Blast Freezer Frosting

[stcornish]

I am looking for another method of eliminating frosting in a blast freezer other than using a desiccant make-up air unit. Any suggestions?

[frank]

I've seen the make up air refrigerated past the ADP and then re-heated thereby reducing the RH to approx 2%.
As I don't work with this type of equipment I couldn't give you any specifics but the end results I've seen have been pretty impresive

[stcornish]

Frank,

Thanks for your response.

It would seem to me that the make-up air temperature would have to be dropped to below the design temperature of the freezer. If the freezer were designed for -15 deg F (-26 deg C) The make up air would have to at least -25 deg F (-31.6 deg C) to avoid frosting on the evap coil where the suction temp is at least 10 deg F (5.5 deg C) below the freezer design temperature. This would require several coils in series with defrost and redundant coils to achieve this result. A through investigation of the cost versus the increase in production revenue would be required to determine if this is ecomonically feasible. My guess is that it would be better to put in redundant coils into the freezer and defrost there.

The problem with desiccant dehumidifiers is they are good to about 7 deg F (-14 deg C) dew point. They can help to reduce the frosting but defrost cycle is still required at low temps.

Steve

[Gary]

Have you looked into Niagara coils? I haven't worked with them, but have seen them operate. As I understand it, they continuously spray the coil with glycol to eliminate formation of frost, and heat the recovered glycol to renew it's strength.

[frank]

Steve

From what I've seen the make up air is reduced to very low RH prior to the air being delivered onto the blast freezer coil. This coil then suffers from less frosting due to the absence of moisture in the air stream.

[stcornish]

Below are my responses to both Gary and Frank.

Gary,

The concept of spray coils with glycol sound interesting. My first concern is that as you defrost the condensate becomes part of your glycol. Now you are adding volume to the glycol and diluting the glycol %. Two problems, one what do you do with the excess mixture and two how do you keep the glycol % up to avoid freezing. If a portion of the coil tubing side was used to pass the glycol passed through this would eliminate this problem. Of course the coil surface would get larger and there would be some problems with the heat transfer efficiency reduction. If this type of coil were used then the hot gas could be used to heat the glycol before returning to the coil. One down side I see to this concept is the addition of another mechanical system not usually there with std. electric defrost. The glycol system would add a pump, a storage tank, control valves, piping, and a mixing tank. The up side is that is allows for continuous operation but does it offer any benefits over hot gas defrost?

Frank,

We are looking a using desiccant dryers to reduce the moisture content of the make-up air to the freezers. Because of the low temperatures single pass cooling through an air handling unit does not seem very feasible. What methods have you seen to reduce the air RH to below the freezer dew point?

Steve

[Gary]

I really don't know much about the Niagara system, beyond casual observation. The customer seemed very happy with it. I found the following website:

http://www.niagarablower.com/nofrost.htm

[stcornish]

Gary,

Thanks. I have visited their website and have will check it out further. The concept looks interesting and very different.

Steve