Correct evaporator air flow... 2 fans or 3 ?

[kengineering]

Hi, I'm just trying to live up to my name. The "experts" I work with can side step design problems, I'd rather try to solve them.
Is there a formula for choosing the correct air flow for an evaporator? I once heard there is a wet bulb / dry bulb , entering air/ leaving air computation that would insure the most efficient use of fans.

I sure wish I worked with the likes of you guys. Thanks again, Ken

[Gary]

Hi, I'm just trying to live up to my name. The "experts" I work with can side step design problems, I'd rather try to solve them.
Is there a formula for choosing the correct air flow for an evaporator? I once heard there is a wet bulb / dry bulb , entering air/ leaving air computation that would insure the most efficient use of fans.

I sure wish I worked with the likes of you guys. Thanks again, Ken

I'm sure there must be a formula for this, as there is for everything. As a service tech, I prefer to work with ballpark guidelines.

First of all we need a good psych chart:

http://www.me.umn.edu/courses/me4131...tricCharts.pdf

Let's say you want a room temperature of 75F @ 50% RH. Find 75F along the bottom of the chart, follow it straight up to where it intersects with 50% humidity, then horizontally to the left where it hits the saturation area.

Assuming minimal latent load, this is the temperature you want on the surface of the coil (55F). Wetbulb, drybulb and surface temperature are all identical at this point, assuming the coil is actually condensing moisture.

Not all of the air that passes through the coil actually touches the surface, so this bypassed air mixes with the conditioned air, causing the wetbulb leaving the coil to be slightly higher and the drybulb slightly more so.

But then, you want the coil to be slightly colder to handle more than minimal latent load. So... as a rough rule of thumb, I would want to adjust the airflow so that the leaving air wetbulb temp would be about 54F, or the drybulb temp (measured close to the coil) would be about 55F, giving me a coil surface temp of roughly 52-53F.

I generally summarize the process by saying that for an A/C application the leaving air drybulb temp (close to the coil) should be about 20F lower than the temp at the thermostat in order to maintain 40-60% humidity.

For other applications, we can follow the same thought processes if we know the temperature and humidity desired.

The important point here is that the coil temperature determines the humidity and the coil temp can be adjusted by adjusting the airflow. The lower the airflow, the lower the coil temp, the lower the humidity. The higher the airflow, the higher the coil temp, the higher the humidity.

But as I said earlier, surely there must be a formula for this. I will leave that for others. I'm just not a formula kinda guy.

[kengineering]

Sorry for the delay in responding , had some computer issues.
I beleive you have given a formula of sorts thanks I will try it on the following.

I am trying to build/ design a refrigerated open merchadiser. So far I have selected a coil, TEV and condesing unit. The compressor is 1/2 hp with 134A and a 1/4 ton TEV. I have used this coil and valve in a closed display case with great results with a 1/3 hp condensing unit . I stepped up the CU to a 1/2hp because of the open design.
So far my presures are very low. I normally see operating presurres for 134A around 34 on cycle 14 off.. I am seeing the pressure hover at aroud 15 then down to 9 to produce the desired temps, 38*f. Thanks , Ken

[Gary]

This being an open display case, there are other factors to be considered:

From an energy efficiency standpoint, increasing the airflow, thus loading up the compressor, would be a very good thing, but it may in fact increase the spillover of cold air into the aisle. If this is the case, then the better solution might be some form of unloading to achieve our ideal temperature/humidity target.

Unloading has an energy cost, but it has the advantage of the ability to pull down under heavy loads, which may or may not be needed in a display case.

The least costly (energy-wise) means of unloading would be compressor speed control (inverter), but this may not be a viable option.

Another choice might be an EPR (Evaporator Pressure Regulator) valve, which gives us a very steady leaving air temperature, while allowing fast pulldown when needed.

Or... if it is decided that pulldown ability is not needed, it might be better to simply downsize the condensing unit.

Balance is everything.

Decisions... decisions...

[750 Valve]

Coolpack has a design tool for merchandisers, looks to only cover evap load. Don't know whether you're talking multideck, service deli style or open well but have seen the first two operating anywhere from 0.8m/s to 1.6 m/s velocity @ air curtain (honeycomb). Haven't had any need to play air games with a wide island freezer but I'd guess about 0.8 m/s relying on cold dense air to fall into well.
As a rule of thumb I'd suggest the more horizontal the air curtain (not ideal) then the lower your air velocity will have to be to ensure the air falls onto the shelves and doesn't blow out the case. If you have a vertical air curtain then you can run a higher airfow, which will likely allow a higher sst and better system cop (energy $) and will require less defrosting. Shelf angle and depth has a big influence on case performance also.
From a design standpoint I think it would be best to address the airflow within the space first, then select a coil based on your heatload and airflow requirements.
Hope this helps

[Gary]

From a design standpoint I think it would be best to address the airflow within the space first, then select a coil based on your heatload and airflow requirements.
Hope this helps

Exactly so. But let's make that "heatload, humidity and airflow requirements".

[750 Valve]

Exactly so. But let's make that "heatload, humidity and airflow requirements".

Of course!

[Peter_1]

How I do it, making that I have a uniform flow over the coil of +/-2.0 to 2.2 m/s (6.5 ft/sto 7.2 ft/s)

[lana]

Is there a formula for choosing the correct air flow for an evaporator?

This is a late response, But I give it anyway.

For air conditioning application try to keep the air velocity around 500 FPM. If you increase the velocity you will get higher outlet temperature from the coil. If you decrease it you will get lower temp.
For air cooled condenser where noise is not an issue then you can increase the velocity up to 1200FPM.
For blast coolers (dry-coolers with huge fans) the velocity must go higher.
Hope this helps.
Cheers