Steam coil on 100% outdoor air

Solid_Fuel_Man

I've installed the controls on a operating room make up air unit. This is 100% outdoor air which goes through a steam coil, and a DX coil. Variable speed blower is controlled to maintain positive room pressure. 

This unit is pretty much always calling for cooling, therefore a discharge temp of 50-55 is usually what is called for. This is all done via electronic controls (BMS). 

This is a new install I did 2 months ago. Trouble is, the freeze stat keeps tripping now that we have below freezing outdoor temps. Tue steam valve is generally at 15-25% for proper discharge temps. Unit is sized for -30F outdoor temp I'm told. 

Would it be best to put the freeze stat up high on the coil? I'd think the would be the hottest. However the bottom part of the coil is where condensate would be that could freeze. Currently I put the capillary for the freeze stat (as per the print) covering the whole coil. Its 15 feet long, coil is roughly 36" x 36". 

SlamDunk

Sounds to me, barring a faulty freeze stat, that the response time for steam is slow. Temps dip suddenly in the 5-7am range before steam valve can open, and you have BMS process time. When the BMS is calling for zero percent steam valve opening, is the valve actually rigged to zero percent? We keep our steam coils hot by keeping the steam valves cracked open at zero percent.

clammy

Was it tube in tube non freeze coil ? When I install freeze stats I also spread the cap tube for best over all cover being it really sensing air temp across not so much coil temp . I don’t know if favoring the top of the coil would see any higher temp . What temp is the freeze stat at and is it located up or down steam side of the coil . Have you tested the freeze stat to make sure it’s not out of calibration ? I would think the supply temperature control should be able to maintain the supply air temp and cycle the steam valve to maintain the indoor air parameters temp / humidity but dx coils are not as easy as chilled water when it comes down to it . Is the bms up to the job of cycling the dx and steam coil to meet set point possible a issue and the need for a seperate temp controller that the bms enables ,just a thought . It s been a long time since I ve worked in hospitals and most all of them where chilled water and three way valves and of course steam . Good luck clammy

pecmsg

What is the lag time between calling for steam and actually getting it?

With a 55°F discharge air that coil should have been hot well before the freeze stat tripped. Is this unit being fired up after a long off cycle?

EBEBRATT-Ed

The steam coil should have a face and bypass damper. Especially with a 100% OA unit. With face and bypass below 38 deg? you open the steam valve 100% to avoid freezing the coil and control the temperature of the discharge air off the steam coil to maintain 55 degrees by modulating the amount of air going through the coil or around the coil bypassing it.

If the engineer didn't spec. face and bypass he screwed up.

The freeze stat has to be low on the coil and the way the freeze stat capilary is strung across the coil makes a difference.

EBEBRATT-Ed

In this day and age using BMS controls some think the controls are capable of overcoming any design flaw in the equipment and they can't

pecmsg

EBEBRATT-Ed said:

In this day and age using BMS controls some think the controls are capable of overcoming any design flaw in the equipment and they can't

Controls were Never able to fix bad design,

SuperJ

EBEBRATT-Ed said:

In this day and age using BMS controls some think the controls are capable of overcoming any design flaw in the equipment and they can't

As a BMS programmer I whole heartedly agree. In the last 20 years, I've seen a major decline in the robustness of the mechanical designs, and a lot of outright ignorance when it comes to accommodating to equipments constraints. The controls guy and the engineer both look bad when things don't work.

Solid_Fuel_Man

As you guessed, I'm the controls guy on the job. I had a long talk with the engineer on this job. I told him the freeze stat is set to 35F per his design. It is covering the entire coil as is standard practice. 

This is the first time working with him, and I said this is the first unit I've ever seen that didnt have a face and bypass damper. Every unit I've ever worked on steam or water we usually use the valve to control output until about 35 OAT or so. Then 100% valve open, And face/bypass from there on out. 

He admitted that this was his issue and that they were going to figure something out. This is all custom ordered equipment made specifically for this job. It was all supposed to happen last fall and got pushed into winter then COVID happened..... so a few balls have been dropped in that long of a timeline. 

You know what we did in the mean time..... the freeze stat is no longer part of the control logic. 

EBEBRATT-Ed

Unfortunately a frozen coil is likely.

I am not a fan of face and bypass. Sometimes with the valve open and the damper shut the coil radiates enough heat to screw things up

But face and bypass is what it needs

Solid_Fuel_Man

pecmsg said:

What is the lag time between calling for steam and actually getting it? With a 55°F discharge air that coil should have been hot well before the freeze stat tripped. Is this unit being fired up after a long off cycle?


Im not sure what the schedule is for this. I believe it is controlled at a constant 67 degrees in the OR and there is not occ/unocc schedule. 

pecmsg

My last facility was all 100% OA. Only newer units had face bypass set up. Something is delaying the call for steam or steam getting to the coil.

That Freeze stat when tripped should Open the steam valve 100% and shut down the fan, is that the sequence?

Has the freeze stat trip point been checked and verified?

If chillers are in play you may have to run a higher preheat temperature and more chilled water to maintain discharge temperature.

Honeywell Gray Manual is a great reference for this project

Solid_Fuel_Man

No chilled water. Just DX on a pump down. 

Freeze stat shuts down fan and opens valve 100%. 

I have not verified stat actual temperature. If I open up the unit to get to the stat, then airflow stops and things change temps drastically. I wish I could see the coil from inside the AHU then i could be in there with my IR gun while it's actually in operation. 

pecmsg

Schedule a shut down
Get a large bucket of ice and water. 

