Calculating flow through (heating water) coil given only pressure drop and coil measurements 1

[Mark_B]

I have a Titus DESV-06 with a single-row heating water coil and would like to be able to calculate flow rate in real time using information supplied by two Dwyer pressure transducers (one at inlet, one at outlet). I was not able to locate a manufacturer table. I will attempt to get the information from Titus, and remain hopeful someone here can supply it, but in the mean time would like to learn about the calculation.

I was reading through a thread here where I came across a specific method of calculation posted by user 25362:

"Professor Lydersen (Fluid flow and heat transfer-Wiley) states that you may use the usual friction drop formula with its [±]10-15% accuracy. He says that the friction factors can be obtained from Srinivasan, Nandapurkar and Holland: Friction factors for coils, Trans. Instn. Chem. Engrs., 48, T156-T161 (1970)."

water flow through a coil - Pipelines, Piping and Fluid Mechanics engineering

Hi all, I'm in a plant were circuit setters on air handlers were not always used (hand valves instead throttled down). THe problem I have , aside from balancing, I can't tell how much flow is running through a coil. I can contact the coil manufacturer and give him the Delta P and he can run...

www.eng-tips.com

This seems to be for a different type of coil, and not to apply to my Titus VAV coil, which I believe is single inlet but not helical. Is there a method of mathematical calculation for mine? Or do I need to cough up $500 for a flow meter, or fill a bunch of marked buckets at different rates and take notes?

 

[Latexman]

The best way, of course, is to get real flow data on your installation (coil, pipe, and fittings - entire system). Then, provide a calculation methodology or a Look-up table or something.

Crane TP410 has a calculation method for coils too. Coils come in all kinds of shapes and sizes. You'll have to look at TP410 and see if it's applicable to your coils.

 

[btrueblood]

Or surf over to the manufacturer's website and look for data. Like this:

https://www.titus-hvac.com/file/1162/esvHWC_singledual_2023_V2.pdf

which shows tables of flow vs. pressure drop for various sizes of coils.

That was found at the page for your Titus DESV-06, under the "Performance" tab, where it lists "ESV Hot Water Coil Data":

desv

 

[georgeverghese]

Check if there is a possibility of calcium and/or magnesium bicarb / carbonate scale buildup in this coil.

 

[Mark_B]

Check if there is a possibility of calcium and/or magnesium bicarb / carbonate scale buildup in this coil.

It's a brand new coil so I think not possible.

 

[Mark_B]

Or surf over to the manufacturer's website and look for data. Like this:

https://www.titus-hvac.com/file/1162/esvHWC_singledual_2023_V2.pdf

which shows tables of flow vs. pressure drop for various sizes of coils.

That was found at the page for your Titus DESV-06, under the "Performance" tab, where it lists "ESV Hot Water Coil Data":

desv

Doh! I looked right at it and didn't catch that. Thank you.

I am still interested in learning if there is some mathematical/geometrical method for the sake of learning. Will look into the Crane TP410 referenced above.

 

[goutam_freelance]

You can also get the relationship between flow and pressure drop using a temporary flow meter(ultrasonic will be convenient) and, of course, the pressure transducers you already have.

 

[georgeverghese]

Do you have a sketch of the coil arrangement in this DESV- 06? - I dont see it on the DESV website

 

[pierreick]

Hi,
Consider the document attached to support your work.
Use the Darcy Weisbach equation with the appropriate f factor for coil as described in the documents underneath.
Iterative calculation on Re number for example.
Note: agree with George about scale formation, very common at my place.
Good luck
Pierre

 

[EdStainless]

How about a 5gal bucket and a stopwatch.
Record the time and the pressures.
It will give you real data point to use to adjust the mfg data.

 

[Mark_B]

My coil is 3/8" tube, but I am connecting with 1/2". To accurately measure pressure drop across the coil, I assume I will need to connect my transducer directly to 3/8" tube to get accurate pressure drop across the coil itself and not also be measuring the reducer etc?

 

[Mark_B]

How about a 5gal bucket and a stopwatch.
Record the time and the pressures.
It will give you real data point to use to adjust the mfg data.

I might do this as well.

