6 replies to this topic

[CH3CH2OH]

Hello,

 

I'm currently working to specify a flow regulator to provide steam to an air handler for humidification.

 

The steam is going to be provided from a boiler at 10-12 psig.  Maximum steam requirements to the air handler will be 500 lb/hr, and the boiler is capable of producing over 700 lb/hr steam.

 

I am working with a very experienced vendor in specifying a flow regulator, and while I trust his recommendation for a regulator, I would like to complete the sizing myself to confirm.

 

For the Spence K5, 1-1/4" regulator, I have data on Percent Travel vs. C_V.

 

I am looking at two equations from the Spence manual:

• Subcritical flow:   W = 2.1*C_V*SQRT((P₁-P₂)*(P₁+P₂))  , when P₂ > 0.58P₁
• Critical flow:  W = 1.71*C_V*P₁  , when P₂ ≤ 0.58P₁

I know my target steam rate (0-500 lb/hr), I know my inlet pressure (10-12 psig), and C_V values.

 

I don't know how to decide my outlet pressure. 

 

The flow regulator will discharge into essentially atmospheric pressure (~0 psig).  My instinct is to use the critical pressure equation since my discharge pressure is less than [0.58*10 psig = 5.8 psig] critical pressure, however, this provide critical flow and the theoretical maximum flow out of the regulator.  If I use something like 80% of the inlet pressure [0.8*10 psig = 8 psig], I suddenly have an undersized valve even for the 100% open situation.

 

Could anyone provide any guidance on this?

 

[Bobby Strain]

Check your formula for the proper units for pressure.

 

Bobby

[latexman]

It's whatever steam pressure is required to push and condense 500 lb/hr of steam in the air handler and get rid of the condensate.  Look at the air handler spec.

 

Like Bobby, are you ABSOLUTE-ly sure on those pressure units?

Edited by latexman, 24 October 2017 - 04:55 PM.

[CH3CH2OH]

Thank you to you both.

 

So pressure ratio needs to be calculated at absolute pressure, then critical pressure would be:

• 0.58*(10+14.7) = 14.3 psia = -0.4 psig -- for 10 psig steam inlet
• 0.58*(12+14.7) = 15.5 psia = 0.8 psig -- for 12 psig steam inlet

Then my discharge pressure is going to be whatever pressure it takes for 500 lb/hr steam to run the rest of the way from the control valve, to the actual air stream that is being humidified in the humidifier.

 

This would be a short run of 1.5" sch. 80 CS piping, then a steam distributor (a series of stainless steel tubes with small holes in them).  Fingers crossed that I can get flow / pressure drop data on this steam distributor though.

[breizh]

http://www.spiraxsar...-tutorials.aspx

 

You may find useful hints using the link above .

 

Breizh

[Pan Nata]

Hi ethanol,

 

It's not about your instinct. Because you are talking about flow here then it is must be related to critical flow. I'm not sure with your given equation, perhaps its better share to us original source of that equation.

[CH3CH2OH]

Hello Pan,

 

The equations are from the regulator manufacturer Spence's engineering manual.

 

The flow will be sub-critical if I assume 10 psig (normal boiler operating pressure) inlet pressure to the regulator.  The flow will be critical if I assume 12 psig (maximum pressure of the boiler) inlet pressure to the boiler.  Either way, it does not make a very big difference in terms of flow if I assume critical, or non-critical flow since I am very close to critical flow in either case.