Chilled water - Air handler

[butelja]

What are the relative plusses and minuses of these 2 control methods within an air hanlder, considering both temperature and humidity control?

a.) Constant airflow through chilled water coil and varying chilled water flow.

or

b.) Coil bypass dampers and constant chilled water flow.

 

[ChasBean1]

butelja, I believe that discharge temperature and humidity can be accurately controlled using either method, assuming a well-tuned BAS (with well-placed sensing devices) operates either the damper or valve, but I prefer Option A. The reason is that if you had a plant full of air handling units that vary bypass air flow versus chilled water flow, you lose on the opportunity to save in chilled water pumping costs. During low-load seasons, for example, a central plant serving a system full of Option B AHUs would still pump full flow even though it may require only 30% of the cooling capacity.

If you think back to thermo basics (Q = m * Cp * dT), the Q, or energy required from the load, will be constant [lower water flow-(m), higher dT, higher flow, lower dT, therefore constant Q] but the difference remains in the pumping losses at the motor (minor), mechanical pump losses(medium), and system friction (major).

I have no concrete evidence, but I think that excess energy losses from Option B would be the deciding factor.

Best of luck, -CB

 

[quark]

Agrees with CB! Energy is a prime factor in decision making. I have some personal reservations regarding air bypass damper system. (That system put me in trouble even with dehumidifiers) Some controls we used were of Landis&Stefa, Honeywell and Sekonic.

Regards,

 

[ChasBean1]

I agree, Quark, and add the following to your point -

Minor damper modulation could cause major swings in air volume changes in either direction, through or around the coil. The standard air volume control is basically by butterfly valve (i.e., damper), where chilled water flow technology is much more refined with different types of valves, such as globe/needle valves with excellent throttling characteristics. A 5% change in position command from a BAS for an air damper might mean a 40% change in air flow, whereas throttling linearity with a typical coil control valve is much better.

In addition, chilled water flow changes are much less likely to produce the type of immediate and drastic changes in outlet conditions resulting from air volume modulation. With CHW flow control versus air volume control, you are basically "steering the ship" with a control parameter that does not dramatically change the ship's course with minor steering wheel movement.

(If that makes sense!). -CB