Steam Coil Turn down - Vacuum pressure needed?

kiser17

I am an engineer new to steam coils in air handling units. I have a 100% outside air unit application designed to heat air from 0F to 80F using 25psig steam (~266F). This is a process air handler, so it needs to deliver 80F all year. All air passes over the coils with no means (or room) for bypass. On a 70F day the steam coil is less than 10% loaded to deliver 80F. Simple heat transfer calculations show a need for 103F "steam" inside the coil to achieve the 70 to 80F rise. However, 103F steam means it is under significant vacuum pressure (-11.3psig). Vacuum breakers would not allow this. How can this coil achieve this turn down?
Thanks, Ed

EBEBRATT-Ed

I seriously doubt that you have to go into a vacuum. I have done a ton of steam coils and have never seen a "vacuum" steam coil. If you run the coil in a vacuum the condensate will never drain. It is possible that during partial load a portion of the coil is under a vacuum.

You will need to install two control valves on the steam inlet piped in parallel. One valve sized for 1/3 capacity and the other valve for 2/3 capacity

I would need to know the lbs of steam required at the design load 0-80 deg for the coil that has been selected

Either way you probably need a pressure reducing valve on the steam line. Your going to find that 25lb steam is going to be very difficult...impossible to control without reducing the pressure.

You are also going to need someone that knows his stuff to pipe this right (traps on coil inlet and outlet and a vacuum breaker on the coil) and do the controls to prevent freeze ups.

You need a freeze stat on the outlet side of the coil. When it trips you shut down the fan close the outside air damper and open the steam valve to the coil full open

Dave in QCA

You have described a rather difficult, perhaps practically impossible situation. While on one hand, the use of a vacuum pump, throttling steam supply valve and a complex system of controls, it is theoretically possible to control the coil based on steam temperature alone. However, I would say that I have never seen a 100% ODA air handler set up this way.

The typical method of controlling such a coil is with a combination of a steam control valve and a face and bypass damper. One of the concerns in such a coil is that it is prone to freezing if controlled only by the control valve when temperatures are below freezing. So, the typical sequence of operation has the steam control valve and the face and bypass damper modulating together at ODA temperatures above 40F, indexing open or closed as necessary to maintain 80F output. When ODA temperatures drop below 40F, the control valve would go to the open position, the face and bypass damper will modulate to maintain the desired 80F output. In this setup, anytime the ODA is approaching freezing, the coil is fully charged with steam and thus protected from freezing.

Are you sure that there is not room for a bypass? Perhaps a Duramix type unit?

kiser17

EBEBRATT-Ed said:

I seriously doubt that you have to go into a vacuum. I have done a ton of steam coils and have never seen a "vacuum" steam coil. If you run the coil in a vacuum the condensate will never drain. It is possible that during partial load a portion of the coil is under a vacuum.

I am confident in the calculations. I have checked them with two different coil selection software programs, and they each confirm that we need to go roughly -11.3psig steam (~103F sat temp) to turn down the coil to 10F delta T (70F to 80F).

EBEBRATT-Ed said:

You will need to install two control valves on the steam inlet piped in parallel. One valve sized for 1/3 capacity and the other valve for 2/3 capacity

Agreed, and this is already there.

EBEBRATT-Ed said:

You need a freeze stat on the outlet side of the coil. When it trips you shut down the fan close the outside air damper and open the steam valve to the coil full open

Yes on the freeze stat, and we also are planning to install a recirculation damper to mix with the incoming outside air. This will allow us to "preheat' the air to 32F+ as necessary. We have room on the VFD to run some more air, so we will try to avoid freezing conditions as much as possible at low part loads.

kiser17

Dave in QCA said:

You have described a rather difficult, perhaps practically impossible situation. While on one hand, the use of a vacuum pump, throttling steam supply valve and a complex system of controls, it is theoretically possible to control the coil based on steam temperature alone. However, I would say that I have never seen a 100% ODA air handler set up this way.

I have come across several of these (preheat coils without face & bypass) in the field. I just have not stopped to figure out how they work at partial load. Somehow the leaving air temperature (LAT) turns down though... This what I am trying to learn, how does it reduce the LAT? With vacuum breaker air in the coil mixed with steam, or by going sub-atmospheric pressure allowing the coil to be cooler than 212F...

Dave in QCA said:

Are you sure that there is not room for a bypass? Perhaps a Duramix type unit?

I have come across F&B steam coils but have not studied them in great detail. The current steam coils fill the AHU and are sized at about 560fpm. I always assumed I needed more cross sectional area for F&B coils than a traditional coil. I will have to investigate the Duramix type and other solutions more.

Dale_3

Off the top of my head thought. Have you considered removing the control valves and putting electronic pilots on PRV's with a PID loop using discharge air temp?

kiser17

Dale_3 said:

Off the top of my head thought. Have you considered removing the control valves and putting electronic pilots on PRV's with a PID loop using discharge air temp?

I have not considered this. Can you expand on why this would be a good option?

kiser17

I should add that we are looking at a Spirax Sarco automatic pump trap (APT). The APT uses motive (25psig) steam to drive condensate from the trap if there is a coil stall due to vacuum pressure as described above. I would love to hear what people think about these devices.

