Principle of operation
When operating with pressurised water, closed-circuit temperature control units are used. The unit, and thus the circulating water, is under pressure. The pressure required depends on the outlet temperature. The expansion of the water is taken up by the expansion vessel/system. Pressure in the unit, and in the temperature control circuit, increases as a result of the temperature-dependent expansion of the water. At about 98 °C/208 °F, the water in the expansion vessel will begin to vaporise, continuing until the saturation pressure corresponding to the set outlet temperature is reached (e.g. 2.6 bar overpressure at 140 °C/285 °F). This means that the outlet temperature is independent of the existing water-circuit pressure.
Figure 1: Regloplas temperature control unit model P160M for temperatures up to 160 °C/ 320 °F:
Principle of operation
41 - Cooler
E21 - Heater
M10 - Magnetic drive pump
Y2 - Solenoid valve (automatic water refill)
Y6 - Solenoid valve (cooling)
Y8 - Solenoid valve (pressure relief)
B8 - Temperature sensor
57 - Safety valve
56.2 - One-way check valve
60 - Filter (water circuit)
S1 - Level control
F5 - Safety thermostat
61 - Pressure gauge
M2 - Filling pump
Y18 - Solenoid valve blow out OFF
16 - System with heater
17 - System with cooler
58 - Bypass
99 - Consumer
Y16 - Solenoid valve compressed air
Pressurised water units can be used for any application in which the advantages associated with water are decisive, even at temperatures above 90 °C/195 °F (e.g. high cooling capacity at small unit dimensions; injection moulding of plastics with mould temperatures above and below 90 °C/195 °F (no switch from water to thermal oil required).
When the water level falls below the set minimum, the level control opens the solenoid valveand switches on the filling pump. The filling pump ensures that the unit can be refilled even when the water circuit pressure is lower than the system pressure in the unit which is determined by the outlet temperature. The water then flows through the one-way check valve into the system. In addition to the components described in Section 2, safety considerations require that temperature control units for operation with pressurised water be equipped with the following elements: A one-way check valve prevents the water from flowing backward when the pressure in the circuit, which depends on the outlet temperature set, is higher than that of the water circuit. The safety valve opens when the pressure in the circuit rises too high. The pressure gauge indicates the system pressure (static pressure) which is a combination of the pressure due to the expansion of the water and the saturated vapour pressure. A pressure relief solenoid valve is used to release pressure in the temperature control circuit so that it can be separated from the consumer without danger. Depending on the configuration of the unit, it is actuated automatically or manually by means of a push-button, once the water has cooled to a minimum of 80 °C/175 °F.
Once pressure has been relieved, the consumer can be emptied by means of suction at temperatures below 80 °C/175 °F. (This limit is necessary as the boiling point of water is lower under suction). The suction solenoid valve must be open to allow air into the temperature control circuit. Leak-stop mode and emptying the consumer by means of suction, or blow-out, are also possible. Pressurised water units with maximum outlet temperatures above 140 °C/285 °F are usually equipped with a filling pump, as at temperatures above 140 °C/285 °F it is not always possible to refill at water mains pressure.
With pressurised water units, it is possible to cool directly, eliminating the need for a separate cooler. Several disadvantages are associated with direct cooling:
- Water cannot be pre-treated with corrosion inhibitor because new water is repeatedly added to existing water (exceptfor closed cooling systems)
- Danger of scaling and contamination in temperature control circuit
- Poor control action (large temperature fluctuations)
- Max. outlet temperature 100 °C/210 °F due to vaporisation of hot water exiting during cooling.
The greatest advantage of direct cooling lies in its significantly higher cooling capacity, which is most useful for applications with low consumer temperatures (e.g. 30 °C/85 °F).
The decisive criterion for evaluating the advantages and disadvantages of any method must be the specific application.
- The good heat transfer properties of water can be exploited for temperatures above 90 °C/195 °F up to approximately 160 °C/ 320 °F.
- High cooling capacity in small dimension units