Carrier ZONE-MIZER 50ME - Manual (Page 38)

Application data
(cont)
zone cooling thermostats are satisfied, economizer controls
arc bypassed and the outdoor dampers are modulated to
the minimum ventilation position.
Refer to Economizer Economics to determine if the
addition of an economizer is justified.
Economizer and exhaust performance
An economizer can be factory installed on the 48MA/50ME
since the damper motor and outdoor air damper are
standard equipment. The economizer package consists of a
return air damper, linkage, plug-in relays, MAT. wiring, and
mixed air thermostat.
When the 48MA/50ME unit is on full economizer control,
the supply cfm to the space drops off slightly since the
resistance of the outdoor air intake is generally greater
than that of the return air ductwork. To partially offset
this, the return air dampers have a built-in bypass.
When the outdoor air dampers are fully open and the
return air dampers /uiiy dosed, the total cfm drops 15%.
The total cfm consists of 70% outdoor air and 30% return air
thru the built-in bypass. If, for example, the unit normally
operates at 10,000 cfm supply air, the minimum supply cfm
when the economizer is operational is 8500. This 8500 cfm
consists of 6000 cfm outdoor air and 2500 cfm return air. As
the ambient temperature drops from 48 F (recommended
economizer setpoint), the proportion of outdoor air to
the supply air required to maintain mixed air temperature is
less, the outdoor air damper begins to close, and return
air damper begins to open. As this happens, total supply
cfm progressively increases from 8500 cfm to 10,000 cfm
(design).
An exhaust damper is also available for use with the
economizer. It is located between the return air plenum and
the condenser fans. The exhaust damper consists of a plug­
in relay, an exhaust damper, and a plug-in jumper. The
damper provides a forced exhaust of indoor air during the
economizer operation. The exhaust damper opens when
the return air damper is 25% closed. With the damper
installed, exhaust relay and economizer relay are energized
simultaneously. The exhaust relay locks out the outdoor
fan motor controls. Outdoor (condensing) fan motors
operate at full speed, discharging excess return air to the
atmosphere thru the open exhaust damper as shown in
Exhaust Damper Operation.
ECONOMIZER OPERATION
SEQUENCE:
1 — Ambient temperature decreases.
2 - Compressor is locked cut by ecoriomizer control tnermostat
3 — Outside air damper is regulated by mixed air thermostat
to maintain fixed mixed air temosraiure.
ECONOMIZER CONDENSING SCHEMATIC
LEGEND
C — Compressor Contactor
CCP — Capacity Control Pressurestat
CHR — Crankcase Heater Relay
CR — Cooling Relay
DLT — Dischar^ Line Thermostat
ECR — Economizer Relay
ECT — Economizer Thermostat
EXC — Exhaust Motor Contactor
EXR — Exhaust Relay
FCPS — Pan Cycling Pressurestat
HPS — High Pressure Switch
HR — Holding Relay
iT — Interna! Thermostat
LPS — Low-Pressure Switch
MCR - Master Cooling Relay
MHR — Master Heating Relay
OFC — Outdoor Pan Contactor
TM — Timer Motor
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