Try Optimizing

There is money to be saved!! The selection of system components using optimizing techniques and abandoning the practice of using “Rules of Thumb” will often accomplish meaningful savings with increased system efficiencies.

Most of you have heard of or even practiced:

400 CFM per Ton
Temperature diffusion range should be 20 °
Specify 45 °to 55 ° water
Chilled water is better than DX
No DX below 45 degree° suction temperature
High fin density reduces the number of rows required
Coils throw water above 600 fpm

All often untrue!!!!

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.

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.

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.

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.

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.

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.

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

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