Heat loss due to air flow

[jbir]

Hi. Great forum!
We have a tunnel type spray washer. In one of the last sections 190 F water is sprayed onto the product at the rate of 30 GPM. Air is flowing through the tunnel past this section at a rate of 2000 CFM. The air properties are: 65 F and 60 RH. I am assuming that the air becomes saturated at this point. I'd like to know how to calculate the number of BTU's per minute (or hour) this air flow removes from the washer section. We are experiencing a 20 F temperature drop in this section and believe the air flow may be the cause.

 

[MintJulep]

You need the air properties both entering and leaving the section of interest. Your assumption of saturation may not be correct.

Look up the enthalpy for entering and leaving condition on a psychrometric chart.

Convert your 2000 cfm to a mass flow rate.

Multiply the mass flow rate times the change in enthalpy.

There you go.

Is your concern only temperature, or is it energy? Are you pouring energy down the drain?

 

[quark]

You have to do precisely what MintJulep has told. You should have the data of Inlet and outlet Dry and wet bulb temperatures or DBT and RH values. Plot the points on a psychrometric chart and obtain the enthalpy values at the two state points. Once you have the enthalpy values use the equation,

Q(btu/hr) = 4.5 x cfm x enthalpy diff. between entering and exit air in btu/lb

We are experiencing a 20 F temperature drop in this section , when you say this what temperature you are measuring? Is it air temperature or water temperature?
Generally, during adiabatic humidification process air temperature gets reduced. (actually, it's constant enthalpy process). But in actual conditions the air temperature may be more than the constant enthalpy process.

A clear description and some data will make your post more clear.

Regards,

 

[MintJulep]

Quark's 4.5 factor assumes standard temperaure and pressure. If your application or location differ substantially from standard conditions, or if you need a more accurate solution, calculate the actual air density.

 

[wilg]

The 4.5 factor or (4.45) is derived:

as 60 min per hour / 13.5 specific volume of air at 70db/50% rh

yielding 4.45 lbs of air per cfm
read your enthalpy directly off of psychrometric chart

Example
This illustrates properties of air at 73 degrees F (Tdb) and 20% RH. By using the psychrometric chart, wet-bulb temperature is 52 degrees F; enthalpy is 21.3 Btu/lbda; humidity ratio is 0.0035 lb/lbda; dewpoint temperature is 30 degrees F; and specific volume is 13.5 ft3/lbda.
wilg5404@aol.com

 

[quark]

MintJulep, you are absolutely right. But I just wanted to trivialize the solution.

Regards,