Carrier ZONE-MIZER 50ME - Manual (Page 39)

The 48MA/50ME exhaust operation is similar in per­
formance 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 previous 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 considered a nominal ventila­
tion rate, slightly pressurizing 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 un­
acceptably high positive static pressures, a relief ventilator
or roof power exhauster may be used. For extensive or
complicated return air duct systems with static pressure
greater than -0.2 in. wg at the return air plenum, duct
mounted return air exhaust fans can be installed.
Return air systems
If the ceiling plenum on a top floor is used as a return air
plenum, the return air is heated from the time it leaves the
room and enters the unit. This added plenum heat is due to
roof load or heat from lighting. The roof load does not raise
the return air temperature significantly and its effect is
negligible when selecting a unit.
Return air light troffers, however, can add considerable
heat to the return air. A 48MA/50ME unit with a return air
light troffer system can impose various design problems
since the purpose of the system is to reduce the supply
cfm to the space by reducing the space load. With the
48MA/50ME this may result in a very low supply cfm —
much lower than the unit was designed for. If the supply
cfm is raised to satisfy the unit, the purpose of the return
air light troffers is defeated. As a general rule, a return air
ECONOMIZER DAMPER CONTROL SCHEMATIC
CHR — Crankcaso theater P.slay
ECR - Economizer Relay
LAT — Low Ambient Thermostat
MUR — Master Unit Relay
PLG - Pius
EXHAUST DAMPER OPERATION
EXHAUST AIR OUTDOOR A!R BYPASS
1 - Return damper closes 25%
2 — The exhaust damper opens
3 — The Ot-Vi {condensing fans) speed controls are bypassed and
fansi'jn ■full speed, exhausting return air to atmosphere
39