topic Understanding pressure loss in the System Inspector for a Hydronic System in Revit MEP Forum

Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Thu, 15 Jun 2017 21:55:17 GMT

Hi All,

I'm working on an example of trying to model a chilled water system. I basically have the default fan coil units with all the piping properly connected. I was able to generate a pressure loss report, and can see the critical path and all that good stuff. I'm at the point where I'm trying to interpret the results and make sense of it. I haven't found any good material on understanding the information presented by the system inspector tool. Can anyone explain the following:

1. In the attached image (this is just one system - chilled water supply), I've selected a fan coil unit and it gives me information on the pressure. You'll notice that only a certain section of piping is highlighted/boxed. Is the total pressure displayed, the pressure at the unit from the start of the highlighted run or up to the beginning of flow for the entire project (not shown)? It appears to be pretty small, so I would assume it is just for the highlighted run. If that is the case, is there a way to determine what the pressure is at the unit from the start of the chilled water supply, or would I have to manually add up all the pressures along that run?

2 a. I know this is a dumb question, but what is the difference between the Static Pressure and Pressure Loss?

b. Not really sure what Excess Pressure signifies. Can someone elaborate on that?

3. The fan coils have a 13 psi drop that I've manually input. Does the pressure analysis include that? It does not appear to be the case.

I would appreciate any help on this, thanks in advance!

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Scott_D_ — Sat, 17 Jun 2017 00:06:58 GMT

At a guess:

Static pressure is the total system pressure the pump needs to be sized for.

Pressure loss is for that specific section of pipe.

Total pressure is likely to be the combined pressure loss up to that point from the pump.

Excess pressure may be the total pressure the pump can deliver minus the amount in your system.

I'm an Electrical Engineer so take that with a pinch of salt.

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Mon, 19 Jun 2017 16:13:59 GMT

Just to be more specific :

Total pressure is the sum of dynamic pressure (1/2*rho*velocity²), static pressure that you can see in the inspector and gravitational potential energy (rho*g*z).

Regards,

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Mon, 19 Jun 2017 20:52:18 GMT

Thanks for the answers!

Is the total pressure then what the chilled water pump has to overcome?? (2.31 psi for this fan coil in this example)

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Tue, 20 Jun 2017 07:52:48 GMT

Hi @Anonymous

Here is a example of an operating point of a fan coil. You can overcome a certain amount of pressure drop with a specific flow. You have to choose a fan coil which can overcome your pressure drop and provide the flow you need.

Calculations are up to you 😉

Regards,

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Thu, 22 Jun 2017 21:24:44 GMT

This is what i'm having trouble understanding. If you look at the image below, the system inspector is telling me that the total pressure on the fan coil along critical path is 13 psi (top image). Yet on my longest run, the total pressure is even greater at 19.68 psi (bottom image)! How can that be? Why is the 13 psi the critical path? How are these values calculated?

My goal is to find the fan coil with the highest pressure drop in my system.

Also, I'm using Revit 2014 in case anyone is wondering...

Re: Understanding pressure loss in the System Inspector for a Hydronic System

Anonymous — Fri, 23 Jun 2017 06:46:08 GMT

I don't know why but you have no pressure drop in your second pipe.

That's why Revit is telling you that the other pipe is the critical path : From the fan to each end, Revit calculate the pressure drop throughout the paths. Thus the critical path is the path with the higher pressure drop.

I suggest you to verify your second pipe cause it should generate pressure drop. (If you have created your own pipe, check that it has rugosity for instance)

Regards,