 

[Latexman]

My coil is 3/8" tube, but I am connecting with 1/2". To accurately measure pressure drop across the coil, I assume I will need to connect my transducer directly to 3/8" tube to get accurate pressure drop across the coil itself and not also be measuring the reducer etc?

Who cares, right? Once you get your system built with the transducers in place, get the system data and develop your flow correlation. Then, the details and intricacies of your system are baked into the data and your flow correlation.

 

[Mark_B]

Who cares, right? Once you get your system built with the transducers in place, get the system data and develop your flow correlation. Then, the details and intricacies of your system are baked into the data and your flow correlation.

I meant if I want to use the manufacturer Head loss vs GPM data (which was apparently in the table I was reading all along). Your point is taken, though.

 

[georgeverghese]

Using a single diff press transmitter reading dp of the coil will give you better results than with 2 PTs'.
For drawn tubing, suggest the Blasius equation for smooth tubing - see Perry

 

[pierreick]

Attached a set of correlations for friction factor applicable for coil, with calculations.
Check with your data.
Hope this is going to help you.
Of course, bucket + stopwatch will validate the model, more accurate result using delta P measurement Vs 2 Pressure gauges.
All instruments must be calibrated prior to use.
Good luck
Pierre

 

[btrueblood]

Crane works, at least to build a model and correlate to changes from what you've tested. Even works for rocket engines and LOX. Every contraction/expansion makes a contribution too. I am a valve designer, and use the K value method all the time...with success, usually

Bucket and stopwatch method: calibrate your bucket with a known beaker/gallon measure (edit: I've found some buckets that aren't quite 5 gallons to the brim, and ones that are more like 6 gallons to the brim). But yeah, even us valve manufacturers use a volumetric method as a sanity check, and leak rates are almost always done with volumetric methods.

 

[Mark_B]

Just an update,

the two pressure transmitters seem to return nonsense readings that are unusable for determining flow. With pump at either 15% or 95% speed, the pressure differential reading is nearly identical, even though It seems apparent there is much more flow. I did my best to clear air from the pipes and dont believe that is the issue, although of course I could be wrong.

I have sort of given up on the dual pressure transmitter idea and am contemplating a Keyence FDQ ultrasonic flow meter. My budget is currently $0 so that will need to wait...

In the meantime I have felt it much easier to focus on the part of the project I enjoy the most which is programming. I have Steve Karg's BACnet development kit and have it running a basic blinky program. Next step is to program it to run the ECM fan and whatever circuit comes long with that. Will need to implement the BACnet stack which will be interesting.

 

[LittleInch]

Not necessarily.

Pump speed and flow are two different things. What else is in your system that might be controlling or restricting flow?

Can you trace pipework from pump outlet to inlet and see if there is a flow control valve somewhere controlling temperature? Or fixing flow.

A diagram would help hugely.

Can you measure pressure drop across the pump? Do you have or can find a pump curve? That would give you some idea if increase in speed is actually increasing flow.

"Seems apparent" isn't very convincing to me.

What are the readings and how much of the span of the pressure sensing device are the readings? Is this a transmitter or a guage? Check calibration and connections for blockages. If you have more flow you will have higher pressure differential. That's physics. Now the issue is why are you not seeing it so it's either not as much flow as you think is there or your readings are not accurate or in error somehow.

 

[Mark_B]

Not necessarily.

Pump speed and flow are two different things. What else is in your system that might be controlling or restricting flow?

Can you trace pipework from pump outlet to inlet and see if there is a flow control valve somewhere controlling temperature? Or fixing flow.

A diagram would help hugely.

Can you measure pressure drop across the pump? Do you have or can find a pump curve? That would give you some idea if increase in speed is actually increasing flow.

"Seems apparent" isn't very convincing to me.

What are the readings and how much of the span of the pressure sensing device are the readings? Is this a transmitter or a guage? Check calibration and connections for blockages. If you have more flow you will have higher pressure differential. That's physics. Now the issue is why are you not seeing it so it's either not as much flow as you think is there or your readings are not accurate or in error somehow.

There is a control valve but I am controlling it and it stays wide open.

The thing that really bugged me was that the pressure differential reading would stay the same for at least several minutes after I turned the pump off. I did zero the transmitters at atmospheric pressure before filling the system.