Heating coil contrlols problem

[remp]

I have a heating coil in an air handling unit that is struggling to maintain room set point without over heating the room. If the control valve is only slightly open say 10% the top of the coil is hot and the bottom is cold. Only when the control valve gets to about 40% open does the entire coil heat up and then it is all of a sudden roasting...... it is giving hugh problems...The poor BMS is struggling to maintain the set ponit and the BMS company is blaming the coil... What you think?? Looks like the modulating valve could be over sized too but this has nothing to do with the coil issue mentioned surely.

 

[ChrisConley]

Coil and control valve sound oversized. With better control on the valve you might be able to gain control of the space. Possible to consider a Pressure Independant valve?

Possible to reduce loop temperature in the system? Lower MWT would have the effect of reducing the 'size' of the heating coil.

 

[remp]

Chris

I agree with your comments thanks, but why is the top of the coil hot and the bottom cold when low flow thro the coil? should it not be even thro out..>?

 

[Zesti]

If it is a lot of outside air that is going over the coil then it might not be oversized. Depending on your climate the heat needed might vary a lot between summer and winter.

You could make a circuit with an extra pump running 100% flow over the coil to have even warming under all circumstances, even if the control valve is at 10%. The control valve will regulate the flow going to the coil loop.

 

[walkes]

Check how the coil is currently piped, is the hottest water entering at the top of the coil closest to the entering air stream? There could be some air entrainment in the coil affecting flow/heat transfer.
It would likely work best if the hottest water entered low on the discharge side of the coil (counter flow).

The pumped coil will work as well assuming the coil is piped and vented properly.

 

[imok2]

Are you sure you have an equal percentage valve which compensates for the characteristics of a hot water application to provide a control that is close to linear. If you have then look to your controls for proper response and settings.

 

[BM4BRANES]

Walkes is on the right track about your coil being mispiped. Counterflow, bottom to top is what you would likely see under normal circumstances.

 

[willard3]

How is the coil circuited?

 

[Zesti]

Be cautious with respect to the RH requirement.

I mean, are these beans "wet" when they come in and will they loose moisture which you have to remove in order to maintain 65% or do the beans absorb moisture which you have to add in the process ?!?

Be sure the building envelope can handle the 65% indoor-RH.
If (points off) the inner walls/construction get below dewpoint condensation will occur.
(Depends of course on what climate you're in)

 

[Zesti]

Oops: ignore my post above, it ended up in the wrong topic...

 

[TBP]

Do we know whether this is a hot water, or steam coil? Did I miss it?

 

[ChasBean1]

It's steam. Has to be if it's hot on top and cold on bottom - pretty classic for these (unless it's HW piped wrong). Probably sized for 100% OA and way too much muscle for part load heating. Are there face and bypass dampers? If so you may be able to monkey with the clamshell positions when at part load. Or use cooler steam... <-(joke that one was)

 

[remp]

Hi Its actually LPHW at 80 degC and it is piped corectly but the unit is only running at about 30% capacity, it was away over sized even at 100% i'd say.... and yes looks like the heating coil has way too much muscle. To sort out the problem I turned down the flow rate to really really low flow, barley cracking the balancing valve and tested to ensure it could still do mid winter conditions whcih it can....not ideal but problem sorted...thanks for all the help....

 

[ChasBean1]

Oh ok - good luck. Usually however high temp and lower flow makes stratification worse. If at all possible, you'd be better off minimizing temperature and maximizing flow (if the system permits).

 

[operatingeng]

Is it one control valve per coil? Maybe a 1/3 2/3 setup would work better. If you had to turn the circut setter down that much the 1/3 valve would most likely do the day to day work with the 2/3 valve there for the extra capacity if it gets really cold.