GB2158927A

GB2158927A - Electric flow heaters

Info

Publication number: GB2158927A
Application number: GB08510209A
Authority: GB
Inventors: Royston Leonard Edwards
Original assignee: IMI Santon Ltd
Current assignee: IMI Santon Ltd
Priority date: 1984-04-24
Filing date: 1985-04-22
Publication date: 1985-11-20
Also published as: GB8410429D0; GB8510209D0

Abstract

An electric flow heater comprises a stainless steel flow conduit 10, a sheathed heating element 11 which has a part in close proximity to the conduit wall and a thermal cut-out device 31 mounted on the exterior of the conduit adjacent to that part of the element which is in close proximity to the wall. <IMAGE>

Description

SPECIFICATION Flow heater This invention relates to a flow heater. It is particularly concerned with a flow heater suitable for maintaining or topping up the temperature of recirculating water in a whirlpool spa bath. Spa baths are usually constructed so that water is extracted from the bath and pumped back into the bath through one or more orifices with or without air entrainment.

When the user occupies the bath for a prolonged period, the heat losses can mean that the water temperature falls below that which is comfortable for the user.

It is one object of the invention to provide a flow heater suitable for such application.

According to one aspect of the invention we provide a continuous electrical flow heater for liquids comprising a heat conductive flow conduit, a sheated electrical element extending into and along said flow conduit from an entry position, said element being located in close proximity to or in thermal contact with the wall of the flow conduit at a point intermediate its entry position and the return end of the element, a thermal cut-out device being mounted on the exterior of the flow conduit adjacent where the element is in thermal contact with or in closest proximity to the wall of the flow conduit.

The power supply to the element is preferably switched by a triac which is mounted on a heat sink which is cooled by air. The operation of the triac is suitably controlled by a zero voltage switching circuit acting in response to signals from a thermistor located in the conduit adjacent the element. The triac and the zero voltage switch circuit are suitably mounted within a control box situated adjacent the flow conduit. A temperature sensor may be provided adjacent the element, which sensor is capable of providing a signal indicative of the fluid temperature to a display remote from the control box. An additional element may be provided for heating the liquid, a first switch means enabling one or both elements to be connected to the power supply. The control box may additionally incorporate means for controlling the power supply to a pump which pumps water through the flow heater.The pump may be adapted to pump liquid at alternatively high or low speeds, a second switch being provided for changing the pump from one speed to another. The operation of said second switch may be by manual means provided at a remote station. Typically the remote display is located adjacent the remote means for operating the switch, on a module within easy reach of the user of the spa bath. The electrical supply for such display should be at a voltage sufficiently low to avoid any risk to he user, and the manual means for operating the second switch to control the pump speed, may be connected by an air-line to an air switch located within the control box. The control box may additionally incorporate means for turning off electrical power to one of the elements if the second switch is operated to increase the pump speed.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side elevation, partly in crosssection, of a flow heater according to the invention, and Figure 2 is a schematic circuit diagram, for the flow heater of Fig. 1.

A flow tube 10, of circular cross-section and made of stainless steel, is connected in a recirculation path for a spa bath (not shown), so that water can be drawn from the bath and pumped through tube 10 on its way back to the bath. A pair of sheathed heating elements of conventional construction extend into tube 10 from above, tube 10 conventionally lying in a horizontal plane. A first element 11, after entering tube 10 turns through 90 so that the major portion of it extends inside tube 10 in the direction of water flow. A second element 12, entering tube 10 adjacent element 11, also turns through 90 so that its major portion extends inside tube 10 in the direction of water flow, but it is spaced further than element 11 from the uppermost wall of tube 10.Only one half of each element 11, 12 and its respective terminals is visible in the view of Fig. 1, the remainder of each element from its return end being directly behind the parts shown.

A control box 1 3 is positioned above tube 10 with the terminals of elements 11, 1 2 extending into the lower part of box 1 3.

Within box 1 3 are the controls and circuitry associated with the operation of the heater.

On one side of the box, and normally covered, are the manual controls including two position switches 14, 15 and 16 and control knob 17.

The position of first switch 14 determines whether the heater is on or off, the position of second switch 1 5 determines whether the speed of the pump (not shown) is set by the operation of the third switch 1 6 or whether it is set by operation of a remote switch, and the position of the thid switch 1 6 determines whether the water pump (not shown in Fig. 1, since it is upstream of tube 10) is operated at a high or a low speed. The movement of control knob 1 7 changes the setting of a variable potentiometer, and this is used as a temperature set-point to vary the power input to the elements.Control box 1 3 is typically hidden from view as is tube 10 and associated pipework and switches 14, 1 5, 1 6 and control knob 1 7 are pre-set. The spa bath user has however access to a remote module 1 8 adjacent the bath which is connected by an air tube 1 9 and a low voltage three-core cable 20 to control box 1 3. Remote module 1 8 has an LED temperature display 21 connected via cable 20 to a 3 pole DIN connector on box 13, and a manual switch 22 connected to one end of air tube 19.Connector 21 takes signals from a digital temperature sensor 23 which extends within a pocket into tube 10 under the control box 13, thereby permitting the water temperture just upstream of heating elements 11, 1 2 to be displayed on remote module 1 8. The temperature displayed will differ only slightly from the water temperature downstream of heating elements 11, 1 2 since these provide only top-up heat to the already warm water, typically being rated at 0.75 watts per element. Sensor 23 is electrically isolated from the power supply by means of a double-wound transformer (not shown) so that there is no risk to the user should the case of the remote module 18 become live eg because of a connection to the LED display 21 coming loose.

