DX VAV CONTROL

[demar]

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!!!

 

[ChasBean1]

Demar, can I assume there's no reheat in the system? On that assumption, I think the temperature reset curve is too steep. Does this mean at 70°F outside the system will try to discharge 45°F air, or does the setpoint always maintain between 50 and 60? Many designs have a 50°F discharge temperature and I've found that most of these systems tend to piss people off, especially if there's no reheat. VAVs close down because the space is satisfied, and diffusers lose their throw characteristics and "dump" cold air on people. Are you sure your problem is at 60° discharge and not at lower temps? The cold supply will cause your people problems. If the system can't handle a higher setpoint it might be oversized. Do the compressors stage properly? Does the economizer operate properly? Have you tried raising the outside air compressor enable setpoint? More questions than answers, I know... but I hope some of this helps.

 

[demar]

Thanks ChasBean1 for your reply. I need to clarify a few things: 1)There is reheat at the VAV boxes; 2) the discharge air is maintained between 50 and 60 depending on OAT; 3) the cold blow problem was mininmized when we added the DAT reset.

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.

 

[php]

I agree with ChasBean1 that Air distribution within the space is of great importance.
Consider readjusting the direction of throw (perhaps away from occcupants) in order to improve the situation.

 

demar, what kinda of condensor are you dealing with? hermatic or semi hermatic? if semi hermatic why aren't you unloading with pressure actuated unloaders. whether they be mechanical or electric. this would help with the problem of over cooling by varying the capacity based on low load conditions. instead of cycling the compressor on temperature. If your hot gas bypass is working properly and your compressor is unloading. your problem should settle down a bit. With out an economizer vav and dx are a nightmare combination...

 

[ChasBean1]

demar, re: new activity on this post - I went through a period where I didn't log on for a while! My apologies. Probably by now, you've solved your problem. Anyway, this sounds like an issue somewhere between the manufacturer's control settings and the size of the unit versus the area served. I've noticed that many manufacturers' controls people with the level of knowledge that would be required to solve a problem of this ilk are generally too busy - and if you've finally get them, they're too expensive! If this is still alive and kicking in the realm of problems that you have to deal with, please re-post with knowledge gained... I'd be interested. Again, sorry for not replying sooner. -CB

 

[235zzzo]

Hello Forum

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

 

[demar]

Thanks everyone for your helpful suggestions. DX VAV without economizer certainly is a nightmare, and something I'll not want to repeat again. I've learned more about the DX controls lately that should help the situation out. The condensing unit is two-stage (2 compressors) with hot gas bypass. The controls contractor has the system set up as follows: The setpoint is 54 degrees with a one degree differential; when the first stage is activated the discharge air drops to near 54 degrees and the hot gas bypass valve opens. The discharge air stabilizes just above 54 degrees most of the time, but with higher loads will rise to 56 or higher. When that happens the controls bring on the second stage which quickly drops the air temp. below 53 degrees which shuts down the whole condensing unit and takes it off line for 5 min. with the short-cycle time delay. The discharge air temp then rises to 66 degrees before the condensing unit comes back on line and the whole process repeats. I 'm thinking about widening the differential control band considerably (50 degrees unit is off; 54 degrees first stage; 58 degrees second stage) to have more reliable operation. If anyone has experience with this, I'd be happy to listen!

 

[ChasBean1]

Demar, I've been too busy thinking about the trees in your problem to see the forest. You mention you couldn't fit an OA duct to feed this unit to allow economizer operation - that means it's a 100% recirculating unit (no outside air), right? I originally thought you meant 100% OA unit due to the OA temp reset. Anyway, assuming this is a full recirc unit, the VAV flow could present a problem during swing season operation; many VAV terminal devices will be calling for reduced flow, but outside air conditions can cause the unit to enable when it's not necessary.

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

 

Demar, Youve described many problems and it is a wonder now what is really going on particularly with the Dx / hot gas operation.
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.

 

[Chillermec]

I am assuming we are talking reciprocating compressors here. Seems the first problem is probably the 1 degree "differential" setting. If there are no unloaders on the compressors at lighter loads you are probably beating the compressors to death on starts, not to mention loss of supply temp control. If there are no unloaders on the compressors determine what the capability is for adding them. I would max out on these. This depends on the # of compressor heads. You will also need to know if the supply temp controller has the stage outputs available. Spread the control band out further ( I would suggest a starting point of 5 degrees if it is split across setpoint ). This ultimately will be determined by the unloading capability you end up with. The more unloading, the smaller the delta-T effect will be of a stage change, and, a smaller control band can be used. The hot gas isn't really doing anything for you ( and shouldn't come into play anyway ).