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". |
Steam Pipe Sizing for AHU