Trane Steam U/V's

Micahel Peet

We have been asked to look at a problem regarding some steam Unit Ventilators installed in a school two years ago. They have had constant problems and have had many different people try to resolve why they won’t work reliably.
The problem appears to happen when the control valve(Modulating) is open to 100% for extended periods of time. It acts like the coil is stacking up with condensate. If you command the valve to 0% and then wait a few minutes then release it back to 100% the unit seems to start working. This seems to clear the coil of condensate and allows it to start heating again. This problem doesn’t usually happen unless it is really cold out and the units require 100% of heat for extended periods off time.
They have a gravity condensate return system. The previous UV’s worked well(for 20 years with pneumatic controls) so I don’t think it is a system problem as far as the central steam system or return. The new U/V’s had been installed with thermostatic traps, but have been replaced with F&T’s, to help resolve this problem. They also have vacume breakers installed on the condensate side between the coil and trap.
If you want I can fax you a copy of how the units are piped. Just let me know what fax number I should send it to.

Dan Law

Trane UV's

Can you elaborate more on the symptoms - beyond "seems like stacking up with condensate". What are the specifics? Does the unit go off on freezestat? Insufficient heating? Hammering? Also, what type controls? You mention the old being pneumatic - are the replacements digital? I'm interested in the type of control valve. Is it one with a digital actuator? If so, what brand?

clammy

i don't know

but by chance did you ever see the old valve.i had seen pnuematic valae that as the condensate stacked it would modalate the supply valae to a closed position.as the condensate cooled the element and it contraced it would bleed air out of the pneumatic line feeding the sopply valve and as steam entered the trap and closer the supply valve would open it also had a check valve on it to.i've done some steam unit heaters in the past .modine units and we usally used f and t traps with a check valve on the outlet.i believe there's a formula for sizing the check valve i'de have to lok it's been a while.check a hoffman manual it might be helpful

Micahel Peet

When the control valve opens to 100% for extended periods of time(a few hours)the coil stops heating. It appears to fill with condensate. The controls are digital, I believe they are Alerton with Belimo Modulating Globe Valves.

Tony Conner

Get A Copy Of...

..."Hook-Ups" by Spirax Sarco. It has loads of excellent information, inlcuding diagrams of installations like this one.

Bryan_3

Vacuum breaker

The vacuum breaker should be installed between the modulating valve and the coil. This is the only way to equalize the coil.

Micahel Peet

I had also thought that it belonged on the Supply side but the piping detail on the Engineered Mechanical Dwg showed it between the coil and the trap. I'll try repiping one and see what happens. Thanks for the help.

Dan Law

Belimo actuators

Michael,

I noticed your email address seems to indicate you work for a control company, so please
don't take this as an offense. We too use Belimo valve actuators on our projects,
and it wouldn't be the first time we missed the rotation setting on the
commissioning. Many of the Belimo actuators have a CW - CCW selector switch
on the actuator itself. Depending on the logic the programmer
used, the action may be reversed. I know this seems too simple, but programmers have a tendency to "commission" from
the user workstation, and not actually go to the unit and check strokes. I think it's
worth a double check. If it is wrong, a simple toggling of the CW / CCW switch
should correct things. Let me know.

Unknown

Trane U/Vs

You mentioned that the steam traps were changed to F&T traps.
Did you size traps properly?

Steam heating Coils Condense water very quickly. This occurs because of the high Delta T. If Fresh air is supplied to the coil the condensation rate increases because of a higher Delta T.

Sizing the steam trap for a coil.

BTU input divided by 240 gives you the EDR.
EDR divided by four gives you pounds of condensate per hour.
Popunds of condensate per hour multiplied by 2.5 (safety factor) gives you the pounds of hour of condnesate that the trap needs to handle.

Look in the steam trap catalog of any manufacturer and you will find the correct size steam trap needed for the coil.

Do not forget to look at the trap schedule where your operating pressure sits at.

Normally 2-3 PSI for the inlet pressure to the coil.

Another item to check out is if you have a back pressure in the return piping that may be caused by poor condensate drainage (partial blockage) or a lot of bad steam traps.

Additionally, make sure the vent opening on the condensate receiver was not reduced in size. back pressure can occur by not being able to vent the non condesibles fast enough.


Jake