Water to air heat exchange - imperial units doing my head in

In summary, the conversation is about a person trying to figure out the energy required to heat a specific volume of air to a specific temperature using a water to air heat exchanger. They are struggling with understanding the measurements and units used, such as BTU, CFM, and pressure drop. They are also trying to determine the surface area of the heat exchanger and the overall heat transfer coefficient. They are considering contacting the vendor for assistance but also want to figure it out on their own.
  • #1
nytrojen
6
0
Hey guys, newbie here and first post. I've got a (hopefully) simple question, but for some reason my brain just isn't working. I'm too used to working with metric, and even though it should be fairly simple, working with imperial units just hurts.

Anyway, here's the problem: I have a specific volume of air that I need to move, and at the same time I need to heat the air to a specific temperature. I'm using a water to air heat exchanger to heat the air, and found what I think is a suitable product.

The heat exchanger product page lists:
size (in inches)
BTU (is this BTU or BTU/h?)
volumetric flow rate of air (CFM)
pressure drop of air (this really confuses me - is it 81% drop of original pressure? ie: the pressure is now 19% of original??)
pressure drop of water (ditto as above)
volumetric flow rate of water (GPM)

I have T1 and T2 of the air that I wish to heat, and I know the velocity that I want the air to be after the heat exchange (I picked the heat exchanger based on the volumetric flow rate - assuming it remains constant through the heat exchange?). I also have the energy required to heat the air from 20 deg C to 32 deg C (the final temp).

How do I figure out the energy into the water? Sorry if this is a stupid question, but it seems like US methods are completely different to what I learned in University
 
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  • #2
Can you post this HE product page or give a reference?
 
  • #3
Yes, here's the product page: http://www.northlanddistrib.com/Water-To-Air-Heat-Exchangers_c_423.html

Basically I want to produce a flow of hot air at 32deg C, assuming T1 is 20 deg C and a volumetric flow rate of approx 1 m^3/s by blowing it through the HE. Trying to figure out how to work out the energy needed to be put into the water.
 
  • #4
The decimals are not % loss. The website is terribly laid out for just having the values (and not being consistent with them...), but if this is meant to be installed with HVAC ducting, the .81 for air pressure loss would generally be read as inH20 (inch water column), which is what they are using here, according to the spec sheet. They use Ft. H20 (feet head) for the water pressure drop.

Have you read the spec sheet? http://www.northlanddistrib.com/WA16X18-Heat-Exchanger--40000-106000-Btu_p_67.html

T2air, and T1water required, are determined by the airflow across the unit. Make sure your system can achieve these requirements (800-1200 SCFM of air at roughly 7-10 ft/s for the 16x18 model)

Also, check the performance characteristics of your selected model, 2100 CFM (1m^3/s) is very high. In this case, it would be best to contact the vendor and discuss your particular application.
 
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  • #5
I didn't even see the specs there :blushing:

It's more for really rough order of magnitude to get an idea of what we're dealing with. 1650 CFM should be just on the lower limit of what we can get away with, so I'm trying to aim for that.

It doesn't help that I'm completely unfamiliar with any of these units of measurement. Maybe I'll try and use the dimensions provided of the HE and see if I can work out what I need from there instead of relying on the empirical data?

I should be able to use the surface area and thermal conductivity of aluminium to work out the total thermal resistance, correct? Then from there I'll have the overall heat transfer coefficient and since I know how much energy I want to put into the air, I can then use that to find out how much energy needs to be in the water?
 
  • #6
I think you'd get further calling the vendor...

You could take the operating data given in the spec sheet and work backwards to find the expected conditions at your operating points, but you'll likely be making lots of assumptions.
 
  • #7
Yeah it's more for ballpark figures, but I take your point... contacting them would probably be the easiest way. I just wanted the satisfaction of working it out myself lol. Thanks for the help!
 
  • #8
nytrojen said:
Yeah it's more for ballpark figures, but I take your point... contacting them would probably be the easiest way. I just wanted the satisfaction of working it out myself lol. Thanks for the help!

Go for it, then, by all means! Even if only to assure yourself that you know what the vendor is talking about. Should be a good exercise to get yourself a pretty darn good understanding of this type of exchanger.

Getting the surface area of these can sometimes be tricky business though.
 
  • #9
Yeah I've already run into a wall with that lol. But I figure 12 fin/in by 3 rows of fins by the height of the HE by the OD of the tube should be close enough to give me an approximate surface area. Now figuring out the rest is hurting my brain :S Thermodynamics were never my strongest subject. It's really interesting, but it's just layers upon layers and layers.
 

1. What is water to air heat exchange and how does it work?

Water to air heat exchange is a process that involves transferring heat from water to air or vice versa. It typically uses a heat exchanger, which is a device that facilitates the transfer of heat between two fluids without them coming into direct contact. In this process, the water and air flow in separate channels, with the heat being transferred through a conductive barrier between them. This allows for efficient heating or cooling of air using water as the source of heat or coolness.

2. Why are imperial units used in water to air heat exchange calculations?

Imperial units, such as British Thermal Units (BTUs), are commonly used in the United States for measuring energy and heat. Since water to air heat exchange is a process that involves the transfer of heat energy, imperial units are often used to calculate the amount of heat being transferred. However, metric units, such as watts or kilowatts, are also used in some countries.

3. How do I convert imperial units to metric units for water to air heat exchange calculations?

To convert imperial units to metric units for water to air heat exchange calculations, you can use conversion factors specific to the units you are working with. For example, to convert BTUs to watts, you can multiply the number of BTUs by 0.293071. It's important to ensure that you are using the correct conversion factors for the specific units you are working with.

4. Can water to air heat exchange be used in both heating and cooling systems?

Yes, water to air heat exchange can be used in both heating and cooling systems. In heating systems, the water is heated and then circulated through the heat exchanger to transfer heat to the air. In cooling systems, the water is chilled and then circulated through the heat exchanger to cool the air. This process can be used in various applications, such as HVAC systems, geothermal heat pumps, and industrial processes.

5. What are the benefits of using water to air heat exchange in HVAC systems?

There are several benefits of using water to air heat exchange in HVAC systems. Firstly, it is a highly efficient process, as water has a higher heat capacity than air, allowing for more efficient heat transfer. Additionally, it can be used for both heating and cooling, making it a versatile solution. It is also a more environmentally friendly option, as it can use renewable energy sources, such as geothermal energy, for heating and cooling. Lastly, it can help reduce energy costs and improve indoor air quality, as it does not introduce outside air into the building.

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