I thought I had this whole temperature difference figured out but my formula is still giving me an inccorect answer. Hopefully it is something easy that I have been overlooking since I have been staring at it so long.
I am trying to calculate the gpm of a coil using a formula that contains CFM, entering and leaving air temperatures and entering and leaving water temperatures. Here is the formula that it should be calculating:
GPM = (CFM * 1.085 * Delta Tair) / (500 * Deta Twater)
Here is how I have it written in Revit using my shared parameters:
GPM = (CFM * 1.085 * (LV. AIR TEMP. - ENT. AIR TEMP. + 0 °F)) / (500 * (ENT. H2O TEMP. - LV. H2O TEMP. + 0 °F))
I used the +0°F to offset the K problem, but the formula is still giving me the wrong answer.
For example, with CFM = 300, LV. AIR TEMP.=85, ENT. AIR TEMP.=55, ENT. H2O TMEP.=140 AND LV. H2O TEMP.=120 the GPM should be 0.98 gpm but revit is giving me 4.97.
Is it something I am missing or can revit not calculate the gpm correctly?
Any help would be great!
Can you report the details on the Shared Parameters or post an image of the dialog box with the formula? Attached is the image of my test. I removed the "+ 0 °F" for 'GPM 1' because that made the results incorrect. I made both of the test parameters Numbers, so they are unitless.
I have attached a screen shot that shows my shared parameters and my formula. One question about yours, why do you divide by 1 cfm at the end? When I tried this, it automatically changes the 1.085 to 1cfm. Also, I wonder if the problem is that I don't have GPM as unitless, I have it set to gpm as the units.
Ok, when I set up a unitless paramater and entered the formula like you did it works. Why won't this work when the units are set to gpm? It seems weird that it needs to be unitless.
Oh well. Thanks a ton for the help! At least know I have numbers that I can work with. I can try to find a work around to get the units added back so I can map it to the coil connections.
Thanks again!
It should work without having to be unitless - I was just doing it fast and didn't go to Project Units to set the tolerance to two decimal places. My question is how did the "+ 0° F" change the equation? Well, if it works, it works. Revit exists through work-arounds...
Sorry to keep harping on this and I really appreciate your help, but I can't figure out why this won't work. The formula works great if the GPM parameter is set to unitless, but if I have the GPM parameter set up as a Pipe Flow type, I get very strange answers out of the formula.
I though I could come up with a work around by having a parameter of GPM unitless and then have a second parameter of GPM with the correct units. The correct unit GPM would equal (GPM unitless * 1gpm). This gives me the correct answer in the family, but once the family is in my project, the correct unit GPM won't update as the CFM parameter is changed. The correct unit GPM always shows 0.
Has anyone found a way around this?
Thanks again in advance!