ACFM or SCFM

[som1973]

Hi All,
I am trying to figure out the problem on our Tunnel oven supplied three years ago to our customer.
Recently, we found that the Water is coming of the Tunnel Oven through Gaps of Each Modules.

I am being asked to verify/check the Exhaust fan CFM rating of existing fans. whether these fans are unable to remove moisture from the oven or not.

I have calculated the Actual CFM ( ACFM) considering all the process involved in the tunnel oven

We have 6 exhaust fans on the oven with 1100 CFM(SCFM)(Existing) Rating each.

Now I need to check & compare whether Fan CFM rating is ok or not.

Do we need to consider ACFM or SCFM for Exhaust fan selection. Pl. Help.

Thanks in Advance.
Som

 

[PEDARRIN2]

Typically concern between ACFM and SCFM is when the air (or gas) is compressed. Since you are talking about fans, the difference is probably slight due to minimal difference effects in temperature/pressure from standard conditions. Temperature is likely within ~10 F from standard and pressure drop in a duct is typically on the order of inches of water column measured in psia, there should not be any difference between SCFM and ACFM.

 

[Wilbur55]

Try evaluating the system using mass flow rates. A centrifugal fan moves an actual air flow rate. A pitot tube reads stagnation pressure of the flowing air stream. The standard numerical constant in the formula v = C * 4005 * sqrt(P_v) may not be applicable without correction for temperature (and elevation, if you are located on a mountain). The value of C may also have some dependence on temperature. Whoever measures the air flow rate through the fans may need to account for the air temperature.

 

[willard3]

First thing you should do ia get an anemometer and measure actual airflow at 70F at atmospheric pressure, then compensate for the conditions in the oven. After three years running this oven, it is unlikely that fans are doing nameplate volume.

 

[MintJulep]

Oven. Moisture apparently condensing and dripping out. Warm moist air at or near saturation is well away from standard conditions. You absolutely need to consider the actual density of what you are moving.

Willard3 is correct, after three years it's very likely that the fan blades are crudded up with whatever you are drying in the oven and are not operating at their design point any more.

Also, the nature of production is "more more more", so the rate of product running through the oven is probably more than the design rate as well.

 

[Compositepro]

ACFM is what is important but is interchangeable with SCFM given the temperature and the pressure. You seem to be on the wrong track with regard to problem solving. A drying oven will have high humidity which will condense when it cools. It seems that you do not have a slight negative pressure in your oven so that leakage is into the oven rather than out through the cracks. This is controlled by exhaust flow and by restricting in-flow. Are there baffles or curtains on the openings to the tunnel oven that are properly adjusted and maintained. Accumulation of crud in any part of your oven can cause this problem.

 

[som1973]

Thanks All for the advice/suggestions.

The Oven is baking oven and not drying oven, there are Manual dampers on the exhaust to control the amount of exhaust going out. I have calculated the actual CFM Required considering all the process involved during baking.

Now I am comparing this Actual CFM with the CFM Exhausting( considering Percentage of Manual Damper opening), there are 6 zones in the oven & 6 have different manual damper opening percentage) & I found that the CFM exhausting(considering Manual Damper opening) is less than the Calculated Actual CFM.

Hence the difference in the Air volume containing moisture is coming out the oven as water after condensation.

Now I am thinking of changing the CFM rating of the existing fans.

Please Comment/Review.

Thanks
Som

 

[willard3]

You still don't know what the actual CFM is.

 

[som1973]

I have calculated Actual CFM.

 

[chicopee]

You should have been more specific as to the purpose of the tunnel oven as most responders assumed that it was for drying instead of baking. Also a little more details about what is contributing to the high humidity during the baking process. Since the problem seems to have occurred after three years of usage than I would tend to agree with MintJulep about the loss of performance of the fans.

 

[IRstuff]

How sure are you that the product hasn't changed? Have they changed the recipe? Going from no water to dripping water is a pretty drastic change that should have had an impact on the product quality itself.

TTFN
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[willard3]

Calculated and actual CFM you will find are different.

 

[som1973]

Thanks Chicopee & IRStuff for your advice/Suggestions, the customer has introduced two new products few months ago which has more moisture in the product ( I got the Product info from customer)& now he is complaining that water is coming of the oven.

In order to remove this excess moisture, I am thinking of changing the CFM Rating of Exhaust fan. I Theoretically, calculated the Actual CFM of the fan considering all the process involved in baking ( including moisture loss from product)& found that the existing fan cfm rating were less than calculated Actual Cfm.

I hope by changing the CFM of fan, this problem of water leaking from oven should solve..

Thanks
Som

 

[ChasBean1]

Sorry to chime in late. You said “Recently, we found that the Water is coming of the Tunnel Oven through Gaps of Each Modules.” I would suggest that you seal the gaps.

To check/verify the exhaust capacity, I would not be concerned so much initially about ACFM vs. SCFM. This might be on the order of 10-20% based on STP vs. warm air.

During the cycle, smoke-stick the tunnel entry. This should be from the more negatively pressurized room because it is the dirty side, where the equipment enters the tunnel. So if the exhaust capacity is insufficient, the dirty side is the side that is more likely to emit steam during the cycle.

On the outset, 6,600 cfm (1,100 at 6 connections) seems very high. If the tunnel entry takes in air during the entire cycle (based on the direction of smoke travel), the volume of exhaust air may be reduced. You should reduce the peak exhaust volume until the airflow between the tunnel entry and the dirty side is roughly neutral during the cycle.