A Guide to Reducing Energy Consumption in Your Food Processing Evaporator Systems

A Common Type of Evaporator
(Reprinted with permission. Copyright, 1981 by Chapman & Hall.)

• A heat source (normally steam) that evaporates the water.
• An evaporation vessel where water is driven out of the product as vapor.
• A vapor separation vessel, where vapor and product are separated.
• A vacuum system that draws water vapor out of the separation vessel. This vacuum also reduces pressure in the evaporation vessel, which reduces the boiling point.

• Installation of additional effects will generally improve the efficiency of an evaporator system. There is a practical limit to how many effects can be used, as each evaporation vessel must have a lower pressure than the previous effect.
• Preheating feed product reduces the heat required to achieve boiling in the evaporation vessel. This requires addition of a heat exchanger appropriate to the characteristics of the feed.
• Vapor recompression takes advantage of the significant heating value of the vapor driven out of the product. Vapor can be reused in the same evaporator by increasing its temperature and pressure close to those of the steam injected into the heat exchanger. This can be done using a steam jet or a compressor.

• Optimize the venting rate of non-condensable gases to reduce steam waste while maintaining appropriate evaporation vessel pressure. Non-condensable gases in the evaporation vessel increase pressure, which increases the boiling point and heat requirements.
• Maintain the optimum pressure profile as provided in the evaporator's design. Excess pressure inhibits evaporation by raising the boiling point.
• Condensed steam can be used to preheat feed product or used in the next effect of a multi-effect system. It can also be fed back to the boiler to offset the use of cold makeup water.
• Pre-concentration of the feed will reduce the energy required to operate an evaporator. For some applications, pre-concentration with separation membranes can save up to 90 percent of energy consumption.

• Prevent air leaks into the evaporators to minimize venting rates. Air is a non- condensable gas and has to be vented to keep evaporation vessel pressure from increasing.
• Clean heat transfer surfaces to allow the most efficient use of energy to evaporate water from the product.
• Inspect and repair or replace wet or damaged insulation as it is found. Also, make sure insulation is the appropriate thickness.
• Keep the vapor separation vessel clean to maintain product yields and pressure pro-files.
• Prevent water leaks into the system to avoid diluting the product, which defeats the process.