In many facilities running liquid ring vacuum pumps, the same pump can reach different vacuum levels depending on the season or even the day. The most common reason behind this is a change in service liquid temperature. This article covers why and how service water temperature limits vacuum level, where that relationship comes from physically, and the typical scenarios encountered in the field.
Table Of Contents
- What Is a Service Liquid's Vapor Pressure, and How Does It Relate to Vacuum?
- Why Does Vapor Pressure Rise So Sharply as Temperature Increases?
- Why Can't a Liquid Ring Pump Go Below Its Own Service Liquid's Vapor Pressure?
- How Does Seasonal Temperature Change Affect Vacuum Performance?
- What Solutions Exist If Service Water Temperature Runs High?
- How Does Using Oil Instead of Water Reduce This Problem?
- Why Is Service Liquid Temperature a Common Cause of Reported Faults?
- Frequently Asked Questions
What Is a Service Liquid's Vapor Pressure, and How Does It Relate to Vacuum?
Every liquid, when in equilibrium with its own vapor in a sealed container at a given temperature, generates a pressure known as its "vapor pressure." Water has a defined vapor pressure even at room temperature, and this value rises as temperature increases. In a liquid ring vacuum pump, the continuously circulating service liquid (usually water) has an exposed surface inside the pump, and vapor evaporates from that surface at a rate corresponding to the liquid's own vapor pressure.
This evaporation means gas is constantly being added to the environment inside the pump. As a result, no matter how well the pump is designed, it cannot bring the ambient pressure below the service liquid's own vapor pressure — because the liquid itself keeps generating vapor at that pressure level.
Why Does Vapor Pressure Rise So Sharply as Temperature Increases?
The relationship between vapor pressure and temperature isn't linear — it's exponential. A small increase in temperature can produce a disproportionately large jump in vapor pressure. This behavior stems from the fact that as temperature rises, liquid molecules gain kinetic energy that increases their rate of escape from the liquid's surface.
The practical result: a relatively small increase in service water temperature (a few degrees, for example) can shift the pump's achievable vacuum level by far more than one might expect. That's why service liquid temperature isn't a parameter that can be dismissed with "a bit warmer shouldn't matter."
Why Can't a Liquid Ring Pump Go Below Its Own Service Liquid's Vapor Pressure?
As covered in our article "How Do You Read mbar, mmHg and Absolute Pressure on a Vacuum Pump Datasheet?", absolute pressure is a measure of the total amount of gas present in an environment. A liquid ring pump's compression chamber contains both gas coming from the process and the service liquid's own vapor; no matter how well the pump is engineered mechanically, it cannot eliminate the vapor the service liquid continuously generates.
As a result, the service liquid's vapor pressure sets the pump's theoretical vacuum ceiling: even if the pump removes all the gas coming from the process, the ambient pressure cannot drop below the service liquid's vapor pressure. Catalog figures are typically given at a specific reference temperature (often around 15°C); if actual operating temperature runs warmer than that reference, the achieved vacuum stays shallower than the catalog value.
How Does Seasonal Temperature Change Affect Vacuum Performance?
In an open-loop system, service water is usually drawn from the municipal supply or a well, and its temperature varies with the season. Cold water may be readily available in winter, while the same source's temperature can rise noticeably in summer. This variation can lead the same pump to reach different vacuum levels between winter and summer — and in some facilities, that difference can be large enough to noticeably affect process performance.
As also noted in our article "How Should You Select a Vacuum Pump for Vapor-Heavy Processes?", processes sensitive to this kind of seasonal fluctuation should favor closed-loop solutions that keep service liquid temperature under control.
What Solutions Exist If Service Water Temperature Runs High?
General engineering practice offers several ways to bring service water temperature under control. As covered in our article "What Separates Water-Circulated and Oil-Circulated Vacuum Units?", closed-loop water-circulated vacuum units cool service water through a heat exchanger before returning it to the system, keeping temperature more stable and independent of ambient conditions.
Switching to a colder water source (deep well water, for instance), installing an additional cooling tower or chiller system, or expanding the capacity of existing cooling infrastructure are other methods worth considering for lowering service water temperature. The right solution should be determined on a project-by-project basis, based on the facility's existing infrastructure and the process's sensitivity.
How Does Using Oil Instead of Water Reduce This Problem?
As explained in detail in our article "What Separates Water-Circulated and Oil-Circulated Vacuum Units?", oil's vapor pressure is noticeably lower than water's at the same temperature. Units using oil as the service liquid are therefore less sensitive to temperature and can reach a deeper vacuum; according to Gücüm Pompa's catalog, the oil-circulated unit can reach down to 10 mbar, while the water-circulated unit reaches down to 33 mbar at 15°C.
This gap explains why an oil-circulated solution is worth considering for processes where controlling service water temperature is difficult, or where a very deep vacuum is the target.
Why Is Service Liquid Temperature a Common Cause of Reported Faults?
When a "pump won't reach the required vacuum" complaint comes in from the field, service liquid temperature is one of the first parameters worth checking — because this issue is often not a failure at all, but a shift in operating conditions. Unlike other possible causes such as leaks, wear, or incorrect sizing, a service liquid temperature problem can be resolved quickly simply by cooling the liquid.
For that reason, measuring the service liquid's current temperature before diving into a more complex fault diagnosis, on any system experiencing a vacuum drop, can quickly reveal the root cause.
Frequently Asked Questions
What Temperature Should Service Water Be?
Gücüm Pompa's catalog uses 15°C as a reference value. Exact temperature requirements can vary by target vacuum level and model, so consulting the supplier for a specific application is recommended.
How Much Does Vacuum Drop if Service Water Temperature Rises by 5 Degrees?
An exact figure can't be given, since it depends on the temperature range in question and water's vapor pressure curve; but because the relationship between vapor pressure and temperature is exponential, the effect of a small temperature increase can be disproportionately large.
What Can Be Done If Service Water Can't Be Cooled?
If service water temperature can't be controlled, an oil-circulated vacuum unit or an alternative system capable of reaching a deeper vacuum can be considered. For an exact solution, sharing process data with the supplier is recommended.
Does This Issue Only Occur in Water-Circulated Systems?
No. The relationship between service liquid vapor pressure and temperature applies to every liquid ring pump application; whether the service liquid is water or oil, temperature always affects the achievable vacuum level.