Control of power supply to heating elements 11, 12 is determined by the operation of a triac 24 (see Fig. 2) which is driven by a zero-voltage switching circuit based on a Ples- sey SL 445A chip provided on a printed circuit board 25. Triac 24 is mounted on a heat sink 26 shown in dashed line in Fig. 1 on the back of control box 13, heat sink 26 being cooled by air. A thermistor 27 fitted in a pocket extending into tube 10 adjacent elements 11, 1 2 is used to actuate the temperature control. The temperature set-point for control can be varied by manual adjustment of control knob 1 7.

The heater of this embodiment is designed to be used where only a limited electric current supply is available. A two-speed pump 27 (see Fig. 2) is employed, and the control box 1 3 contains controls which switch the second heating element 1 2 off when the high pump speed is selected, thereby limiting the overall current drawn. This is achieved by connecting the coil of a relay 28 (see Fig. 2) to the high speed terminal X of the two-speed pump 27.When remote switch 22 is selected by switch 16, the user can operate switch 22 which causes air within tube 1 9 to actuate an air switch 29 within box 1 3. If the user wishes to increase the water flow into the spa bath, the operation of swich 22 will cause the pump to be switched to its higher pump speed, but this automatically cuts out the second element 12 by means of the relay 28.

This configuration is of course only necessary where there is a limit on the current suppply, since otherwise a single element which remains on could be installed.

The heater is protected against malfunction by a conventional miniature circuit breaker 30 and a manually re-settable thermal cut-out device 31 which is positioned so that it extends from the base of the control box 1 3 into thermal contact with the upper part of tube 10 above elements 11, 1 2. The surface of tibe 10 is flattened where cut-out device 31 contracts it to ensure contact over the base area of the device, and the first element 11 is bowed upwardly adjacent the cut-out device 31 so as to bring element 11 very close or actually into contact with the flattened wall of tube 10. In Fig. 1, for clarity, the connections to elements 11, 12, air switch 29, circuit breaker 30 and the thermal cut-out device 31 are omitted.

Claims

1. A continuous electrical flow heater for liquids comprising a heat conductive flow conduit, a sheathed electrical element extending into and along said flow conduit from an entry position, said element being located in close proximity to or in thermal contact with the wall of the flow conduit at a point intermediate its entry position and the return end of the element, and a thermal cut-out device mounted on the exterior of the flow conduit adjacent where the element is in thermal contact with or in closest proximity to the wall of the flow conduit.

2. A heater as claimed in Claim 1 in which the power supply to the element is switched by a traic which is mounted on a heat sink which is adapted to be cooled by air, the operation of the triac being controlled by a zero voltage switching circuit acting in response to signals from a thermistor located in the conduit adjacent the element.

3. A heater as claimed in Claim 2 in which the triac and the zero voltage switch circuit are mounted within a control box heater adjacent the flow conduit.

4. A heater as claimed in Claim 3 including a temperature sensor provided adjacent the element, which sensor is capable of providing a signal indicative of the fluid temperture to a display remote from the control box.

5. A heater as claimed in any preceding claim wherein an additional element is provided for heating the liquid, and a first switchd means is incorporated which enables one or both elements to be connected to the power supply.

6. A heater as claimed in Claim 5 is which the control box further incorporates means for controlling the power supply to a pump which pumps water through the flow heater, said pump being adapted to pump liquid at alternatively high or low speeds, and a second switch being provided for changing the pump from one speed to another.

7. A heater as claimed in Claim 6 in which the second switch may be operated by manual means provided at a station remote from the control box.

8. A whirlpool spa bath installation incorporating a heater as claimed in Claim 7, whereby the remote display is located adja cent the remote means for operating the switch on a module within easy reach of the user of the whirlpool spa bath.

9. A whiripool spa bath installation as claimed in Claim 8 wherein the electrical supply for the remote display is at a voltage sufficiently low to avoid any risk of electrical shock to the user, and the manual means for operating the second switch to control the pump speed is connected by an air-line to an air switch located within the control box.

1 0. A whiripool spa bath installation as claimed in Claims 8 or 9 in which the control box additionally incorporates means for switching off the electrical power supply to one of the elements of the second switch is operated to increase the pump speed.