All thermally heated evaporators will lose a considerable amount of waste heat as a normal consequence of operation. Whether a single effect direct steam heated design, or a multiple effect with Thermal Vapor Recompression design, there is no avoiding the fact that all of these designs will generate waste heat.
This waste heat is in the form of saturated water vapor that is released from the evaporator, usually from the final effect and at the lowest temperature. For most typical dairy evaporators, this temperature will be 110ºF, or higher. For a single effect design operating at 150ºF product boiling temperature, the waste vapor will discharge at ~150ºF.
The higher the system capacity, the greater the quantity of waste heat being lost.
Example 1: An evaporator concentrating 100,000 PPH of 9.0% TS skim milk to 50% TS will evaporate 82,000 PPH of water. If this were a 6-Effect TVR design, the amount of waste vapor flow can be ~8,200 PPH, which would be exhausted to a condenser at 110ºF. While this evaporator is quite efficient removing 10 lb. water/1 lb. steam, the waste heat is nevertheless substantial at ~8 MM BTU/hr. This is typically transferred to cooling water and then to the ambient air via an evaporative cooling tower, i.e. wasted to the atmosphere.
Example 2: A much smaller evaporator concentrating 45,000 PPH of 36% TS skim milk to 50% TS will evaporate 12,600 PPH of water. If this were a 1-Effect TVR design, the amount of waste vapor flow can also be ~8,200 PPH, which would typically be exhausted to a condenser at 140ºF. Same as with Example 1, this amount of heat is wasted to the atmosphere.
For either case, some of this energy can be recovered. A separate heat exchanger can be used to condense some of this water vapor while heating a colder medium. We can refer to this as a waste heat recovery unit (WHRU). The colder medium could be a cold feed to the evaporator, or perhaps cold water that will be stored and used later for CIP.
Interestingly, a novel condenser design is possible that incorporates the WHRU in the same shell, achieving a very cost effective and space efficient design. Both the condenser and WHRU consist of shell-and-tube units, where the WHRU is configured to preferentially condense the waste vapor before the surplus can be condensed in the main condenser section. The WHRU can handle either a sanitary medium or a non-sanitary medium, as circumstances require.
What is the benefit of such a novel condenser? Simple, free energy is recovered to the extent practical by transferring this heat to a useful product stream, lowering the net operating cost of the process. Simultaneously, the operating cost of the cooling tower is reduced since less waste heat transfer to the atmosphere is needed. This can be a valuable tool for de-bottlenecking a cooling tower as a secondary benefit. The novel integrated design also lowers the net capital cost.
For Example 2 above, if we consider that the feed to this evaporator is cold shipping condensed at 40ºF, it would be theoretically possible to heat this stream to 135ºF in the WHRU, assuming we can condense sufficient waste vapor at 140ºF. The quantity of waste vapor needed is 3,750 PPH. This represents 46% of the available waste heat, so 46% of the waste heat (3.75 MM BTU/hr.) can be recovered.
If you think that your evaporator system may be a candidate for significant waste heat recovery using this novel condenser design, please give us a call for an evaluation.
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