Batch Heating of Whey

[buckman1122]

I have an application that requires cooling 2700 gallons of whey from 132° F to 85° F in 20 minutes using a double tube heat exchanger. The problem I am encountering is the fact that my customer is only pumping at 75 gpm through the heater and this results in only 1500 gallons of the whey passing through the HX (after 20 minutes).

I need to know how to calculate the temperature at which the fluid exiting the HX should be so that the remaining 1200 gallons of 132° F whey will achieve the 85° F required. Obviously this is a transient heat transfer problem but I'm having difficulties finding a good model to perform the calculations with.

 

[Latexman]

The equations for this are in Donald Q. Kern's "Process Heat Transfer". I believe they are also in Perry's 6th and 7th Editions.

Good luck,
Latexman

 

[dcasto]

I assume batch cooling. Won't the whey coat the cooling tubes and decrease the heat transfer coefficient? It's gonna be a long drawn out affair. After you do the calculation, I'd set up a 1 gallon batch in my house to see how bad the heatransfer changes too.

 

[speco]

Buckman1122,

You left out an important ingredient in the problem. You need a coolant inlet temperature and flow rate.

You hit the nail on the head about the recirculating whey flow rate. I would bet that your coolant temperature is high enough that, even if the whey were cooled to the temperature of the coolant, it would be impossible to achieve your objective final temperature in 20 minutes. You should probably have a pump capable of recirculating the entire 2700 gallons a few times an hour.

Regards,

Speco

 

[StoneCold]

buckman1122
You could easily just do a heat balance on the material to figure out the required outlet temperature of the heat exchanger to cool the entire batch to 85F. That being the ideal case, the real case is more than likely worse do to recycling of previously cooled material. If I was the one trying to do it I guess I would change out the pump and I would try to pump the 2700 gallons through the exchanger at least three times in the twenty minutes. Which turns out to be 405 gpm. That is a lot but there are a lot of if's in batch processing. I would also shoot for a as high a tube velocity as I think the piping and pump could tollerate to help mitigate the heat transfer problems with the whey.
Speco is probably right about the cooling water being to high a temp to be able to only recirculate part of the material.

Regards
StoneCold

 

[HEC]

buckman1122
Dairy industry, what a wonderful industry to work in, the general attitude goes along the lines of, "we want to try something new, and have these pieces of equipment left over from the last time, we don't want to spend any money but we want it to work perfectly!". Rant over.
Stonecold is on the right track, look first at the overall heat balance and rate at which the heat must be removed from the batch. This will provide a minimum heat transfer rate through the h/x. from this a "first pass" outlet whey temperature can be derived. Use this as a mixture in the bulk tank from which you are recirculating. As the tank temperature comes down the h/x capacity will also reduce due the changing temperature of approach.
From the "first pass" outlet temperature the cooling water or more likely chilled water temperature requierments will be determined.

It sounds like whey directly off the cheddaring belt, is there any opportunity to recover this heat to the cheese milk pasteuriser preheater. A hot water and cold water well system is used here to buffer the "batch/continuous" nature of the cheese making process. This will reduce the initial whey temperature you need to treat.
Cheers

Mark Hutton