check my plan

jacobsond

Im at a college. Steam coils on most AHUs. Ive been working on steam systems for a whole 2yrs. They still consider me the steam expert. Kind of sad to be considered the expert, but I will take it. This sight and Dans books have been more than helpful. Summer repairs on a little AHU. 2 steam coils only about 24x30. They kept having the distribution coils freeze. Finally gave up and let the wash-bay that it served just run on the makeup units in the ceiling. We got the go ahead on replacement coils. Things I noticed after I removed the coils. No vacuum breaker after the control valve. The common condensate line would back up through the traps of this AHU when the ceiling units were under heavy load because they have a tendency to leave the large doors open. My plan is to add a vacuum breaker. Then its easy enough to make a separate condensate run because the tank is 10 ft away. Would a check valve work also? Not sure why the condensate backs up That line runs to a Tee where the condensate from the rest of the building comes to the tank. No easy way to do anything there.

dopey27177

Not sure how the return line is piped to the condensate tank.
Is the steam trap an F&T trap. If so the condensate return line should be piped into the the condensate receiver above the water line of the condensate receiver. The vent from the condensate receiver must be open to the atmosphere. The vent allows air to be removed from the system and will break any vacuum that may b e formed.

If the condensate tanks vent is tied into a vacuum system a vacuum breaker will upset the operation of the entire operation of the heating system.

The best way to prevent a coil from freezing is to set the control to normally to be open at all times except when the temperature of the room or bays reaches the desired temperature,at that point the valve will close.

Send pictures of the piping from the coil to the condensate tank.

Jake

jacobsond

The line to the tank is approx 8ft above the tank. Tank and pump is open to atmosphere. Each coil has old dunham bush 1inch F&T. One of the problems is the 1 1/2 inch line going to the units in the ceiling in the wash bay backs up into these F&Ts. Not sure of the cause of that but its welded pipe so cant do much there. The plan is to run a separate condensate line into the tank. Do I need a vacuum breaker after the control valve or will running a different line to the tank be enough. Cannot do much with the valve control. Pneumatic valve normally open and closes with pressure. The system uses 100% fresh air so cannot really regulate the incoming air.

dopey27177

As I read your system the fan coils should never operate at 100% fresh air in the winter.
10% fresh air in the winter is more than enough especially when garage door bays are not fully closed during operations.

You speak about welded piping that may be a problem, if the run of welded piping is extensive and it is not insulated there is possibility that when there is no steam flow because the cooling pipe and condensate can cause a vacuum and draw up water from the condensate receiver if the return line to the condensate tank ties in below the water level of the tank.

Another thing you can try is to reset the float switch to empty the tank sooner. Possible that your tank is receiving more condensate at certain times than originally accounted for.

Jake

Run that separate return line. Leave out a T for the vacuum breaker if needed in he future.

jacobsond

here are a couple pictures. Please note they will not let me change any of the yellow insulated condensate line. My only option is going to run a new line to the tank. Note how close the traps are to the bottom of the coil. We run 8psi which I think is to high, but with setups like this might be needed.

Derheatmeister

Could you install a HX between the Condensate return line and the AH and glycol the AH side ?
P/S piping may be the way to go on the Condensate side..

dopey27177

Setting the steam pressure for the coils depends on the size of the coil.
Steam pressures range from 5 Psi to 10 PSIG.
Call the manufacturer about the steam pressure needed for the coils in this application.

My gut tells me it is more likely 5 PSI.

The condensate tank appears to small. Try adjusting the float to start the pump sooner.
Another try is to tie in the condensate lines into vent side of tapping and use T for the vent.
Because the tank is so small the current inlet is to low and that could leave less room for the collection of water.

Jake