Steam Line Sizing

[psugreen]

When sizing a steam line to an AHU coil, should the branch lines going to the ahu coil be the same size as the coil connection size or is it ok to use a smaller size? When using the size charts it looks like a smaller size would be acceptable but I think I remember reading somewhere that individual branch lines should be the same size as the coil connection.

 

[katmar]

I doubt whether there is a uniform method used by the manufacturers of AHU coils to size their connections, so it is difficult to make a blanket statement on whether you should make your branch connection the same size as the coil connection. If the sizing charts show a smaller size I would go with that.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ione]

Agree with Katmar: do not blindly rely on the coil connection size.
Perform a pressure drop calculation along your steam pipeline and evaluate the most suitable pipe diameter in order to ensure your AHU coil is fed with the required steam flow rate at the required pressure.

 

[willard3]

It's usually a mistake to size piping to any coil to the inlet/outlet size of the coil.

Size for pressure drop and increase/decrease to coil piping size.

 

[Zesti]

With steamlines it's usually the flowspeed of the steam that is the limiting factor, not pressure drop (IMHO).

 

[ione]

Zesti,

Pressure drop is related to velocity. I agree there are recommended velocities that one shouldn’t exceed because of erosion issues (25 m/s for saturated steam and up to 70-80 m/s for superheated steam). Anyway the coil is rated for a certain pressure of steam, that is for a set temperature, and not for a set velocity.

 

[Zesti]

Ione,

I guess we are both thinking exactly the same thing.

Properties of the flow change at every point along the pipe: hands up anyone who is solving the integral for any old steamline going to the odd steamcoil instead of just getting out a simple table based on maximum flowspeed and kg/s going through the pipe?

 

[TBP]

Do your own calculations to figure out what size steam line needs to be run. Do not use the "size of the hole" method - it almost always leads to gross oversizing. Coil & heat exchanger manufacturers will standardize as much as they can on their units. The connection size on your coil is likely sized large enough that you can use low-temp hot water. If you size a temp control valve on the steam line off the coil connection size, it's almost guaranteed to be way too large, and will "hunt". The higher the steam pressure you run, the smaller your line gets. Size control valves separately. They'll typically be one size smaller than the correctly sized steam line.

Short lines can be sized on velocity. Long runs, like several hundred feet, normally need to be sized on pressure drop.