Fans, Chamber Size & Static Pressure--Co-relationship/How to Control?

It seems that you have either Blocked the Return Air Flow to the Fans or the Exhaust stream from the Chamber, when you reduced the height of chamber.

In any comfort air conditioning system, we use approx. 90% return air and 10 % fresh air which may vary depending on the application. Hence approx. 10% air is exhausted out to atmosphere.

I suggest to provide a return air ducting or passage in the chamber and also an outlet for exhaust. Some fresh air is always needed for any airconditioning system to maintain proper ventilation. The use of return air will save energy.

Hope it HELPS.....

Thanks but the present case relates to a blast freezer operating at minus 25/30C and there is no provision or need to exhaust some air.

I also place a sheet above the evaporator grid to act as a overhead duct for aligning the air flow from suction to discharge and back but even this didn't work. To my astonishment, with heavy fans operating I could not see them stir even a leaf!! That was amazing and the thought still haunts my mind.

For your information the evaporator coil/grid discharge was fully open (as you find in a home window Air conditioner) while the suction air was let pass through the fans only. Failing to get this work I even reduced the discharge by placing a metal sheet against it but to no avail. It seemed as if some black magic was holding down air movement!

Science surely is magic in itself...I love it!!

Lot of information missing. What was the static and total pressure before and after? First you stated chamber hight was 6 feet but later you say "I thought of reducing its height from 12 feet to 6 feet". What is volume flow rate, temp, TR etc? A simple line diagram would help more.

The blast freezer was working efficiently at 12 feet height but was taking a bit too long time. So I thought of reducing the chamber size to make it keep more cooling and to give the target temperature sooner. I did that and that was kinda my fruitcake moment. I badly failed.

As for other factors you pointed out to we can assume them to be constant before and after except ofcourse the reduction of size of the blast freezer chamber which really matters.

Axial fans respond to the increased load imposed by constrictions on either their inlet or outlet side by requiring increased power and reducing their rotational speed. If you had cut off the air on either side of the fans, you would be alerted by a change in the sound as they run, or by tripped circuit breakers or blown fuses if the additional load is sufficiently great.

It's possible that air passage through the evaporator is blocked by ice. That would certainly restrict circulation. An accurate sketch or diagram would reveal other possibilities. A written plan accompanied by drawings is a good beginning for any project of this kind. If you skipped that step, it's still not too late.

I take your point on usefulness of drawings and i will see if can post any.

Here's an excerpt from a reply I posted here to an other member...and hope it will help clarify the setting..

'''The air is thrown into a finned evaporator from axial fans attached to it at the back and discharged air comes out from the opposite open side of the evaporator (ie from the front). Product to be blast frozen such as fish is placed right in front of the evaporator grid. The strong air thrust coming through the evaporator hits the fish and freezes it down to about minus 30 degrees in 10 hours or so, depending upon the thickness of the fish , its initial temperature and the way it is stacked. There is only one door located right in front of the evaporator grid which serves both as entrance or exit .''

You are having a situation similar to the one in which the discharge damper of the Fan is fully closed. Fan is running, is drawing current and is also developing static pressure, but NO FLOW..

For a trial just let some air exhaust in the atmosphere and see the flow..

Hope it helps

Two thoughts- evaporator iced up in fins and not readily apparent blocking airflow or blower motors running backwards(if 3 phase and wiring was disconnected/reconnected during work)

maybe your 2 fans make to and from airflow if u tampered with electricity

the fans were not even touched!

they operated perfectly well before the rood height was lowered. After I raised the room height they again became functional as ever!

Don't look for "Exotic" solutions, look for basics first. Was the evap moved when height was lowered. If so, check operation and rotation of both fans. Don't just look. One fan operating will make the other spin, or one runnig backward will circulate air round and round in the fan area and never discharge it to the box. -- JHF

thanks.....but

Allow me to inform all here that

1. No, the evaporator was not moved.

2. Fans were not touched.

3. Evaporator was not choked. In fact it was clean and dry before operation.

4. Fan rotation was correct.

5. No electric changes were made.

6. Suction and discharge openings were clear.

7.The blast freezer was also operated with a 4 ft wide door open.

In short, nothing was changed except the height of the blast freezer chamber roof which was lowered. The ''Exotic' or the 'Occult'' side to this that the air stopped to move and even two heavy fans couldn't even move a straw!!

Hi,

I am of the opinion now that the 2 Fans are working against each other with the Reduced Volume of Air available for Suction.

You must take a trial with ONLY 1 FAN RUNNING AT A TIME and see IF FLOW OF AIR IS RESUMED.

Low Buffer Volume for the Suction side may be a possible cause when you reduce the height of chamber.

Hope it solves yr problem,.....

Thanks, I think that's a good idea.

Yes, it appears that the fans are too big for the reduced space and starved of suction air, hence no discharge. However, only a test will tell.

I would start by requesting the the vendor of the evaporator to get an engineer from the evaporator manufacturer to inspect the installation. If the product being frozen is stackerd tightly to the lowered ceiling it could be the fans, propeller type I presume, are at their "cut off" point. Propeller fans are very sensitive to back pressure.

Get the help you paid for when you bought the evapoprator. I cut my teeth in the HVACR industry with a manufacturer and it seemed that was where the buck always stopped when one our products didn't perform, regarless of whose fault it was.

Good luck...Lou Bindner

There is no after-sales-service available for the equipment, no warranty either. But the fact is that the evaporator was performing well before the ceiling was lowered.

Stacking was not the problem nor the back pressure on the fans...because I ran the blast freezer both in vacant and stacked positions but got the same odd result. The two heavy fans wouldn't even move a feather!! But when i pulled off the lowered ceiling and got back to the initial height all went well.