Carrier ZONE-MIZER 50ME - Manual (Page 40)

temperature rise of 5 to 10 F does not cause a problem, and
special ratings can be made available.
In addition, when the supply cfm is reduced, as above,
the outdoor air quantity remains constant. This results in
a higher than normal percentage of outdoor air which the
48MA/50ME unit may not be capable of introducing For
return air light troffer systems, exercise care when using the
light manufacturer’s data concerning the amount of heat
actually returned to the unit because with the higher
return air temperature, a portion of the heat is trans­
mitted back to the space thru the ceiling.
Application data
(cont)
As an example of economizer economics, the Econo­
mizer Break-Even Point 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 E outdoor changeover temperature
75 E room design
55 E supply air temperature
Compressor C.O P. of 3.3 (100 F condensing
temperature and unloaded compressor were
used to obtain this value)
Economizer economics
Economizer control on a multizone unit does not neces­
sarily 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 con­
trol system.
The amount of heat required to neutralize the over­
cooling 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 out­
door 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 venti­
lation rate, operating cost is reduced at lower ambients. A
high ventilation rate also reduces the reheat requirement
and associated cost. The following example illustrates the
need for a careful analysis of job requirements before
arbitrarily selecting economizer control
Example.
A 48MA/50ME unit is operating with economizer 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 cooling The
remaining 5000 cfm of 55 F air, including 1000 cfm of ventila­
tion air, is going to zones with either no load or a heating
load and must be neutralized.
Although 4000 cfm of low-cost cooling is obtained, an
extra 4000 cfm of air must be heated to some degree above
that of a unit without economizer controls
For an identical unit without economizer control, only
4000 cfm of the 5000 cfm needed for cooling requires me­
chanical 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 eco­
nomical 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.
The relative cost figures are in $/Btu input for gas and $/kwh
electric cost converted to Btu
Example:
$.25/100,000 Btu (input) — gas cost
$.031/kwh — electric cost
Convert electric cost:
100.000 Btu
100.000 Btu
$.031/kwh x kwh/3413Btu x
= $.91/100,000 Btu
Cost Ratio.
$.91/100,000 Btu _
.25/100,000 Btu
For 48MA units, the cooling load must be greater than
48% to justify use of economizer.
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.
The cooling load for this comparison is the internal load
(lights and people) minus the negative transmission at the
changeover temperature (48 F). To determine the percent
cooling load, compare this value to the unit design cooling
capacity
48MA/50ME ECONOMIZER BREAK-EVEN POINT
40