Set the control on a low temperature setting 
Immerse the sensing bulb in ice bath. Only need a inch or 2 wait 1 Minuit slowly raise trip point until it trips. 1/2 turn warmer is generally close enough.   

SuperJ

Thinking out loud....

Can you use a little feed forward biasing (based on OAT and fan status,) on your valve to raise the minimum on the valve or bias the PID loop upwards, or maybe start with a high discharge temperature setpoint (say 75f, that slowly ramps down to 55f over 20minutes).

Is your temperature sensor downstream of the DX and fan? Perhaps install a temp sensor on the face of the DX coil, this will remove the thermal mass of the DX coil from the control loop, allowing you to be more aggressive with the steam valve control.
I've had big AHU's in hospitals that take forever for the discharge temperature to reflect a change in valve position due to the thermal mass of sometimes multiple water/glycol filled coils. (preheat/heat wheel/reheat/cooling/etc).

We have some systems that use a time delay relay on the freeze stat, so the freeze stat can flicker a bit and not trip. Once it actually trips it needs a manual reset though.

Valve sizing can be a big problem too, sometimes you have use a 1/3, 2/3 valve arrangement to get good control, otherwise it behaves like a light switch.

SuperJ

pecmsg said:

Schedule a shut down
Get a large bucket of ice and water. 

Set the control on a low temperature setting 
Immerse the sensing bulb in ice bath. Only need a inch or 2 wait 1 Minuit slowly raise trip point until it trips. 1/2 turn warmer is generally close enough.   

Good idea to calibrate the freezestat.

EBEBRATT-Ed

@SuperJ

Couple of other things to check.

Make sure the coil has the correct steam trap should be sized to handle the steam load on the coil at a 1/2psi differential maximum plus a safety factor (I would use a safety of x2)

Keep in mind that a coil close to freezing being hit by steam will condense much more than it's rating.

Make sure the condensate line has little (or preferebly no) back pressure and that the coil is "free draining".

A vacuum breaker on the coil between the control valve and the steam trap is a must.

Any condensate hanging in the coil .....if when the steam valve opens and if they are relying on steam pressure to push the condensate out this could be delaying heat and possibly causing the FS to trip

Is this a one row non freeze steam coil ? (I hope)

Not a return bend type coil

If it is a non freeze steam coil it should have 1" tube the coils with the smaller 1/2" tubes doesn't drain as well

pecmsg

SuperJ said:

pecmsg said:

Schedule a shut down
Get a large bucket of ice and water. 

Set the control on a low temperature setting 
Immerse the sensing bulb in ice bath. Only need a inch or 2 wait 1 Minuit slowly raise trip point until it trips. 1/2 turn warmer is generally close enough.   

Good idea to calibrate the freezestat.

Without it your guessing!

Solid_Fuel_Man

I didnt do any of the design or install of this. I was just there for controls on this job. 

I'll get more information next time I'm on site. At this point, and we all know the likelihood of the results, the freeze stat is out of the control logic. 

I'm just hoping I dont have to go back on the roof with a snow shovel.....

D.ROSCHELLI

We have several steam coils at our facility and some good comments were made. The best control we have with a steam coil is IFB integral face and bypass. Keeping even heating over the coil is important and the steam valve should be fully open after the temp drops near freezing. Most coil manufactures want a min. pressure in the coil and have manuals that cover this issue. With a bypass coil that is over or under the steam coil it does cause tripping of the freeze stat and you could freeze the chill water coil if you had that instead of DX. Stratification of the air stream is very common on this type of unit too.

EBEBRATT-Ed

@Solid_Fuel_Man

LOL I would be more worried about an ice pick!!

Jackmartin

The engineer should get a kick in the back portion of his upper legs. The freeze stay you have is a capillary sensing type. In the event even one inch is exposed to the trip pressure it will go into fail. These controls have to be positioned correctly they cannot be exposed to untreated air. The coil is not heating rapidly enough the response from a steam coil installed correctly is almost instant. Check the CV on the supply valve if it is marginal this will happen. The trap you are using could be wrong check the trap against the manufacturers specifications
 In the event all else fails get the software to give you a five minute grace period for initial warm up this will allow the coil to get up to temperature controlbefore the freezestat comes into play. We are in a part of Canada that regularly seesinud forty if you did not have a lag for the freezestat nothing would work here. Make sure the capillary is not close to the edges of the coil this will cause it to trip. Stay Well and Be Blessed Jack

pecmsg

Jackmartin said:

The engineer should get a kick in the back portion of his upper legs. The freeze stay you have is a capillary sensing type. In the event even one inch is exposed to the trip pressure it will go into fail. These controls have to be positioned correctly they cannot be exposed to untreated air. The coil is not heating rapidly enough the response from a steam coil installed correctly is almost instant. Check the CV on the supply valve if it is marginal this will happen. The trap you are using could be wrong check the trap against the manufacturers specifications
 In the event all else fails get the software to give you a five minute grace period for initial warm up this will allow the coil to get up to temperature controlbefore the freezestat comes into play. We are in a part of Canada that regularly seesinud forty if you did not have a lag for the freezestat nothing would work here. Make sure the capillary is not close to the edges of the coil this will cause it to trip. Stay Well and Be Blessed Jack

AARON units had the control mounted in there un - heated control box. 

Brilliant!