AHU LAT Variation 48-56 deg F 3

[Rhonda2]

With conventional (pressure dependent) control valves installed at large cooling coils, what leaving air temperature (LAT) variation should be expected relative to setpoint in operation with modern controls? Does it vary depending on the location of the coil in the system?

We are seeing numerous situations where LAT varies significantly due to differential pressure variations across control valves. This leads to variation in chilled water flow and subsequently to variation in LAT.

When LAT falls below setpoint, reheat masks the problem but causes additional ton-hours to be consumed. When the LAT rises above setpoint, fans work harder to sustain comfort conditions and humidity issues may arise. Many facilities have raised the LAT setpoint in operation because of inability to meet the original value.

I'd like to hear about some experiences in the field and what design options are being employed to address these concerns.

Thanks,

 

[lilliput1]

If coils are new or clean they should achieve design temperature differentials providing: control valve not leaking, coil is piped counterflow (direction of air flow opposite to direction of water as it goes through the rows of coil). Maybe some coils are not piped counterflow.

 

[Rhonda2]

"Should" the key word. Design delta T can be achieved when system differential pressure across the valve is constant and flow is steady, however, in most large hydronics applications, this is seldom the case. You do have a good point about counterflow.

Furthermore, design delta T at part load is less than optimum. 19 deg delta T should be achieved on a 12 deg F coil for most load conditions (80% or less). The coil sets the delta T at design flow but the control valve sets the delta T at all other load conditions.

Flow Control Industries recently provided data from multiple installations showing me how a simple control valve retrofit alone substantially raised the delta T.

 

[imok2]

Rhonda2
"As I previously stated, there is a time lag associated with repositioning a control valve when it experiences a change in system pressure".

It has been my experience as a service mechanic that Positive positioning relays on valves and pneumatic motors are very powerful and there is no lag time to speak of, like lilliput says they get full main line pressure and are very sensitive to change without hunting. in my estimation Pneumatics are the best controls for end devices I love Hybred systems