heating coil capacity

[317069]

Hello all
I have a question:
I am traying to select an AHU ( hot water for heating, DX for cooling) for an application.
For the cooling side th unit is ok
but, for the heating side the capacity of the coil with the nominal water flow mentioned in the catalog is much higher than my requirment.
example: my requirment is 130000 BTU/hr
equipment coil has capacity of 200000 BTU/hr. at 20GPM flow rate
The catalog didn't give the BTU output at a different flow rate.
What would happen(beside the 3 way control valve will cycle on/off too much.)
if I go with 20GPM and size the pipes with keeping the unit fan in [On] mode all the time?
any suggestions
Thank you

 

[Drazen]

If your design load is 13000, than the actual load can be much lower and you can have troubles with regulation abilities at lower load.

I don't see selecting AHU based on nominal capacity only as a viable solution, you need to either use manufacturer software for simluation of several load situation, or ask their techinical service to do that (if they don't offer adequate selection software).

The other solution is to precisely design three-way valve, but you need regulation curves for both, heating coil and the valve to create proportional regulation system characteristic.

Otherwise, it can end with "flooding" at low loads (too much flow or too much transimition that cannot be controlled). There is one solution for that, tow use two paralell regulation lines with valves with different Kvs for full and low loads, but it is really not possible to describe it in detail within the post, and it is for use in very special cases anyhow.

 

[MintJulep]

Order a unit with a smaller, properly sized, coil.

 

[ChrisConley]

With a larger coil you could use a higher delta T through the coil, which if you are using condensing boilers would be a plus.

 

[imok2]

If you use a 20*F TD you would need 13 GPM flow or if you want a 25*F TD you would need a 10 GPM flow with 130,000 BTU

 

[walkes]

A pumped coil could help at the low load times as well by maintaining good water flow through the coil. The three-way valve will modulate as needed to inject hot water.