Theory: Delta T and Flow

Unknown

As long as no other variables change.

Mark_46

Fuzzy Math

Can somebody confirm my assumptions?...

When GPM flow increases, Delta T decreases.
When GPM flow decreases, Delta T increases

Brad White

Warm and Fuzzy Math

Yes.

Empire_2

That's a Good Bingo.

Check out the B&G Web site. They have an awesome System syzer that will give you ALL the answers. If you have a local B&G Dist. in town, they should be able to give to the slide SS calculator....

Hey Brad:? I have to tell you that THAT is the Shortest ANSWER i HAVE EVER SEEN YOU GIVE...:-)

Mike T.

Mark_46

Thanks gents

Brad White

Then you never saw me answer

no.

Weezbo

*~/:)

100%

Glenn Sossin_2

but they do change

thats why they call them variables

Unknown

Correct

That's why the answer to this post should be no. It can but not always.

scott markle_2

delta and transfer

For me the interesting and somwhat confusing thing about this relationship is the effect on transfer.

Delta is indicative of transfer yet a low delta high flow can transfer more energy because the average surface temp of the emmitters are higher

Seems to me that conventional wisdom was to overpump and maintain low deltas. thus avioding derating calculations for long loops or boiler condensation issues.

A more progressive aproach is to embrace high delta, as we can move energy with less pump power, Although there are design chalenges that come with this

Unknown

You're not confued

That's the way things work in the real world.

Mark Eatherton

Which would move more energy...

1 GPM at a 100 degree delta T, OR 100 GPM at a 1 degree delta T...

I agree with Scott. The correct answer is, it depends.

If you are talking a single fluid stream then yes, your assumptions are fairly safe. If you are talking two fluid streams, then probably not.

It just depends....

ME

bob_46

BTU

Q=W*C*T1-T2 the temperature doesn't seem to have anything to do with it.

Dave_12

Flow vs Delta T

Mark:

Yes, flow is directly related to delta T. Firing rate, flow, and delta T are related as follows:

BTU/HR = GPM x 500 (constant) x Delta T

(The 500 constant is from 60 sec/min x 8.33 lb/gal)

If you know any two, you can compute the third. If firing rate is the same, then you can see that if GPM goes down, then delta T goes up in proportion.

This is a fundamental formula that anyone designing hydronic systems should commit to memory. It is extremely useful.

Hope this helps.

Rocky_3

Using formula for BTU/Hr

If the constant of 500 is derived from the density of water at, what, 60 degree, 65 degrees? Then if you change the density of the fluid, ie add glycol or change the temperature from 60-65, then the constant also changes, yes? So if I'm on a job, and have a handy reference chart of densities of various glycol solutions at different temperatures, then I should be able to calculate the exact heat transfer rated for any fluid I am trying to pump. Or, is there so little difference that I'm better off just sticking to the rule of thumb?

thanks,
Rocky

bob_46

Rocky

Go to B&G's site and click on Syzer. You can plug in all kinds of variables and play "what if".

Mark_46

Thanks!

Dave,

Yes that formula helps in more ways than one. Thanks.

Separate from why I started this thread, I have been curious to find GPM for both my zones at a given pump speed. That formula seems to do that if you know 2 of three variables, as you said, rather than calculating head feet by pipe length and components? Whats confusing to me is when finding head feet a GPM must be assumed. Im trying to FIND GPM!

jp_2

pump curve

if you know the head in feet use the graph of the pumps head vs GPM curve

ALH_4

System curve and pump curve

You have to plot your system curve on the pump curve to find the operating point. To do that, assume several gpm numbers and calculate the head loss and plot those points on the curve for your pump. Draw the curve through those points and read the head a gpm from the intersection.

scott markle_2

head

Mark, I think your misunderstanding something, as far as I know there is no way to calculate gpm without an assessment of the pipe work, if knowing this to a high degree of accuracy is important a flow meter may be easer than adding up all the fittings and footage's. The pump and the pipes determine the flow. Btu's have no effect on flow. What the equation Gpm= btu/500xdelta tells you is the gpm required to move a given btu at a given delta. 20 is a convenient delt to work with because the math is easy 15000 btu = 1.5 gpm etc.

Mark_46

One more

One more question.

When a component is specified at, eg, 6 head feet that means 6 head feet not 6 feet of equivalant pipe length, correct?

[Deleted User]

Korrect...

feet of head is a pressure term. It, one foot of head, is equal to .433 PSI. So in your case, 6 feet of head equals 2.6 PSI differential pressure, or pressure drop.

ME