More on Vacuum and Venting in Your Evaporator

Vacuum and venting issues are one of the first things to investigate if you are having trouble maintaining consistent control of your evaporator.

Vacuum Control

There are several effective methods for maintaining good vacuum control of an evaporator system:

  • Air bleed control valve on the shell of the condenser
    • The tail end of the condenser is the point of deepest vacuum (and lowest temperature) in any evaporator system. An air bleed control valve is effective for controlling the absolute vacuum level inside the surface condenser, which directly affects product boiling temperature, and can quickly increase the boiling temperature by reducing the vacuum level in the surface condenser. It takes just a small air bleed to quickly increase the absolute pressure (vacuum level) in the condenser and thus throughout the entire evaporator system.
  • Throttle cooling water flow
    • Reducing the cooling water flow to the surface condenser reduces the rate that waste heat vapors are condensed, leaving a larger volume of gas for the vacuum pumps to pull from the surface condenser causing the vacuum level in the condenser, and therefore the entire evaporator system, to decrease.
  • Restricting the vapor flow into a surface condenser with a throttling damper valve will cause the main evaporator system to operate at a higher temperature (higher absolute pressure or less vacuum) than inside the surface condenser. Opening the damper will pull the vacuum level in the evaporator closer to the surface condenser’s vacuum level, thus reducing the evaporator’s operating temperature.
  • For Mechanical Vapor Recompression (MVR) Evaporators — injecting make-up steam into the system can quickly reduce the vacuum level (create a higher absolute pressure level) in the evaporator system
    • MVR Evaporators operate close to a neutral energy balance with low rates of waste heat rejection to a surface condenser. Injecting steam will overload the surface condenser with excess waste heat, increasing the system pressure and boil the product at a higher temperature.

No Dead Ends

All shell-and-tube heat exchangers, including pre-heaters, need to be well vented at both ends to prevent collection of non-condensable gases that block heating vapors (steam) from effectively contacting available heat transfer surface area.

Condensate must not be allowed to block vents where it can obstruct venting of non-condensable gasses out of the heat exchanger. We recommend including a sight glass on the side shell of each heat exchanger, at the condensate outlet level, which allows you to see that the condensate level is not high enough to obstruct the vent outlet. This sight glass also allows visual confirmation of splashing condensate flow, indicating active condensation of heating vapors inside the heat exchanger shell.

At Caloris, we utilize optimally sized orifice holes at each heat exchanger vent connection to achieve a controlled rate of gas venting out of the heat exchanger. Using valves to control venting rates can be problematic because they are adjustable, which affects consistency. Orifice holes are more permanent for consistent day-to-day evaporator performance, unless the orifices are inadvertently removed.

Number 1 Symptom

If your evaporator is running hot, this is the most common symptom of a vacuum leak or insufficient system venting.

With any vacuum leak, non-condensable gasses entering the system quickly expand to a significantly larger volume and choke the flow of gasses through the venting lines to the surface condenser, and the capability of the vacuum pumps to pull those non condensable gases out of the system. Read our previous blog post on identifying vacuum leaks.

Another potential culprit to consider when running hot is excessive fouling of the surface condenser. The condenser and/or cooling tower may be an issue (sealing, capacity, fill in tower, etc.). The vacuum pump should be looked at as well — is the service water sufficient? What about capacity?

Vacuum Leak Monitoring

Caloris recommends a system for vacuum leak monitoring with an airflow meter on the vacuum pump discharge. All air that leaks into the system exits past the meter. The meter allows you to document normal airflow volume when the system is known to be “tight” and then routinely monitor the air flow rate to identify a deviation from the normal.

Caloris Can Help

If you are experiencing vacuum and venting problems with your evaporator, a review of your system is in order. Caloris can help with such evaluations, following up with recommendations for system performance improvement. Give us a call at 410-822-6900 or send email to to arrange an inspection/evaluation of your equipment.


About Caloris

Caloris transforms process systems design for the better with unexpected, future-focused solutions that solve real operational problems. We engineer efficient, dependable and productive solutions that also improve environmental sustainability where practical. Designed with the end user in mind to drive plant productivity. Caloris offers evaporation, membrane filtration and spray drying process systems for food and dairy industries, as well as for water reuse and wastewater treatment applications. Both Caloris’ custom and pre-engineered packaged systems are designed to meet our customers’ specific concentration/purification needs utilizing the most cost effective and energy efficient technologies available. Whether concentrating dairy, food or juice products at a high-volume production facility, or processing industrial wastewaters to reduce transportation and treatment costs and recover re-usable water, Caloris provides a range of options and technologies.