GB2154402A - Fluid heater - Google Patents

Abstract

Fluid is heated by passing it through an electrically conductive pipe coil 18, 19, 20 and subjecting the coil to a cyclically changing magnetic flux. <IMAGE>

Description

SPECIFICATION Method and apparatus for heating a fluid The present invention relates to the heating of fluids using electrical energy and is primarily concerned with the heating of liquids.

According to a first aspect of the invention, there is provided a method of heating a fluid wherein the fluid is caused to flow in contactwith an electrical conductor and the conductor is subjected to a cyclicly changing magnetic flux to generate in the conductor heat which is imparted to the fluid.

Electrical energy is commonly used for heating fluids by means of immersion heaters. During use of an immersion heater, a convective current offluid is usually established but the rate of flow over the outer surface ofthe immersion heater is relatively low. In a method in accordance with the present invention, flow of the fluid is usually caused buy a pump or by a difference in levels and a relatively high flow rate can be used. The flow rate is selected according to the electrical power available for conversion to heat in the fluid and the required temperature rise ofthe fluid.

In an immersion heater, the element th rough which the electric current is conducted to generate heat is usually enclosed in a metal case and separated from the case by a layer of electrically insulating material.

Thus, heat generated in the conductor must be conducted through the electrical insulator and through the case to the fluid which is to be heated. In a method in accordance with the present invention, the fluid which isto be heated is in direct contact with the electrical conductor, there being no intervening electrical insulation, and heat can be transferred to the fluid conveniently at a high rate.

The fluid is preferably confined to a flow path by boundary surfaces, with substantial flow occurring at all positions between the boundary surfaces.

According to a second aspect of the invention, there is provided apparatus comprising a core of a material with a relatively high magnetic permeability and primary and secondarywindings of electrically conductive material on the core, wherein the secondary winding istubularand defines an inlet and an outlet forfluid.

Apparatus in accordancewith the second aspect of the invention can be used in a method according to the first aspect by causing the fluid to flow through the interior of the secondary winding from the inlet to the outlet and causing an alternating electric current two flow th rough the primary winding.

The secondary winding preferably has a small numberofturns, relative to the numberofturns in the primary winding, and is preferably disposed outside the primary winding. Adjacent turns of the secondary winding are preferably spaced from each other.

An example of apparatus in accordance with the second aspect ofthe invention and which is used in a method according to the first aspect ofthe invention will now be described, with reference to the accompanying drawing,which represents diagrammatically an elevation ofthe apparatus.

The apparatus is arranged generally in the manner of a transformer having primary and secondary windings. The apparatus shown in the drawing comprises a core 10 of soft iron or other material having a high magnetic permeability and which may be laminated. In the particular example illustrates, the core has three integral limbs carrying respective primary windings 11,12 and 13. The primary windings are formed of insulated copper or other metal wire and may be arranged in the same manner as the primary windings of known transformers. The primary wind ingshaveterminals 14to l7forconnectiontoa three-phase supply.

The apparatus includes three secondary windings 18, 19 and 20 which embrace respective ones of the primary windings 11,12 and 13. Corresponding ends of the secondary winding are connected together electrically atjunctions 21 and 22 respectively. Each of the three secondary windings preferably comprises not more than ten turns: whereas each primary winding preferably comprises more than fifty turns.

More preferably, the number of turns in each primary winding is at least twenty times the number of turns in the corresponding secondary winding. There is between the primary and secondary windings an electri cally conductive screen having a terminal (not shown) for connection to earth.

The secondarywindings 18,19 and 20 are formed of electrically conductive tube, preferably copper tube, which define a fluid inlet 23 at the junction 22 and further defines a fluid outlet 24 at the junction 21. In the examples illustrated, the secondary windings are connected for flow of fluid through these windings in parallel with one another. However, an alternative arrangement could be used.

The apparatus illustrated in the accompanying drawing is intended primarily for heating water. Water which is to be heated is caused to flow, by means of an extraneous pump (not shown), into the secondary windings at the inlet 23 and from the secondary windings at the outlet 24. Pressure or flow-sensing means (not shown) are associated with the water flow path, for example at the inlet and the outlet, to provide a signal indicative of whether or not there is substan tial flow ofwaterthrough the secondarywindings.

