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daviddsims
25-06-2012, 02:36 AM
I am looking for some advice on one of our large AHU units at my plant. It is a 35k cfm unit with return louvers, exhaust, and outside air dampers with a chilled water coil first and then a reheat coil. The chilled water circuit is 44 degrees and the reheat circuit is 195 hot water circuit we get from recovered steam. The chilled water is controlled by a valve positioner with an adjustable setpoint and the hot coil is controlled by return air temp. This is a high humidity room that we try to cool with equipment such as dryers, fryers, and lots of moisture on the floor. I currently have to reheat the return air up to 80 degrees so that the concrete ceiling does not sweat if I output air in the 40's. Any advice on controlling a high latent heat room and providing the best IAQ that you can?

Tesla
25-06-2012, 09:22 AM
hi David
Firstly the outside air (economy dampers) should be controlled on enthalpy difference at either 100% or minimum fresh air. The final supply air temp (SAT) setpoint should match the load requirements as far as enthalpy is concerned. The chilled water supply temp is important in reducing the humidity, this needs to be calculated by what your coil can do in the real world ie... supply 100% RH at x temp how much moisture will be removed at what temp. Is there opportunity to vary the speed of the fan for given conditions or is the load constant? The reheat is simply to increase the final temp to a lower humidity. For IAQ what are the requirements for people & product - does the product matter? Check the ASHRAE 65 comfort chart ( I think that's what it's called) it gives a wide range of acceptable space conditions for us humans for a given outside temp hum value. If it were possible to exhaust from the source the fatty or steamy air - this could save the max on costs & provide excellent close conditions. What type of control is used on this system? Humidity is not easy to measure as sensors are not very accurate and tend to drift with time - generally need calibrating every 6 months. Summing up
1 outside damper, exhaust air damper & RA return air damper is controlled by enthalpy difference between outside air and return/exhaust air damper.
2 chilled water is set to remove moisture valve position wise but the chilled water temp also needs to match load. Where at max heat and moisture of load may require chilled water at lowest say 6degC, then a lower load may only require 12degC chilled water.
3 The reheat temp only needs to match the load to increase the SAT to required set point.
Give us a few more details and advise if they are measured or rated.
A sensor measuring the ceiling temp and humidity could be use for the control system to eliminate condensate.