Dave in QCA

OK, you're using a recirculating duct to keep the temperature of the air entering the coil above freezing - an important detail that you left off of your original description.

It seems the issue you are left with is a rather simple control of a steam coil. These typically are controlled with a modulating valve, or 2 valves staged as 1/3, 2/3 set up. There is usually not a need for special trap configurations as long as the condensate drains from the trap by gravity to a vented condensate receiver. In partial load operation, only a portion of the coil will be heated. That's pretty standard stuff.

pecmsg

rather then play with the steam install Face Bypass dampers!

Dave in QCA

As the ODA temp gets close to the 80F discharge temperature setpoint, face and bypass dampers alone will be troublesome. This is because a fully charged steam coil will give off heat to the bypass air as it washes down the backside of the coil.

This is another reason that 100% ODA steam coils are typically controlled by both steam control valve(s) and face and bypass dampers.

pecmsg

Dave in QCA said:

As the ODA temp gets close to the 80F discharge temperature setpoint, face and bypass dampers alone will be troublesome. This is because a fully charged steam coil will give off heat to the bypass air as it washes down the backside of the coil.

This is another reason that 100% ODA steam coils are typically controlled by both steam control valve(s) and face and bypass dampers.

I agree

As a engineer I would assume the OP was at least aware of other options

kiser17

Dave in QCA said:

OK, you're using a recirculating duct to keep the temperature of the air entering the coil above freezing - an important detail that you left off of your original description.

Sorry, I was not trying to leave things out. I am really just trying to understand how normal steam coils work at part loads that require the steam to be at temps less than 212F.

Dave in QCA said:

In partial load operation, only a portion of the coil will be heated. That's pretty standard stuff.

This is the part I was trying to understand. How does a coil only use a portion for heating? Is the other portion unheated because it is filled with vacuum breaker air that serves no heating value? Therefore, the vacuum breaker air also contributes to setback heating control...???

Thanks again, Ed

Dave in QCA

Sometimes visualizing things helps to understand. So, imagine that you have a coil in place, the air handler is running, and its moving 1000 cfm of air. The steam valve is closed and the air moving through the coil is 50F. Next, you crack open the steam valve, just a little bit. As the steam flows into the coil it pushes out an equal volume of air. As the steam quickly condenses it flows down the coil and out through the trap. If you feel the coil, you would find that the very top of the coil is near 212, but an inch or so down, it would be only mildly warm, and lower, it would be the temperature of the airstream.

What is in the rest of the coil where steam is not? There would be a small amount of condensate flowing down the coil tubes, and the rest would be air. It was never pushed out. Now, if you were to fully open the valve, the coil would become fully charged. You got that part. But next, if you partially close the valve again, say back to the original described condition, what would happen then? The steam would condense and since the incoming steam would not be sufficient to replace the steam that was condensing, a vacuum would be formed. This would be relieved by a backward flow or air through the trap, or through a check valve and vacuum relief loop around the trap, or through an independent vacuum breaker if one is present. Of course, the air in the coil has no thermal effect. Reducing the capacity of the coil from its full rated specifications is dependent on reducing the flow of steam.

What I alluded to in my first reply, which I think is NOT a good solution, is if the coil, valves, and controls were setup to operate in the manner of a Dunham VariVac heating system. Through a system of vacuum pumps, modulating valves, and controllers, the VariVac system produces steam/vapor at a wide range of temperatures. It is used to control the temperature is large steam heated buildings in which there are no individual room controls. Again, it is interesting to think about in theory, but in no way a practical application to your present project.

kiser17

Dave in QCA said:

Sometimes visualizing things helps to understand. So, imagine that you have a coil in place, the air handler is running, and its moving 1000 cfm of air. The steam valve is closed and the air moving through the coil is 50F. Next, you crack open the steam valve, just a little bit. As the steam flows into the coil it pushes out an equal volume of air. As the steam quickly condenses it flows down the coil and out through the trap. If you feel the coil, you would find that the very top of the coil is near 212, but an inch or so down, it would be only mildly warm, and lower, it would be the temperature of the airstream.

What is in the rest of the coil where steam is not? There would be a small amount of condensate flowing down the coil tubes, and the rest would be air. It was never pushed out. Now, if you were to fully open the valve, the coil would become fully charged. You got that part. But next, if you partially close the valve again, say back to the original described condition, what would happen then? The steam would condense and since the incoming steam would not be sufficient to replace the steam that was condensing, a vacuum would be formed. This would be relieved by a backward flow or air through the trap, or through a check valve and vacuum relief loop around the trap, or through an independent vacuum breaker if one is present. Of course, the air in the coil has no thermal effect. Reducing the capacity of the coil from its full rated specifications is dependent on reducing the flow of steam.

Thanks for the feedback Dave. I've always associated air in a coil as a bad thing coming from hot water & chilled water. But I now understand it is manageable in steam coils, as it can be pushed out by the steam when the control valve restores pressure. The air should be removed by the air vents or F&T trap down stream of the coil. I assume the copper coil will not corrode with the air/water mix, but the black iron condensate pipe between the coil and the trap may be a concern. I assume this is just part of the design, and that pipe will need to be replaced a little more often then other pipe in the system.

