VIFB Steam coils and temperature overshoot

[NEplantengineer]

Hi folks,

I'm looking for other's experience's with integral-vertical face and bypass steam preheat coils with respect to temperature overshoot when in 100% bypass mode.

I've got a new VIFB steam preheat coil in a brand new 100% outside air custom unit. When the outside air (OA) is above 40F, no problem, modulate the steam valve to get the 50F discharge air setpoint. Below 40F the dampers are modulated to get the 50F discharge temp. Problem is at 35F outside air temp, the dampers are in full bypass but the discharge air is 51F. This means that the air stream is picking up 16F with the dampers fully closed! There doesn't seem to be anything wrong with the steam valve or the dampers.

What have other folks seen for temp rise when the F&B dampers are in full bypass?

 

[ChasBean1]

Is this a return air system? A 16°F rise is quite high. The F&B dampers might be functional, but do the 'clamshells' compress together well during full bypass?

If it's a 100% outdoor air system, just initiate F&B mode at 35°F and it shouldn't be a problem becuase you would have to provide that heating from the coil anyway. If it's a return system where the return air should be providing the heating, you maybe should dig deeper into the F&B operation. I would say a 3-7°F rise might be normal, not a 16°F rise... -CB

 

[NEplantengineer]

CB- thanks for the input. Yes this is a 100% OA unit. And yes I can just change the cross over(F&B dampers to valve) point to 35 F. It just bothers me (and others) that this temperature "override" (this is what the manufacturer's call it) is 2 or 3 times what they publish in their literature.

I'm going to have the installation contractor do a "tightness test" on the dampers today.

-NEPE

 

[ChasBean1]

Good idea. Let us know the outcome. I'd also look at design sizing, meaning the design vs. actual airflow through the coil. If the coil design is for 600 fpm and you find it's only 300 fpm, higher residence time would mean higher heat gain (although magnitude? I'm not sure)...

 

[UtilityLouie]

I have also seen the vertical blades warping a bit and thus preventing closure.

I have been finding out that - no matter what - working with 30-35 F outside air (100%), there is no perfect solution. VIFB does work well for freeze protection, but I haven't had a lot of luck with temperature control with steam valve 100% open and dampers controlling outlet temp.

If you are using higher pressure steam (>15 psig) maybe try setting your valve open percentage something lower than 100%. You will be preventing modulation, the big issue with coil freezing, but the coil temp will go down a little bit.

I've tried this with some success.

I know this is remote, but any possibility of a steam leak?

 

[lilliput1]

Did you lower the discharge temperature setting. It may be controlling for 50°F +/- 2°F. Is the coil oversized? Is the steam pressure too high? Consider adding a PRV upstream of the coil. Usually minimum operating pressure is 5 psig. Verify this is adequate to operate the trap.

 

[cme]

Why are you modulating the valve at 40 deg and above? The valve should be fully open and the dampers modulate.

Those coils work well for a couple years then performance tapers off.

I'm doing a 20,000 cfm system with one now.