Steam Pipe Sizing for AHU

[onlinetips]

Need to install an AHU in a mech room for 40000 cfm
Unit is 100% OA
Entering : 15 degF
Leaving: 55 degF
(40 deg temp diff.)
1760 MBH = 1833 lbs/hr
steam pipe size: 4” at .5 loss per 100 lf and 6000 fpm
cond. Return pipe: 1-1/2” at .25 loss per 100 lf and 3000 fpm

1)Is this ok?

we have 60 psi (medium pressure) steam available
existing equipment in the room uses a pressure reducing station to get 10 psi steam.
We could install a new station to get 5, 10 or 15 psi steam

2) What is the difference in using these three different pressure ratings?

The length of pipe from the 60 psi steam pipe to the pressure reducing station is about 30 ft. (this would be 60 psi also) The length of pipe from the reducing station to the ahu is 120 feet. If the coil is sized at 15 psi and the length of pipe from the pressure reducing station to the ahu is 120 feet then at .5 in psi drop per 100 ft, the 15 psi steam would be down to 14.6 psi.

3)Can this pressure be increased?

4)Do we use pumps on a steam pipe to increase pressure?
Or do we increase the steam pressure to 15.6 psi at the pr. Red. Station and it will be 15 psi by the time it reaches the ahu?

5)Can the size of pipe be reduced to increase pressure at the coil? Or use another device to reduce the cross section area and increase pressure back to 15 psi?

6)Do we need to install steam traps on the 120 lf length of pipe?

7)What happens at part loads when some of the steam is not used by the coil?

8)Is there a bypass for the steam if it goes thru the coil and all of it does not change to condensate return or is that controlled by the control valve and the trap? (what happens to the unused steam)?

9)Does either of the pipes (steam or CR) need to be sloped?

10)Is it ok to use just one coil instead of two (preheat and heat). The 55 degree discharge air is serving an un-occupied space, which we need to keep above 50 degree F.?

11)What is the advantage of using face and bypass coil control (air-side)?

12) Do we need to consider warm-up and operating loads for pipe sizing above?(steam)

13) How do we find out the back pressure of condensate return? Is it measured in GPM(liquid) or also in lbs/hr

 

[TBP]

If you have a 60 PSIG steam supply, you can likely run a 3" line to the PRV station to feed the unit. Check the capacity of the existing PRV station. Most steam PRVs are seriously oversized. You can quite possibly tie into the downstream side of the existing valve.

Check the rating of the coil you're planning on installing. It's probably rated for 15 PSIG steam. If it is, then that's where the safety valve must lift, and you won't be able to run at that pressure. Operating pressure will likely be 10 - 12 PSIG in that case. One advantage to having the safety valve lift at 15 is that it gets you out from under ASME code requirements.

Steam at lower pressures has more latent heat per pound. The price paid for using low pressure steam in any give application, is that piping, control valves, coils, heat exchangers, etc must be quite a bit larger to handle the lower pressure, since each pound of steam has expanded with the pressure drop.

A 4" line operating at 10 PSIG will handle the flow you need to this coil. A pretty good rule of thumb is that the condensate line is half the size of the steam supply.

After the steam leaves the boiler, you can't increase the pressure. You can knock it down at PRV stations, but unless you buy a steam compessor (rarely required/justified, and big $$$$), pressure falls throughout the system.

There are two ways of controlling the air temp off the coil. You can use face & bypass dampers to move more or less air over the heating coils, or have the air flow over the coils at all times, and control the steam by means of a temperature control valve. Condensate removal from the coil is critical, especially in applications involving heating of outside air, and a control valve on the steam supply. The condensate needs to drain by gravity to a vented receiver. You MUST have a vacuum breaker on the steam coil. These coils will tend to freeze in spring & fall, rather than the dead of winter. At low loads, just below freezing, the steam valve is throttled-in, because not much heat is required. While this is a TCV, you get a pressure drop across it, just like a PRV. If there's any back-pressure at all, such as a slightly pressurized condensate system, or a rise in the return lines, condensate will back up into the coil. I can tell you from direct experience that this will freeze very quickly once it backs-up into the coil exposed to cold air, and it will split the coil.

Slope the steam line to the TCV, and install a trap just ahead of it. This will keep the line clear of condensate at low loads when the TCV cycles shut periodically.

There won't be any excess steam to deal with. A PRV station controls on pressure. It doesn't look at flow. It will pass enough steam to maintain whatever it's set point is, then throttle closed. The TCV will be the same. It just looks at air temp downstream of the coil, and throttles to maintain that temp. Be aware that there will be some slight fluctuations in pressure after the PRV, and air temp off the coil. If you set the PRV for 10 PSIG, for example, you'll likely see it move slowly between 9 and 11.

The steam that gets to the coil will be held back by the steam trap until it gives up it's latent heat and condenses.

You can't have ANY backpressure on units like this that are heating outside air. For every 500 lbs of steam condensed, you have 1 USGPM of condensate to handle.

There are some very good publications available that explain all of this. Most manufacturers like Spirax Sarco, Spence, Armstrong, Hoffman/Bell & Gossett, etc have excellent literature available. I like Spirax Sarco's "Hook-Ups", and carry a copy with me. They have a publication aimed at people without much steam experience called "Steam Utilization".