Immersion heater circuits

Immersion heaters for heating tea water are available but are not recommended They work to begin with but do not stand up to the damp conditions in the vessel and short circuit after a time. 

No particular difficulties are experienced in controlling temperatures in the region from room temperature up to 300°C. Bimetallic strip devices, Wheatstone bridge circuits (thermistor activated) or mercury thermometers with capacitance devices connected to relays can be used for control of bath temperatures. For good control, the immersion heater should have the lowest possible heat capacity and the bath should have as large a volume as possible. Should the bath volume be small for whatever reason, the heat capacity of the heater should also be low to prevent flirther bath temperature rise after the current is shut off. A thin Pt wire, woimd aroimd a frame, may be used as a heater in these cases it may be placed directly in the bath without any Insulation. Vigorous agitation of the liquid in the bath is important. Bath temperatures somewhat below room conditions may be maintained by means of an immersed copper coil with a constant... 

If the container itself does not include a heating element, any immersion heater should be insulated to avoid the potential of electrical shock and should include a cut-out device if the temperature exceeds the set point. Alternatively, a second temperature sensor should be placed in the heat bath to act as a circuit breaker to cut off power to the heater if its thermostat fails. The thermostat clearly should always be set well below the flash point of the heating liquid in use. A thermometer placed in the bath at all times it is in use is recommended to provide a visual indication of the actual temperature of the bath. Digital controllers normally provide this additional information. 

A TN supply feeds a domestic immersion heater wired in 2.5 mm PVC insulated copper cable and incorporates a 1.5 mm CPC. The circuit is correctly protected with a 15 A semi-enclosed fuse to BS 3036. Establish by calculation that the CPC is of an adequate size to meet the requirements of lET Regulation 543.1.3. The characteristics of the protective device are given in lET Regulation Fig 3A2(a) of Appendix 3. 

A small, single-point over-sink-type water heater may be considered as a permanently connected appliance and so may be connected to a ring circuit through a fused connection unit. A water heater of the immersion type is usually rated at a maximum of 3 kW, and could be considered as a permanently connected appliance, fed from a fused connection unit. However, many immersion heating systems are connected into storage vessels of about 150 litres in domestic installations, and the On Site Guide states that immersion heaters fitted to vessels in excess of 15 litres should be supplied by their own circuit (OSG Appendix H5). 

Therefore, immersion heaters must be wired on a separate radial circuit when they are connected to water vessels which hold more than 15 litres. Figure 3.55 shows the wiring arrangements for an immersion heater. Every switch must be a double-pole (DP) switch and out of reach of anyone using a fixed bath or shower when the immersion heater is fitted to a vessel in a bathroom. 

The single pressurizer serves all four coolant loops. It serves to hold the primary circuit pressure constant and to allow for expansion and contraction of the coolant volume caused by changes of load. An electric immersion heater of 1800-kW capacity is used to raise the pressure to, and maintain it at, the required value during negative pressure surges caused by an increase in load demand. In the event of a positive surge, caused by a reduction in load demand, a spray system, fed from the cold leg of one of the reactor coolant loops, condenses steam in the pressurizer to avoid tripping the pressure relief valves with which it is fitted. 

Circuit 3. A thermostaticaiiy controiied 3kW immersion heater as described in Exampie 5, previousiy having a maximum demand of 13.04A and protected by a 16 amp MCB. 

The bath temperature may be controlled by heating with an immersed loop of Nichrome resistance coil, such as the heating element from an electrical appliance. The coil is connected to a 110-volt line in series with a 27-ohm 5-ampere variable resistor and a 600-watt heater element, which may be cut out of the circuit to lower the fixed resistance. An ammeter in the circuit helps to determine the proper adjustment of the sliding contact of the resistor. To avoid an excessive load on the coil,... 

The stability of the pressure of the primary system is assured by the pressurizer (8). This is a vertical vessel whose volume is normally 60 per cent filled with water and 40 per cent by steam. The lower part of it (filled with water) is connected by a surge line with one of the two primary cooling circuits electrical heaters are immersed in the water. The upper part (filled with steam) can be sprayed by cold water. 

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