Carrier 48MA - Manual (Page 15)

m
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 for proper airflow. However, return air
exhaust fans add to the operating cost and increase
noise level. More efficient duct design methods
should be investigated to eliminate the need for
special return air exhaust fans.
Economizer Economics — Economizer control on a
multizone unit does not necessarily reduce
operating cost as it would on a single zone unit. A
single zone unit either heats or cools; a multizone
unit can do both simultaneously. Therefore, in a
multizone, the economizer operates to maintain a
mixed air temperature low enough to cool a zone
with a high internal load. The remaining zones
requiring less cooling or heating must have heat
added to offset cooling capacity available but not
needed. This is true of any multizone with any
type of control system.
The amount of heat required to neutralize the
overcooling capacity is dependent on:
1. The percent cooling capacity required from the
unit, and
2. The mixed air temperature required to satisfy
the zone with the highest internal load.
As the ambient temperature drops, the percent
of outdoor air needed to maintain a mixed air
temperature is less. Since the reheat or wasted heat
added is a function of the difference between
outdoor air introduced and ventilation rate,
operating cost is reduced at lower ambients. A high
ventilation rate also reduces the reheat requirement
and associated cost. The following example illus­
trates the need for a careful analysis of job
requirements before arbitrarily selecting on econo­
mizer control.
Example:
A 48MA/50ME unit is operating with econo­
mizer control and supplying 10,000 cfm of 55 F
mixed air. The normal ventilation rate is 2000 cfm.
Assuming a realistic cooling load of 50%, 5000 cfm
of the 55 F air is used for cooling. Since the
ventilation rate is 2000 cfm, half is sent to the
cooling zones leaving 4000 cfm of low-cost cool­
ing. The remaining 5000 cfm of 55 F air, including
1000 cfm of ventilation air, is going to zones with
either no load or a heating load and must be
neutralized.
Although 4000 cfm or low-cost cooling is ob­
tained, an extra 4000 cfm of air must be heated to
some degree above and beyond that in a unit
without economizer controls.
For an identical unit without economizer con­
trol, only 4000 cfm of the 5000 cfm needed for
cooling requires mechanical cooling, since the
1000 cfm of ventilation air is already cooled. Of
the other 5000 cfm, 4000 cfm is return air and is
neutral, and 1000 cfm is ventilation air to be
heated. In the final analysis, it must be determined
if it is more economical to heat 4000 cfm from
55 F to 75 F, or to cool it from 75 F to 55 F. The
answer depends on the efficiency of the cooling
and heating source.
An example of economizer economics is illus­
trated in Fig. 17. The graph plots percent cooling
load versus relative energy cost (electricity to gas)
and is based on the following typical assumptions:
48MA028 — 10,000 cfm, 15% outdoor air
48 F outdoor changeover temperature
75 F room design
55 F supply air temperature
Compressor changeover point (COP.)
of 3.3 (100 F condensing temperature
and unloaded compressor were used
to obtain this value)
The relative cost figures are in $/Btu input for
gas and $/kwh electric cost converted to Btu.
Example:
$.10/100,000 Btu (input) - gas cost
$.015/kwh - electric cost
Convert electric cost-
$.015/kwh X kwh/3413 Btu x | q 5 qqq
= $.44/100,000 Btu
Cost Ratio:
$.44/100,000 Btu _ , ,
.10/100,000 Btu
Therefore, if cooling load is less than 45%
(from graph) at the changeover temperature, the
economizer is uneconomical for 48MA units.
For 50ME electric heat units, the cooling load
break-even point is 70%; the internal load must be
greater than 70% to justify economizer control.
Fig. 17 — 48MA/50ME Economizer
Break-Even Point
15