Condensate rate of water for an air conditioning cooling coil

[bkireland]

TL;DR

I need to calculate the volume of water condensing from a air conditioning cooling coil.

I need to calculate the amount of water condensed from a DX cooling coil per hour given the size of the expansion coil (the total condensing surface area), the incoming air temperature, the amount of air flow from the fan, the BTU capacity of the compressor and the incoming air humidity. There are lots of condenser calculators around but they all need the air flow and incoming and outgoing humidity and then give a total volume of condensed water but I need more than that. The size of the coil and the air speed across the coil must impact the volume of condensate? Alternatively if I could calculate how much water condenses from a flat plate given the size of the plate, the incoming and outgoing air temperature and the humidity that would help. Or, if I could find what the outgoing temperature of the air exiting the expansion coil is given the size of the coil, the incoming air volume and speed and BTU of the compressor. Thank you very much for your help.

 

[Lnewqban]

Welcome!
Number of rows in the coils is also important.
Calculation is very difficult, due to formed film of water around the tubes (different for height and depth location of each tube and for number of fins).

The best way is to have the performance data for that specific coil, which the manufacturer gets by experimentation.

 

[bkireland]

Thank you for your answer. I am realizing that the calculation is difficult, which is why specific information is so hard to find. Thank you again.
Best wishes,
Bill

 

[berkeman]

Paging @russ_watters @Chestermiller

 

[russ_watters]

I need to calculate the amount of water condensed from a DX cooling coil per hour given the size of the expansion coil (the total condensing surface area), the incoming air temperature, the amount of air flow from the fan, the BTU capacity of the compressor and the incoming air humidity. There are lots of condenser calculators around but they all need the air flow and incoming and outgoing humidity and then give a total volume of condensed water but I need more than that.

This is relatively easy to calculate by hand. Are you familiar with the psychrometric chart? You can manually look up values in the chart (you should at least get familiar with it) or get them from a website with a form. You need absolute humidity and enthalpy. From incoming air enthalpy minus condenser capacity you get the leaving coil enthalpy, then go back into the chart for leaving coil absolute humidity. Let me know if this is enough to go on and if not I can walk you through it (there's some assumptions involved of course). What are the actual given values?

The size of the coil and the air speed across the coil must impact the volume of condensate?

A bit, but it is very had to calculate how much. The safe assumption is that your leaving air is saturated and the coil is effective enough that all the condenser capacity is used. In reality the condenser capacity isn't fixed, and the leaving air dewpoint is a touch below saturation/dry bulb.

Or, if I could find what the outgoing temperature of the air exiting the expansion coil is given the size of the coil, the incoming air volume and speed and BTU of the compressor.

This is the way, per above.

 

[bkireland]

Thank you very much. I will look at all the things you suggested.

 

[bkireland]

Hi Russ: I looked at your website with great interest. I have a MS degree in physics from UCSB. My minor was astrophysics and I enjoy the subject immensely. My major was solid state and I worked for many years developing semiconductor materials at Fairchild Semiconductor Company.

Anyway back to my current project. I have a small consulting business with a client who builds and markets equipment to make drinking water from air. The equipment they use was designed some 10 years ago pretty much by throwing darts at an air-conditioning company catalog. The client now wants to fine tune the system for efficiency and production. In order to do that I am trying to code an Excel spreadsheet to simulate the process but am having a lot of difficulty with the calculations.

I have written many simulation programs, power generation, solar, wind and hydro, optimizing silicon manufacturing processes and others but this one is giving me problems. I have downloaded a psychometric chart and will try and make sense of it. Thank you.

Regards,
Bill

 

[Chestermiller]

You need to be able to calculate the outlet temperature of the condenser. For this, you need to know the heat transfer coefficient between the air and the heat transfer surface. This is a function of the flow velocity and the condenser geometry. There may be experimental correlations of heat transfer coefficient (for your system as a function of air flow rate. Otherwise, you would need to use a generalized correlation of Nussult Number as a function of Reynolds Number and Prantdl Number for similar geometries. See Transport Phenomena by Bird, Stewart, and Lightfoot (sections on heat transfer).

 

[bkireland]

Thank you.

 

[Dullard]

Not precisely responsive, but may be useful to sanity-check results:

The theoretical maximum water production rate is approx 5.6 Kg/Hr per Ton of refrigeration. That's a fairly expensive way to produce water...

 

[russ_watters]

Not precisely responsive, but may be useful to sanity-check results:

The theoretical maximum water production rate is approx 5.6 Kg/Hr per Ton of refrigeration. That's a fairly expensive way to produce water...

That assumes all of the energy extracted is for condensing water. You can do something like that with a more complicated cycle/system, but for a standard air conditioning unit, if you start with saturated inlet air you still have about 2/3 sensible cooling and 1/3 latent.

 

[Lnewqban]

Thank you for your answer. I am realizing that the calculation is difficult, which is why specific information is so hard to find. Thank you again.
Best wishes,
Bill

You are welcome.
Condensation formation and dripping flow on tubes and fins, which depends mainly on humidity content of air, tend to complicate the heat transfer calculation.

Please, see:
https://achp.readthedocs.io/en/latest/ACHPComponents/WetDryHeatExchanger.html

https://pdf.sciencedirectassets.com...3165e53560d05060252&rr=976ccbc0dcdd4c09&cc=us

 

[bkireland]

Thank you.