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Economizer and exhaust performance — an, Economizer and exhaust performance – Carrier 48MA User Manual

Page 14

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OA

damper

adjust

MIXED AIR THERMOSTAT

MOTOR POT

LEGEND

CHR

— Crankcase Heater Relay

ECR

— Economizer Relay

LAT

— Low Ambient Thermostat

OA

— Outside Air

Fig. 15 — Economizer Damper Control Schematic

Economizer And Exhaust Performance

— An

economizer can be readily factory installed on the
48MA/50ME since the damper motor and outdoor
air damper are standard equipment. The econo­
mizer 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 econo­

mizer 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.

With the outdoor air dampers fully open and

the return air dampers fully closed, 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 set-
point), 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 (see Fig. 13). As

this happens, total supply cfm progressively
increases from 8500 cfm to 10,000 cfm (design).

An exhaust damper option is also available for

use with the economizer. It is located between the
return air plenum and the condenser fans. The

option consists of a TPDT plug-in relay (EXR), 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, ECR and EXR are energized
simultaneously. The EXR locks out outdoor fan
motor (OFM) controls (32LT on OFMl and FCPS
on OFM2 and OFM3) and outdoor (condensing)
fan motors operate at full speed, discharging excess
return air to the atmosphere thru the open exhaust
damper (see Fig. 16).

The 48MA/50ME exhaust operation is similar

in performance to a relief damper except that the

exhaust dampers are mechanically linked to the
return air dampers and the condenser fans operate
to produce a pressure differential which aids the
exhaust cycle. At approximately 0 in. wg at the
return air opening, the 48MA/50ME units exhaust
between 150 to 200 cfm/ton. With positive return
static, more air is exhausted. At -0.40 in. wg
(.25 in. wg on the 016 unit) return air static,
exhaust capabilities of the units drop to zero.

In the example, the 4000 cfm exhausted at

0 in. return static accounts for all but 1100 cfm
outdoor air introduced by the economizer outside
air section. In practice, this excess cfm is con­
sidered a nominal ventilation rate, slightly pres­
surizing a building to eliminate drafts and

unwanted air seepage. This excess air filters out of

the building thru doors and window spaces. The

^slight positive pressurization of the building aids

the exhaust fans in removing air. If, however, the
balance between the building static and exhaust
system leaves the building with unacceptably high

positive static pressures, a relief ventilator or roof
power exhauster may be used. For extensive or

SEQUENCE:

1

— Return damper closes 25%.

2 — The exhaust damper opens

3 — The OFM (condensing fans) speed controls are bypassed and

fans run full speed, exhausting return air to atmosphere

Fig. 16 — Exhaust Damper Operation

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