AHU'S COIL PRESSURE DROP

[naifmbo]

hi every body,
ahu with the following previous data at 46 c outdoor:
chilled water in : 7.0 c
out: 12.0 c
pressure drop : 7 psi
booster pump
indoor 22-23 c
NOW after adding :
1 3-way globevalve
1 balancing valve
stop booster pump
increasing the water flow rate (may be 1.5)
we have :
chilled water in : 7.0 c
out: 9.5 c
pressure drop : 15 psi
but!!!!! indoor 25-26 c (at the same load)
why?
is it true :
1. increasing flow however will reduce supply air temp.
2. lower in-out means lower air temp.

 

[imok2]

If you assume a constant load; then increasing the waterflow through a coil will decrease the delta "T" of the water and increase the air temperatue off the coil just as your results show. So the answers are no and no. Why? well the answer is that the water has less time to pick up heat because is going faster through the coil and this effect means that the air will come off the coil warmer. If you slow down the GPM and keep the same load profile the air off should be colder the formula you are looking at is
BTUH = GPM x (Tin-Tout)x 500 you can see if you increase the gpm the btuh will increase...BUT that assumes that delta "T" remains constant; in your case it goes down like said because of the reason above.

 

[naifmbo]

but i think the air will phase coil with lower temperature wall(higher convection heat transfer due to low coil temp).
really,it is not clear to me that increasing flow will increase air temp!

 

[imok2]

The longer air or water are in contact with each other the closer they become to equilibrum, so if you want colder air supply you need to slow the velocity of water or the velocity of air that will increase the contact time of the elements to each other.

 

[quark]

One thing not clear is that how can you increase flow after stopping booster pump and installation of flow restriction devices?

Actually increased flow increases turbulence and so overall heat transfer coefficient.

In my opinion your flow rates might have reduced.

 

[naifmbo]

because the main pump was suppling water for another big side(120 m3/hr)which we canceled and concentrated the flow for the other small side (45 m3/hr),so the flow will be higher even with those restriction devices.in addition to that if we have lower flow we will have a higher temperature diff.

 

[lilliput1]

You have to measure db & wb of the air entering & leaving the coil to make sense of the heat balance. Heat absorbed by the water should equal total heat (latent + sensible) given off by the air.

 

[quark]

Where are you checking the return temperature? Is it after 3 way valve by accident? If so, your actual return temperature will be offset by the increased flow from bypass.

Regards,

 

[naifmbo]

the bypass is closed by butterfly valve!
note: we have 12-15 psi water pressure drop, is it normal?

 

[imok2]

Correction on my first post; ans to questions are yes and yes
sorry bout that!

 

[imok2]

In load bypass applications such as radiators, coils, and air conditioning units, the pressure drop should be 50 to 70 percent of the minimum difference between the supply and return main pressure at design operating conditions and a manual balancing valve may be installed in the bypass to equalize the load drop and the bypass drop.

 

[naifmbo]

realy,i forgot: we changed(due to space) water flow direction from counterflow to parallelflow.

 

[lilliput1]

You should newer use parallel flow. You will never get your leaving air temperature low enough to approach the temperature of the entering water. You need to pipe it back to counterflow.

 

[imok2]

All H-X that I'm aware of should be piped counterflow

 

[quark]

That is the reason why you are getting high air temperature. Keeping other variables constant, LMTD in parallel flow is lower than that of counter flow (around 30%). This reduces your coil capacity by the same amount.

Regards,