When flow through the secondary windings has been established, an alternating current power supply is connected to the terminals 14 to 17. Typically, the power supply has a frequency of fifty cycles per second, although a higher frequency, for example up to 1000 cycles per second, may be used. It will be noted that it is not necessaryto use a high frequency alternating current source. A source with a frequency of up to 250 cycles per second is convenient.

The cyclicly changing magnetic flux established in the core 10 by the current flow in the primary windings 11,12 and 13 induces currentflowinthe secondary windings. This latter current flow generates heat in the secondary windings which is conducted directly to the water flowing in the secondary windings. The water flow rate can be selected to avoid a substantial boundary layer at the internal surface of the tubular secondary windings so that there is no significant impediment to flow of heatfromthe secondary winding to the water. A high rate of heat transfer can be achieved with a high efficiency, generally greater than 90%.

The velocity of the water in the secondary windings is in excess of 10 metre per minute, preferably at least 20 metre per minute, and more preferably at least 40 metre per minute. The secondarywindings define an elongated flow path, the aggregate cross-sectional dimensions of which are small, as compared with the length ofthe path between the inlet and the outlet.

Hot water emerges from the outlet 24 almost immediately the electrical power source is connected totheterminals 14to 17. There is then a continuous supply of hot water until the power source is disconnected.Thepressureorflow-sensing means is arranged to disconnect the power source from the primarywindings in the event offlow of waterthrough the secondarywindings ceasing orfalling belowa threshold rate.

it will be noted that both the primaru windings and the secondary windings are fixed with respect to the core 10.

Dimensions and other characteristics oftwo alternative examples of apparatus as illustrated in the drawing are as follows: Example 1 Example 2 Rating kVA (Nominal) 30 50 Phases 3 3 Primary Volts 380 380 Primary Amps 45.6 76.0 Primary Turns 117 94 Primary Windings Cu (solid) 7mm x 4mm 5.89mm dia.

Secondary Turns 3.5 3.5 Secondary Winding Cu. (tube) 6.5mm OD x 5mm ID 6.5mm OD x 5mm ID Core Height 490 430 Core Width 430 690 Core Thickness 90 130 It will be noted that the current flow and rate of heating are independent ofthe nature of the water or other fluid which flows through the secondary windings. In these examples, the velocity of the water in the tubes ofthe secondary windings is in excess of 200 metres per minute.

Whilst the apparatus illustrated in the accompany- ing drawing is suitable for use with a three-phase electrical supply, the apparatus may readily be modified to operate on a single-phase supply. Use of electrical power at 50 kVA, which would usually be from a three-phase supply, enables water to be delivered at a temperature of at least 7G"C and at a rate of 15 litres per minute, using a water supply at ambienttemperature. Highertemperatures can be attained by reducing the water flow rate and higher water rates can be used if a lowertemperature output is acceptable.

Thefeatures disclosed in the foregoing description, or the following claims, orthe accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (11)

CLAIMS:

1. A method of heating a fluid wherein the fluid is caused to flow in contact with an electrical conductor and the conductor is subjected to a cyclicly changing magnetic flux to generate in the conductor heat which is imparted to thefluid.

2. A method according to Claim 1 wherein the magnetic flux changes with a frequency not exceeding 1000 cycles per second.

3. A method according to Claim 1 or Claim 2 wherein the the electrical conductor is a tubular winding around a core of magentic material.

4. A method according to any preceding claim wherein the velocity ofthe fluid adjacent to the conductor is at least 10 metre per minute.

5. Apparatus comprising a core of material with a relatively high magnetic permeability and primary and secondarywindings of electricallyconductive material on the core, wherein the secondary winding istubularand defines an inlet and an outletforfluid.

6. Apparatus according to Claim 5 wherein said secondary winding informed ofcoppertube.

7. Apparatus according to Claim 5 or Claim 6 wherein said secondary winding has substantially fewerturns that does the primary winding.

8. Apparatus according to anyone of Claims 5to7 wherein the secondary winding is disposed outside the primary winding.

9. Apparatus substantially as herein described with reference to the accompanying drawing.

10. A method substantially as herein described with reference to the accompanying drawing of heating a liquid.

11. Any novel feature or novel combination of features disclosed herein and/or in the accompanying drawing.