Dave in QCA

Yes. You got it. Condensate piping can be a point of corrosion in a steam system. If this is an industrial steam plant, they may be adding a condensate conditioner to minimize condensate pipe corrosion, or coil erosion for that matter. What is generally used is some type of volatile amine that is evaporated with the boiling water and travels with the steam. It keeps the PH of the steam and the condensate higher than might be the case when condensate in the presence of carbon dioxide can become slightly acidic. In most cases, even without treatment, the problem is negligible.

EBEBRATT-Ed

Having the recirculating air duct changes everything. Your coil conditions have changed completely, your no longer 0-80 degrees.

Since the coil already fills the air handler face and bypass is out.

As @DanHolohan says "high pressure goes to low" If you run the coil in a vacuum I don't see how your going to get the condensate out of the coil and with outside air that looks like suicide to me.

and if you have a vacuum breaker or trap bypass (it's the same thing) you will have 212 degree steam.

I agree with much of what @Dave in QCA said but you only get the Low temperature you want by burping the steam valve open and closed pressurizing the coil and then braking the vacuum to achieve the you need.

Also, I suggest non freeze steam coils with 1" tubes. Some are available with 1/2" tubes which dont drain condensate as well. Coils with tube bends don't work well with steam

Dave in QCA

I agree with everything that @EBEBRATT-Ed says above with the exception of "burping the steam valve open and closed." Open and closed operation of the coil is not going to work satisfactorily as it will result in wild swings in temperature and you probably need a continuous flow of 80F air. What is needed in this application is a proportional receiver/controller that will operate by utilizing a signal from a temperature sensor/transmitter and run an output signal to a modulating control valve. With a proper setup, valve selection, etc., the controls should be able to maintain your desired output temperature plus or minus 2F without any difficult. It will be steady and smooth control.

I worked in the world of pneumatic controls and what would have been used was a Johnson Controls T-5800 single input receiver/controller, a T-5210 temperature sensor transmitter of the appropriate temperature range, and a modulating valve sized for the pressure and steam capacity needed. Now, everything is probably DDC, but the scenario would still be the same.

EBEBRATT-Ed

@Dave in QCA

I am just thinking that if his load requires "103 deg steam" that everything is going to have to be perfectly sized and the thing looks like it will be a bear to control.

Ah, pneumatics, the good old days. Now regarded as old junk to be ripped out and replaced by every DDC control technician on the face of the earth.

It's worst problem was neglect, water not drained from air tank or auto drainer not working, belt falls off the compressor. The stuff was pretty rugged and reliable with a little bit of maintenance it was fine. I was never a pneumatics expert. I could do a receiver controller and a transmitter and calibrate stuff. Wasn't that bad if you didn't have a huge pneumatic panel without a drawing LOL

jumper

Don't like vacuum breakers. Do like correctly designed controls. This is a classic exercise in control theory. Don't need no stinking bypass or face dampers.

EBEBRATT-Ed

@jumper
no vacuum breaker on a coil that takes 100% outside air is a gauaranteed frozen coil. Face and bypass opens the steam valve wide open at 40 deg and below and controls the air temp downstream by having air bypass the coil to prevent freeze up.

They sometimes have a problem overheating though

jumper

Frozen coils are not unusual. Why doesn't safety switch off fan?

EBEBRATT-Ed said:

@jumper
no vacuum breaker on a coil that takes 100% outside air is a gauaranteed frozen coil. Face and bypass opens the steam valve wide open at 40 deg and below and controls the air temp downstream by having air bypass the coil to prevent freeze up.

They sometimes have a problem overheating though

Another question is, how effectively does frigid air dry the coil?

EBEBRATT-Ed

@jumper
What's supposed to happen (but not always done) is a freeze stat (which is an averaging sensor) is mounted on the outlet side of the coil down low (that's where the coldest air is).

If the freeze stat set at 38-40F trips it should send the steam valve wide open, shut the fan off and close any outdoor air damper to protect the coil from freezing

mikeg2015

Can’t you just use a modulating valve? That’s how they always did it at a corn refinery I worked at. Coil will only get partially heated. No need to heat the entire coil. The air will mix downstream but the sensor needs to be a good distance away. I think typically 10-15 duct diameter/widths.

I think they were using Fisher/Emerson control valves. Typically pneumatic.

A modulating valve would effectively act like a regulator so steam will attempt to fill the radiator at whatever pressure that flow rate generates and then it will only partially heat the coil and condensate will be sub cooled before reaching the trap, but not much heat there relative to the steam load.

mikeg2015

Freezing problem would be if the condensate lines have pressure on them due to failed traps. But hte vacuum breaker would prevent condensate from hanging up in the coil.

Better design might be 2 coil sections and 2 control valves. 2 or 3 stage control would be cheaper than modulating, but the temperature will swing a little and lag.

jumper

Often hear about pressure required to "blow condensate" through trap. Better to design for condensate draining thanks to gravity. That can be difficult when condensate goes to waste and coil is under vacuum. Even then there are methods besides vacuum breakers.