# Thread: supply pressure versus pressure drop

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## supply pressure versus pressure drop

Consider a water piping system where water is being supplied to condenser coil of an air conditioning unit.

If the coil is new (no fouling) , the pressure difference between the inlet and outlet of the coil can be used to calculate the flow rate through coil.

I have often talked to air conditioning equipment guys who say 25 psi pressure is too low, you need at least 35 psi of pressure.

Shouldn't the only thing that matter be the difference between inlet and outlet pressure, and not the absolute value of supply pressure at the coil inlet?

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Here's my thoughts, based on electrical theory..

As long as the restriction is known across the coil, DP can be used to determine GPM. The higher the inlet pressure, the greater the DP will be. The manufacturer may have a chart for this.

However, everything goes out the window once additional restrictions come into play.. I imagine this would include using a liquid with a density that differs from what the manufacturer expects to be running through the coil. Fouling may be the reason for higher absolute pressure at the inlet, though that may open a whole 'nother can of worms.. temp transfer from one medium to the other may suffer because of the velocity of the fluid, for example. Insulating properties of the fouling may be another.

Hope this helps.. any thoughts?

3. System pressure is usually based on rise to insure there is pressure through out the system. 2.31 ft rise per pound pressure (sea level)
Pressure differential creates flow in the system. The pumps job.

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Originally Posted by hvacker
System pressure is usually based on rise to insure there is pressure through out the system. 2.31 ft rise per pound pressure (sea level) ..
And/or enough to not cavitate at the pump inlet?

5. Originally Posted by mixsit
And/or enough to not cavitate at the pump inlet?

Very true. You have to maintain a net positive suction head NPSH. for several reasons.

6. Originally Posted by mm20252
...Shouldn't the only thing that matter be the difference between inlet and outlet pressure, and not the absolute value of supply pressure at the coil inlet?
Yes that is it. I did test and balance for 10 LONG years, and when we did water balance you really don't care what your static (standing water pressure) is in the system. As long as I connected to the coil and I got within 10% of design Dp and my temperature drop, I'm done.

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well the pressure moving the cooling medium through the condenser will control how much temp gain there will be to that medium.... if the medium flows too slowly the rise of the temp of the medium will be too great... the higher the temp of the medium flowing through the condenser gets then the closer the temp gets to the medium you are trying to cool the less the efficiency of the heat transfer.... so all that just means less capacity, efficiency, or performance. The boys with the slide rules use those long haired formulas to determine the rate of heat transfer based on a certain flow... and in the case you describe the pressure at the inlet of the condenser will determine the flow and therefore determine the ability of the coil to operate within the advertised capacities and efficiencies.

8. What's a slide rule? Just kidding.
Velocity pressure and system pressure need to be separated as just saying pressure could lead to a misunderstanding.
I'm not sure if the original post mentioning hvac guys saying the pressure needs to be at least 35# were talking about system or velocity pressure.

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