DX VAV CONTROL
DX VAV CONTROL
(OP)
I have a VAV air-handler with a DX cooling coil (two-circuits, intertwined) with a two-stage condensing unit (operates on its own pressure controls). The unit also has hot gas bypass (mechanical type) to allow compressor operation at low loads. Due to 'cold blow' complaints when the boxes are at minimum flow and the thermostat is satisfied, we added discharge air reset to the control strategy (based on OA air temp.: 60deg OA, DAT=50; @40deg OA, DAT=60). However, the compressor cycles off more often (with 5 min. time delay) resulting in poor DAT control and more complaints. I think this happens because the DAT is at a higher value, say 60, and the hot gas bypass valve doesn't have a chance to operate before the DAT differential shuts the compressor down. If you have any experience with this, I would sure appreciate some help!!!





RE: DX VAV CONTROL
RE: DX VAV CONTROL
The main problem, now, is related to capacity as you mentioned. At low cooling loads the DAT is reset to as high as 60 degrees, however, the hot gas bypass valve will not function until the evaporator temp. is approx. 45-50 degrees. Then the compressor is commanded off by the temp. controls way before the hot gas valve starts opening up to maintain a load. When the compressorgoes off it has a 5 min. time delay to protect itself from short-cycling and control of DAT is lost. Then we have overheating problems. Additionally, this AHU does not have economizer since it was a retrofit job in the interior of a large building and we didn't have access for a large OA duct (economizer would solve all of my problems!!!).
Anyway, I'm thinking about an electronic Hot Gas valve that would energize whenever it was needed to maintain the current DAT setpoint.
Thanks, for any further ideas you might have.
RE: DX VAV CONTROL
Consider readjusting the direction of throw (perhaps away from occcupants) in order to improve the situation.
RE: DX VAV CONTROL
RE: DX VAV CONTROL
RE: DX VAV CONTROL
If I understood:
- You have a DX coil unit,
- which throws air at too low temperatures into the space,
- most of times, the diffusion air system (is not able!) doesn't have to carry on with all bad/ or less-good solutions like those of a thermal production nature,
- so you are forced to add more complex solutions, as VAV systems, or semi-hermetic compressors and its rather difficult way to deal with parcial loads..., and so on, as you mention above.
So "The Question" is: - What to do with these "irrational" kind of units, the DX coil units, of the ON/OFF type, which the market likes so much, sells so much, because the most of customers does not know really ( I mean at all!!!), what they are buying, in a perspective of an efficient equipment??
An attempt to answer: Why not to consider a higher range of evaporation temperatures for thermodynamic gas, (stop using the range [44 ~ 47ºF]) and adopt : 53 ~ 56 ºF. I mean just to trim the respective pressostat already exists in the unit (don't have to add anything else). Of course, you have to pay (just!) a small bill, which is the lesser output cooling power you get, or you have to admit more air flow rate for the same cooling power, you need.
But nevertheless one important advantage: You have much lesser problems concern with the lack of moisture in the air. You know all what I mean at the end of the day in your office....
May I ask for comments,...
Kind regards.
zzzo
RE: DX VAV CONTROL
RE: DX VAV CONTROL
Two main problems I see here: 1) This unit receives a key operating input (OA temperature) to establish operating control, but OA temp has no direct effect on the unit's service requirement if outside air is not provided to the unit. 2) The VAV action has to be examined as to minimum terminal box settings to determine if some sort of air volume bypass is required. Significant airflow reduction across a VAV coil, such as from VAV boxes having low minimum setpoints (e.g., 1000/200 cfm, or 400/150 cfm, etc.) can tend to freeze up a coil or cause the problems you mention. Changing these minimum settings to 1000/750 or 400/300 might help to resolve this.
Also, if I'm correct in assuming this is a full-recirculating unit (no outside air), outside air temperature or enthalpy needs to be removed from the control equation, and return air enthalpy needs to be sensed to properly control the unit.
I hope I'm not off-base with these suggestions. I get paid for making these suggestions, but usually have the facts straight beforehand! Again, feel free to re-post.
Thanks, -CB
RE: DX VAV CONTROL
WE have seen where Dx has been problematic many times before with or without VAV operation.
Symptom
With regards to temp control for DX ( cold blast ),
the reset control is a good step, but I would not key it to OSA as this is an energy waste. The prblem with DX units especially at light load is they are either on or off and they "overshoot". Your VAV box mimimum will be influenced by this unessecary cooling as will your reheat energy.
Recommnedation
You will get better control of the Dx discharge by placing the temperature control in the return air. Call your local control expert about set up if you have questions.
RE: DX VAV CONTROL