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Air Make Up unit with steam - design

Air Make Up unit with steam - design

Air Make Up unit with steam - design

I have a requirement of an outdoor Air Make Up unit. Air capacity: 11,000 cfm. Location Toronto: Ontario. Room is does not have human unoccupancy (Indoor Temp: 65 Farenheit). I am exhausting all the air with negative pressure in this mechanical room. Medium pressure steam (50-100 psi) is available at this floor. Therefore its availability dictates the use of a steam coil in the unit.

I have a few questions in this regards:

Can I place the unit outside where it can be as cold as -30 degree centigrades?

How much steam pressure is good enough and how do I size my unit/coil?

RE: Air Make Up unit with steam - design

Coils like this tend to freeze in the spring/fall, rather than in the dead of winter. The available supply pressure of the steam is not really an issue as far as freezing the coil at very low outdoor temperatures is concerned. The temperature control valve on the steam supply will throttle-in as the air temperature approaches it's set-point. Even though this is not a pressure reducing valve, the effect is the same. If you have any backpressure at all in the condensate system, you  lose the pressure differential across the trap, and condensate backs up into the steam coil. It can freeze and split the coil in only a few minutes. You also require a vacuum breaker on the steam side. When the TCV closes, the steam in the coil collapses into a vacuum. Now now condensate can't drain, even by gravity.

The main thing is to drain the coil of condensate under all operating conditions, especially low load, when the outside air is near the freezing point. During very cold weather, the steam valve tends to be open to meet the demand, so there's always pressure to keep the condensate flowing. The coil needs to have a gravity drain to a vented receiver for the condensate, and a vacuum breaker.

Spirax Sarco's main office in Canada is in Toronto. I'm sure that someone there would be happy to get you a copy of "Hook-Ups", which has (among many other things) specific information on installations exactly like yours. A coil  manufacturer would be happy to make a recommendendation about the actual coil itself.

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