Influence of Water Temperature in Vacuum Pumps Used for Air Removal in Steam Sterilizer

Abstract

The temperature of the water used in vacuum pumps of steam sterilizers has the potential to cause failures in sterilization processes due to reduced efficiency and compromised vacuum levels. This study measured the impact of water temperature on a liquid ring vacuum pump (monobloc design) in a steam sterilizer. A sterilizer with a 566-L chamber was used for the tests. The water temperatures tested were 10°C, 20°C, 30°C, 40°C, and 50°C, and the following outcomes were analyzed: vacuum depth achieved in the drying phase (mbar), total cycle time (min), vacuum rate in the conditioning phase (mbar/s), and water consumption (L). Water temperature influenced the performance of the vacuum pump in all outcomes analyzed. Considering total cycle times, the performance with water at 10°C can be up to 50% better compared with performance with water at 50°C. The highest vacuum rates were obtained with water between 10°C and 20°C (up to 13 and 20.24 mbar/s, respectively). In cycles performed at 10°C, average water consumption was 33.3 L. In contrast, for cycles carried out at 50°C, the average water consumption increased to 94.2 L. The temperature of the water used in vacuum pumps influences the efficiency of sterilization cycles, which can result in longer cycles and higher water consumption.

Steam sterilization is widely used in hospitals, clinics, laboratories, and pharmaceutical facilities, with equipment ranging from tabletop to large-volume autoclaves. Steam sterilization also provides many advantages for thermoresistant devices, including eliminating toxic residues and providing ease of control and monitoring, fast cycles, and excellent penetration ability in lumens and sterile barrier systems.¹

Although the sterilization process can vary according to the sterilizer and load, it can be summarized in three phases: conditioning, sterilization, and drying. In the conditioning phase, the air must be removed for the steam to come into contact with the load. In general, air is removed via vacuum pumps, liquid ring vacuum pumps in single- and two-stage versions (both of which are subject to the influence of water temperature), dry-claw vacuum pumps, and oil-sealed rotary vane pumps.

In healthcare services, the most common pumps are liquid ring pumps. Basically, during pump operation, the water continuously flows inside the pump through the movement of an eccentric rotor, with water consumed during this operation. The centrifugal force exerted by the rotation forms a “liquid ring” as it projects the fluid onto the walls of the compartment. Air, condensate, and steam are drawn into the area between the impellers and the liquid ring, then removed from the chamber.^2–4 In the drying phase, when the steam from the internal chamber is exhausted, the vacuum pump is also activated to assist in the evaporation of the condensate.⁴

For efficient pump operation, the potable water used to form the liquid ring must be cold.^5–7 The optimum temperature stated in the EN 285:2015 standard is a maximum of 20°C, with a hardness value (Σ ions of alkaline earth), between 0.7 and 2.0 mmol/L.⁶ Values above these limits can cause scaling and corrosion.^5,7 In practice, the temperature of the water is a potential cause of failures in sterilization processes, as it can reduce the efficiency of vacuum pumps and compromise specified vacuum levels.^7,8

Recommendations also exist for maintaining temperature at 15°C, as an increase of 5°C can result in a loss of up to 20% of suction efficiency, thereby affecting performance.⁹ Therefore, temperature should not exceed the value specified by the manufacturer and is among the criteria evaluated in the installation qualification.^5,7

A change in the performance of the vacuum pumps can also result in wet loads, which is a frequent problem encountered by sterile processing departments (SPDs). A pilot study demonstrated that the configuration of the vacuum system, such as the number of vacuum pulses in the conditioning phase, vacuum depth in the conditioning phase, vacuum speed in the conditioning phase, and vacuum depth in the drying phase, can reduce condensate formation by 5.8 times.¹⁰ However, the authors did not obtain data related to variation in water temperature.

According to a study that obtained opinions from 73 sterilization experts, the low efficiency of the vacuum pump was strongly associated with wet loads; however, disagreement remained regarding the impact of water temperature on the performance of vacuum pumps.¹¹ Therefore, further investigation of the role of water temperature is required. In the authors’ experience from constructing, installing, and qualifying sterilization equipment, water temperature is rarely controlled in healthcare service settings, even in countries with temperatures above 30°C. For this reason, the current study measured the impact of water temperature on the vacuum pump of a steam sterilizer.

Methods

A steam sterilizer with a 566-L chamber (model 6212; Cisabrasile, Joinville, Brazil), which is used for laboratory tests, was adapted to reproduce the equipment used in the daily routine of healthcare services. The sterilizer was equipped with a liquid ring vacuum pump in monobloc design (Dolphin LX 0030–0055 B; Busch, Maulburg, Germany).⁴ The tests were carried out without a heat exchanger to reproduce the worst-case scenario in the daily routine of service provision.

To avoid the influence of the load configuration, weight, composition, and conformation of medical devices, the tests were performed with the chamber empty. The cycles were performed sequentially in triplicate, proceeding from lowest to highest temperature. The outcomes were analyzed using the mean obtained in the triplicate cycles.

To carry out the experiments with water at varying temperatures, a 200-L tank was used with temperature measured by a class A wire PT100 resistance temperature detector, with a tolerance of ±1°C. Water pressure was maintained at 0.1 bar. For tests at colder temperatures, ice was added to the container until water reached the preestablished temperature. For tests at warmer temperatures, a heat exchanger heated by saturated steam was introduced directly into the water to increase temperature.

