The vacuum pump is the heart of a vacuum evaporator system and 95% of all evaporators use them. You can find them in evaporation units used for heat-sensitive products such as food and dairy, but also in any type of multi-effect evaporator to maximize energy efficiency.
If the vacuum pump stops working, the temperature cannot be controlled and will increase until the vapor temperature is at least 212 °F and above atmospheric pressure. It is unusual that the vacuum pump stops working completely, but it might run inefficiently or at reduced capacity, resulting in excessive power and cooling water consumption.
The Liquid Ring Vacuum Pump (LRVP) has been around for over 100 years and is the workhorse in nearly all vacuum evaporator applications. It is pulsation free, has no valves and only one moving part — the impeller. It is very reliable. Water is used as a seal and creates the effect of a plunger, compressing the gas from vacuum to atmospheric pressure.
A steady supply of soft, low temperature service water is required for operation. Evaporator condensate that is cooled can be excellent service water. The LRVP cannot produce vacuum lower than the saturation (boiling) pressure of the seal water. For deeper vacuum, a lower service water temperature is needed. In the pump, the service water is heated by the impeller input energy and from water vapor condensing.
There are three types of service water systems:
- Once Through: The discharged water is discarded after use.
- Partly Recirculated: Part of the discharged water is mixed with fresh water and used again. This is the most common system for evaporators.
- Closed Loop: The discharged water is cooled in a heat exchanger and recycled back to the LRVP.
If the service water is too warm or the condenser pressure is very low the LRVP can start to cavitate. This is when water vapor bubbles form in the low-pressure side and then collapse in the compression side. Then the LRVP will be noisy, start to vibrate and over time the cavitation will destroy the pump. There are cavitation prevention devices that can be installed in the LRVP. They will bleed in a small amount of air, preventing too low of a pressure.
A better way to avoid cavitation that Caloris is using is speed control or Variable Frequency Drive (VFD) on the motor. The pump can run at a lower RPM–saving service water and energy–and it is quieter.
If the service water is not soft, calcium can precipitate in the pump due to the increased temperature. Scaling will buildup on the housing and impeller, lowering efficiency and eventually seizing the pump. If the service water is not low in minerals the LRVP will need de-scaling with acid on a regular basis. Caloris’ standard LRVP supply have stainless steel impeller and housing. The pump will not be damaged by de-scaling acid or by CIP chemicals if accidentally carried over to the LRVP.
The service water tank has a vent blowing out non-condensable gas evacuated by the vacuum pump. The flow of non-condensable gas from the vent is approximately equal to the amount of air leaking into the evaporator. Caloris vacuum systems have a gas flow meter installed in the vent and the operator can monitor and record the flow. Leaks can then be detected early and fixed before they become a problem.
Interested in Learning More?
Contact us by calling 410-822-6900 or sending email to email@example.com and we will put you in touch with one of our engineers.
This post was written by Caloris Senior Process Engineer Theo Reis.