Research Facility overcooling then reheating

JoeMass

I am looking at ways to save energy in a research facility. They have one 150hp supply fan (on vfd) that blows 50 degree air throughout (outside air is tempered with chilled water coils) which is then reheated at VAV boxes with hot water coils in each zone. They use campus steam and campus chilled water.



How can I improve the system?



It appears to me that the number 1 thing that they can do is to increase the supply air temp stream from 50 to 60. What are your thoughts?

Jean-David Beyer

I am not a HVAC professional, but...

... I had a big air conditioner for a computer room that got 40F chilled water from the building and ran it through a large water-to-air heat exchanger like a car radiator, but much bigger. There was a proportional thermostat and a proportional valve controlling the water flow. This unit also controlled the relative humidity. There was an open pot with three cal-rod units and a toilet tank valve to keep it full of water. When the humidity was too low, they boiled the water. When it was too high, they turned on lots of reheat which indirectly ran lots more chilled water through the heat exchanger, condensing the humidity out of the air and it ran down a drain.



The reason for "overcooling" is to get the relative humidity down.

Brad White

Some questions

Is the unit a mixed air or 100% outside air unit?

If 100% OA then you can re-set your discharge air temperature (DAT), in summer provided you closely monitor your outdoor dew point. In humid weather, you must cool it down as JDB noted. In winter, if 100% OA, heating it up to 60 may not make sense.



Now, if the unit is mixed air (return air and OA in some proportion), you will likely have an economizer mode, favoring OA over RA when the conditions are within a certain range. Enthalpy control is ideal.



If you are using VAV and it is in MA, our energy code stipulates that the reheat cannot come on until the terminal boxes are set at their minimums. Only then can you add reheat. Exceptions are made for processes of course- you cannot turn down air supply to an area with fume hoods based on cooling demand, for example.



There are MANY ways to go about this. In-unit energy recovery, wrap-around heat pipe coils, DDC controls. Lots of things we can talk about.