Based on variations in temperature according to geographical area, climatic season, and the location where equipment is installed, the study was carried out with water temperatures at 10°C, 20°C, 30°C, 40°C, and 50°C. The outcomes analyzed were:

• Vacuum depth achieved in the drying phase (mbar). This outcome analyzed the deepest level of vacuum reached during the drying phase of the sterilization cycle. The vacuum level was measured using an absolute pressure transducer, according to the EN 607701:2011 standard.¹²

• Total cycle time (min). This outcome was defined as the complete duration of the sterilization cycle, involving all stages such as conditioning, sterilization, and drying. Time values were recorded by the steam sterilizer controller.

• Vacuum rate in the conditioning phase (mbar/s). During the third vacuum pulse in the conditioning phase, this rate expressed the speed at which the vacuum was generated. Vacuum rate was calculated as the difference in initial and final pressure (e.g., 1,300 mbar – 150 mbar = 1,150 mbar) divided by the time at which the vacuum pump was active to achieve the pressure change.

• Water consumption (L). This outcome reflected the amount of water used by the vacuum pump during the cycle. It was calculated by multiplying the pump’s water consumption specification (5 L/min) by the amount of time the pump was running.

Results

Table 1 summarizes the results at each water temperature tested. Considering total cycle times, the performance with water at 10°C can be up to 50% better compared with performance with water at 50°C. (Figure 1).

Table 1.

Mean (±SD) of the outcomes analyzed, according to water temperature.

Figure 1.

Cycle profiles, according to the water temperature of the vacuum pump.

Water temperature also influenced the performance of the vacuum pump for all outcomes analyzed (Figure 2). The highest vacuum rates were obtained with water between 10°C and 20°C (up to 13 and 20.24 mbar/s, respectively). For triplicate cycles performed at 10°C, average consumption was 33.3 L. In contrast, for cycles carried out at 50°C, the average consumption increased to 94.2 L.

Figure 2.

Vacuum pump efficiency, according to water temperature.

Discussion

The results confirmed the influence of water temperature on the outcomes analyzed. Even with water temperature controlled, the temperature of the mixture of air, steam, and condensate from the sterilization chamber also influenced the performance of the vacuum pump by heating the pump compartment and, consequently, the liquid ring via heat transfer. The higher temperatures of this mixture decreased efficiency and extended cycle time. For this reason, using heat exchangers that use cold water to cool the condensate before entering the vacuum pump is essential. This process assists in maintaining a lower and more constant temperature in the vacuum pump, improving its overall performance.

Cavitation is a phenomenon that occurs when the pressure inside a vacuum pump reaches values lower than the steam pressure, resulting in the formation of bubbles that can implode and cause damage to the pump. This effect is more prevalent at higher water temperatures, which reduces pump efficiency and consequently results in higher maintenance costs and shorter lifespan.¹³ Practical recommendations are provided to prevent cavitation from occurring, such as equipping the pump with an atmospheric air ejector, manufacturing the impellers from materials with anticavitation properties, using a working fluid with low saturated steam content, and lowering the water temperature.¹⁴

According to ISO 17665:2024, the minimum water pressure and maximum temperature should be specified and then established during installation qualification.⁵ Because variations occur in water temperature due to different weather seasons or changes in facilities that can influence pressure or flow, measuring water pressure and temperature more frequently is recommended. This will allow the healthcare technology management department to monitor the variations and assess whether they meet equipment installation requirements.

Although heat exchangers favor the maintenance of water temperature uniformity, it is not a requirement listed in technical standards. In practice, heat exchangers can recirculate cooling water recirculated, resulting in water savings. However, cooling the water may incur an increased energy cost.¹⁵ In addition, existing infrastructure may not support integration of advanced temperature control technologies and substantial upgrades may be required.

In instances water supply system failures, dry pumps are not dependent on water supply, and carrying out temperature control is not necessary. In general, the return on investment is accelerated due to the continuous reduction of water expenses and simpler maintenance.

Regarding wet loads after the sterilization cycle, SPD professionals and maintenance teams commonly increase the drying time of the load without a clear rationale. As the drying phase also uses the vacuum pump, the water consumption will also increase and can reach 450 L.¹⁶ In this case, longer cycles can increase the total processing time of medical devices and generate water waste, which often is not properly identified. A study showed that the water consumed during the drying phase has a major impact on total water consumption. For each additional minute in the drying phase, an approximate increase in water consumption of 10 L was observed, representing an increase of 100% for each additional 15 minutes of drying.¹⁶

In the current study, the vacuum depth (i.e., the lowest level of absolute pressure that the pump can reach or maintain within a hermetically sealed system) achieved in the drying phase was shown to be quite sensitive to water temperature. Thus, considering this control in the evaluation of the performance of liquid ring vacuum pumps in steam sterilizers is essential.

The application of the results to daily routine in the healthcare setting should consider that air removal is influenced by sterile barrier systems and conformation of the devices contained in the load.¹⁷ In addition, the weight of the load has been shown to be a determining factor in the duration of the sterilization process, as greater weight requires more energy to promote heating. Therefore, more time and steam supply will be required.¹⁸

Limitations

This study involved certain limitations. Tests were not performed on equipment that included a heat exchanger. Also, the impact of pressure variations on temperature setpoints was not measured.

Conclusion

The temperature of the water used in vacuum pumps influences the efficiency of sterilization cycles, potentially resulting in longer cycles and greater water consumption. Solutions exist for controlling water temperature; however, each SPD needs to evaluate the technology that best suits its infrastructure and financial resources.

Considering that the tests described here were carried out in an empty chamber, future research should consider the most challenging loads sterilized in SPDs and the magnitude of local temperature variation during climatic seasons.

Disclosure

R.Q.S. has received consulting fees from Solventum and is an education services provider for Karl Storz.