If you are experiencing problems with your evaporator, we highly recommend investigating for vacuum leaks.
There is no such thing as a small vacuum leak, when it comes to evaporator operation, with as much as 90 percent of all evaporator performance problems traceable back to vacuum leaks.
While not all evaporators need to operate under vacuum, most evaporators used in food processing do operate under vacuum in-order to minimize product heat exposure.
Here are a few basics that are useful to understand:
- Why do we operate under vacuum (i.e. less than atmospheric pressure)? It allows us to “boil” and evaporate water from a product at a lower temperature.
- Steam is an efficient means of delivering heat for an evaporator system, as 1 Lb of steam delivers ~ 1000 BTU’s of Latent Heat energy when it condenses from vapor to liquid phase.
- By comparison, air is a non-condensable gas that can only deliver Sensible Heat to a heat transfer surface, making air a much less effective gas for heating an evaporator.
- The specific volume of steam dramatically increases under vacuum, meaning 1 Lb of steam occupies significantly more volume inside the shell of a heat exchanger when it is under vacuum.
- Air, a gas that is denser than steam, also expands in volume under vacuum, comparable to steam in its amount of volume increase.
- Even a small volume of air leaking into an evaporator under vacuum will significantly expand, and displace steam away from contacting the heat transfer surfaces of the evaporator heat exchangers where the steam needs to condense in-order to deliver its Latent Heat.
- Non-condensable gases tend to collect at the ends of shell-and-tube heat exchangers, also referred to as “calandrias” when used in a falling film evaporator. Proper venting of the non-condensable gases from the shell-and-tube heat exchangers is critical to maintain efficient evaporator performance.
- Air can also create an insulating layer at the heat transfer surfaces, acting as an insulation layer that impedes efficient heat transfer across the heating surfaces unless proper venting draws that non-condensable gas (air) away from the heat transfer surfaces.
Now that we know why air leaks in your system are a concern, where should we try and find them?
Common sources of air leaks into your evaporator include:
- Manway gaskets/sightglasses
- Tri-clamp fittings that are not secure
- Pump seals
- Feed Product
What to Look For:
If you have a new piece of equipment and have the ability to shut it down overnight, do the following:
- Run the system, pull vacuum and then shut the unit down. The next day, what is the vacuum level? It is normal to lose 1/2″ mercury per hour — anything higher and you most likely have a leak.
- If you are in a quiet environment, walk around while the system is under vacuum and listen for the “hissing” sound of an air leak.
For existing equipment, it’s often not possible to shut everything down to inspect as above. Ultrasonic leak detectors are the most effective method for inspecting an entire evaporator system for possible leaks, although they do require training and some experience to use properly.
One simple method to assess a suspected air leak location is to use a squirt bottle containing soapy water. Spray around suspected leak sites, just like you would inspect a tire for a leak. Where you see the bubbles being sucked into the machine, you have found your leak.
Alternatively, with shell-and-tube heat exchangers you can CAREFULLY touch the outside surface of the ends of the heat exchanger. Air will tend to get trapped at the ends of a shell-and-tube heat exchanger due to insufficient venting (and most commonly the bottom since air is denser than steam), preventing steam from getting into that end of the heat exchanger and allowing the surface of the shell to become cool to touch.
We Are Here to Help!
If you are experiencing lackluster evaporator performance, a review of your system will help locate leaks. 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 firstname.lastname@example.org to arrange an inspection/evaluation of your equipment.