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400 CFM per
Ton -
A properly plotted psychrometric chart will prove that the 400
CFM per ton assumption is only valid when a certain ratio of internal
sensible heat load to internal latent heat load exists. Processes
that cause moisture to dissipate into the conditioned space result
in lower ratios and the CFM per ton is reduced. In addition, higher
outdoor air quantities increase the total tonnage while at the
same time reduce the required CFM per total ton.
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Temperature
diffusion range should be 20 °
-
The diffusion range (temperature difference between the room and
the air temperature delivered to the room) may be increased and
the total CFM required reduced when the air temperature leaving
the cooling coil is a close to saturation as possible. The depth
of the coil in the direction of air flow determines this approach
to saturation.
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Specify 45
° to 55 ° water
-
The chilling of water to 45 ° requires more HP per ton than
does the chilling of water to say 50 °. The water flow quantity
and costs are reduced. Total optimization might reduce the compressor
size by as much as one model size. Additional savings are realized
when circulating pump, pipe sizes and insulation are reduced and
lesser power requirements are present.
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Chilled water
is better than DX -
The use of chilled water systems in lieu of direct expansion systems
is commonly misunderstood. When a common refrigerant supply must
serve multiple evaporators, the control and distribution complexities
justify the selection of chilled water (or glycol). When only
one or two evaporators must be served, the use of a direct expansion
system is often justified. The increased efficiency of using only
one heat exchange instead of two, the reduced compressor HP requirements
and the elimination of circulating pumps can be quite advantageous.
The historic claim that DX cannot be easily controlled or modulated
has long since been overcome and eliminated as a concern.
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No DX below
45 degree° suction temperature
-
The use of evaporator temperature regulation has eliminated the
concern for freezing of condensate on coil tubes and between coil
fins. The optimized use of coil depth and fin density allows for
safe use of leaving air temperatures as low as 35° F.
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High
fin density reduces the number of rows required
-
Only in the cases where dehumidification does not occur and condensate
does not gather between the fins should high fin densities be
used. When the space between fins becomes narrow enough for drops
of water to touch an adjacent fin surface, the added capillary
attraction increases the quantity of water being held up in the
coil. This increased water quantity reduces space for air flow
and increases the net air velocity between fins. In addition,
it reduces the ability of the coil to transfer heat in those areas
that are blocked by water. It becomes apparent that water will
be "thrown" into the air stream at lower velocities
while static resistance to air flow is increased.
Our tests at the National Bureau of Standards indicated that when
the air flow was stopped, twice as much water drained from a 12
fin per inch coil than did from an 8 fin per inch coil after both
had been operating under similar psychrometric conditions.
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Coils throw
water above 600 fpm -
It is false thinking to believe that there is a common air face
velocity above which dehumidifying coils will throw water from
their face. More critical is the fin density and the face height
above the drain pan. A coil with a high face dimension allows
more water to run down its fins than does a coil with a lesser
face height. The more water that collects between fins means that
the water tends to be pushed from the coil face at lower velocities.
The shorter the face height and the longer the finned length,
the higher the face velocity that can be tolerated. This explains
the need for intermediate drain pans.
Engineering
Bulletin -Volume 2, Issue #4
by:
Kenneth W. Wicks -ASHRAE Fellow
10-07-02
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400 CFM per Ton