GB2389890A

GB2389890A - Flow boiler control system

Info

Publication number: GB2389890A
Application number: GB0209916A
Authority: GB
Inventors: Robin Wood
Original assignee: Electroheat PLC; ELECTROHEAT PLC
Current assignee: Electroheat PLC; ELECTROHEAT PLC
Priority date: 2002-04-30
Filing date: 2002-04-30
Publication date: 2003-12-24
Anticipated expiration: 2022-04-30
Also published as: GB2389890B8; GB2389890B; GB0209916D0; GB2389890A8

Abstract

A control system for an electric flow boiler comprises a sensor system for measuring the rate of increase of temperature of fluid flowing through the boiler and means to reduce or remove the electrical power for heating should the rate of increase of temperature exceed a predetermined limit. The control of electric power includes varying the electric current, or activation of different numbers of heating elements. Before heating power is applied, a sensor detects the presence of water or other heating fluid in the system by applying a high power pre-heater to the system, a high rate of temperature rise being indicative of the presence of air, and a temperature operated switch, in contact with the boiler casing, being used in place of a comparator system. The pre-heater can be one of a plurality of main heater elements which is activated for a short time (50 to 60 seconds) and then deactivated for a similar period of time. If air is present, as indicated by excessive rise in temperature, an alarm and a safety system are activated.

Description

- 1 FLOW BOILER CONTROL SYSTEMS

This invention relates to control systems for electric flow boilers and in particular to systems which ensure safe operation in the event of malfunction of supplies or components. Flow boilers comprise an enclosed heating element over which water or other thermal transfer fluid flows continuously removing thermal energy from the heater. The fluid flow and the energy supplied to the heating element is controlled so a to give a desired temperature increase from over that of the fluid when it enters the boiler. With a constant power input the temperature rise is dependent on the fluid flow rate and in the event of a reduction in this rate or the fluid flow ceasing the temperature can rise to dangerous levels causing boiling and rupture of the boiler and its associated pipe work. For this reason flow boilers require safety systems that ensure that power to the heating element id reduced or removed in the e event of flow failure.

The present invention provides and improved system for monitoring the fluid flow and controlling the heating element to ensure safe operating conditions at all times.

According to the present invention there is provided a flow boiler having safety systems including at least one sensor system for measuring the rate of increase of temperature of the fluid flowing through the boiler and control means responsive to the sensor system which may reduce or remove the power supplied to the boiler in the even that the rate of rise of temperature exceeds a predetermined limit.

Control of the energy input into the heater element for the water may be carried out in various ways. The current supply may be varied over a wide range using known solid state power control systems or a series of elements may be used in which the number of heating elements activated depends upon the

l l 2 - energy input required. The latter system only requires on/off control devices rather that proportional control devices.

The input temperature of the fluid, usually water, in a closed circulating system, such as a central heating system, will depend on how long the system has been operating and the ambient temperature. When first switched on the water may require a high heat input to raise it to the desired exit temperature for the system. After the circulating system has been operating for some hours a more or less steady state will -

be reached with water entering the boiler at the ambient temperature of the area being heated. The amount of heat energy required by the circulating water will in consequence be reduced. A further factor which will alter the amount of heat energy required can be a reduction in the circulating volume of water caused by the operation of thermostatic valves on radiators served by the system. These valves restrict or cut off the flow of water to the associated radiator. -

To overcome the need to switch large amounts of power, e.g. 12 kW, to maintain the desired water temperature a series of heating elements can be used which are successively disconnected from the system as the water reaches its operating temperature. In this manner a desired operating temperature may be maintained by switching a considerably lower load, e.g. -

3 kW, than that required for the start-up of the heating system. A signal to operate the change from using all the heating elements to fewer or even a single element can be obtained by measuring the rate of temperature rise as the water flows through the system. If the rate is rapid then one or -

more heaters can be disconnected until the rate of rise of temperature drops to a predetermined point.

It is a prerequisite of all flow boiler systems that a -

heat transfer fluid, e.g. water, is present before the system is allowed to operate. Were a heating element to be switched

- 3 on in the absence of fluid the temperature rise would be rapid leading to catastrophic failure. Before energizing any heating elements a sensor system is required to ensure that water or other heat transfer fluid is present in the system.

One sensing system that has been used comprises a low power pre-heater fitted to the pipe work prior to the main heating element. The temperature rise caused by the pre-heater is measured and in the event of air being present more rapid rise in temperature caused by the pre-heater is used to inactivate the main heater. The detection of the changes in the rate of temperature rise uses a comparator system and tends to lack sensitivity. Consequently it produces false signals due to its poor signal to noise ratio.

In has been found that using a high power pre-heater a temperature operated switch may be used in place of a comparator. This is a simple and reliable device which can rapidly detect a rise in temperature caused by the absence of water or other heat transfer fluid. The preheating can be achieved by using one of the main heating elements. The element is energised for a short period of time, e.y. 50 to 60 seconds, and then switched off for a similar period of time.

The temperature switch for detecting the rise in temperature caused by operation of the heater can be in thermal contact with the boiler casing. In the event that water is absent the rapid rise in temperature of the casing will cause the alarm and safety systems to operate. The system reduces the number and complexity of the components involved in the prior system.

Claims

( - 4 - CLAIMS

1. A control system for an electric flow boiler comprising: electrical heating means, operative to heat fluid flowing through the boiler; sensor means, operative to measuring the rate of increase of the temperature of the fluid flowing through the boiler; and control means, responsive to said sensor means, operative to control the amount of electrical power employed for heating the fluid for the rate of increase of the temperature the fluid not to exceed a predetermined limit.

2. A control system, according to claim 1, wherein said sensor means comprises a temperature sensor and a comparator system.

3. A system, according to claim 1 or claim 2, wherein said control means comprises means to vary the amount of electric current flowing through said heating means.

4. A system, according to claim 1, 2 or 3, wherein said heating means comprises one or more heating elements, and wherein said control means is operative to control the number of said one or more heating elements employed at any time to heat the fluid.

5. A system, according to claim 2, wherein said one or more heating elements comprises a plurality of heating elements.

6. A system, according to any one of the preceding claims, further comprising air detection means, operative to detect the presence of air in the boiler system, operation of said electric flow boiler being inhibited if air is detected.

7. A system, according to claim 4, wherein said air detection means is operative to activate an alarm if air is detected.

f - 5

8. A system, according to claim 6 or claim 7, wherein said air detection means comprises: pre-heating means, operative to apply a finite amount of heat to the fluid; and a temperature operated switch, said temperature operated switch being activated if the temperature rise of the fluid, in response to said finite amount of heat, exceeds a predetermined limit.

9. A system, according to claim 8 when claim 8 is dependent upon claim 4 or claim 5, wherein said pre-heating means comprises: one of said one or more heating elements; and operation time control means, operative to control the amount of time for which said one of said one or more heating elements is operated.

10. A system, according to claim 8 or claim 9, wherein said temperature operated switch is in contact with the casing of the boiler.

11. A system, substantially as described.