­
connect the wire at spade terminal number 4 of non-critical
zone cooling relay. This will prevent the non-critical zone
from energizing the economizer. Mechanical cooling and
heating are not affected.
If a zone thermostat calls for cooling while in economizer
mode, a set of cooling relay contacts close, energizing the
economizer relay as shown on the Economizer Condensing
Schematic.
For economizer damper control, the economizer relay
locks out the outside air damper adjustable potentiometer
and shifts the damper control to a Mixed Air Thermostat
(MAT.). The MAT. sensor, located in the fan section,
adjusts the outside air damper to maintain a preset mixed air
temperature as shown on the Economizer Damper Control
Schematic.
The 48MA/50ME economizer operation provides
economic use of outdoor air for low-cost cooling. When all
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