GB2293031A - Improvements relating to temperature control systems for barrier layers - Google Patents

Abstract

A barrier layer comprises a base layer A, a screed B incorporating heating elements 1 and temperature sensors 2. Above the screed B is an insulant layer C separated by a vapour seal D, and a wearing slab E. The cold area F is above the wearing slab E. It is desired that the 0{C isotherm should be kept generally at the position G indicated within the wearing slab E. The heating elements 1 will be heated individually and in sequence as necessary to maintain the temperature of each heating element within a range of +3.0 to +4.5 DEG C through circuitry controlled by the sensors 2. <IMAGE>

Description

"Improvements relatina to temDerature control systems There is a need to provide heating to act as a barrier to the Zero degree C isotherm penetrating from a cold area, (typically -2 to -200 C) such as a cold storage warehouse, liquid gas storage tank, ice skating rink or other similar areas. There is a danger of the 00C temperature isotherm causing ice to form in the subsurface, expansion resulting in physical damage to the floor, base or other part of the structure of the installation which could be in contact with water molecules.The system of using a heat barrier to prevent the passing of the 0 C isotherm is well known, by using a single energy source, (e.g. a transformer) to supply a single matching heating element controlled solely by sensed temperature and which therefore results in wasted energy in a number of cases.

According to the invention there is provided a temperature control system for a barrier layer associated with a cold area, the system comprising a number (N) of individual heating elements or linked multiples thereof, a single energy source for supplying energy to heat any one of the heating elements or multiples, sensing units for monitoring the temperature in the vicinity of each heating element or multiple, and a switching device for selectively connecting the energy source sequentially over time intervals less than 1/N to each heating element or multiple for heating thereof if the related sensing unit registers a temperature below a predetermined minimum.

By this means the output of only one energy source, e.g. a low voltage high/current transformer is routed to one of the matching heating elements or multiples in sequence determined by time and to a level determined by temperature so that the barrier temperature created by the heating elements stays within predetermined temperature limits of say +2 C to +4.5"C utilising the physical attributes of the material containing the heating elements, namely the base or other parts of the structure in contact with moisture, to heat up quickly and to cool slowly in the ratio in time of over 1:3 (for a preferred system employing three heating elements or multiples).By using time as well as temperature to control the distribution, the invention makes it possible to use only one transformer of a third of the required overall power for three heating elements or multiples instead of three transformers as in the past to achieve the same heat barrier.

The routing can be achieved by using solid state semiconductor technology which makes it possible to switch indefinitely, the low voltage/high current to each heating element or multiple in turn to control the temperature accurately. The solid state electronics have a very long life comparable to the life of the whole installation and structure.

The invention also extends to a method of controlling the temperature of a barrier layer associated with a cold area wherein a number (N) of individual heating elements or linked multiples thereof are located just above the barrier layer, a single energy source supplies energy to heat any one of the heating elements or multiples, sensing units monitor the temperature in the vicinity of each heating element or multiple, and a switching device selectively connects the energy source sequentially over time intervals less than 1/N to each heating element or multiple for heating thereof if the related sensing unit registers a temperature below a predetermined minimum.

The invention may be performed in various ways and a preferred embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a typical floor base construction for a cold area; Figure 2 illustrates a curve of temperature surrounding a heating element of a temperature control system in the base construction; and Figure 3 is a circuit diagram for components of a preferred temperature control system of this invention.

The base construction shown in Figure 1 includes a screed B incorporating heating elements 1 and temperature sensors 2, laid over a base layer A of imported filler.

Above the screed B is an insulant layer C separated by a vapour seal D, and a wearing slab E. The cold area F is above the wearing slab E. It is desired that the 0 C isotherm should be kept generally at the position G indicated within the wearing slab E.

The heating elements 1 will be heated individually and will tend to be subject to a temperature curve of the type as illustrated in Figure 2. The objective is to maintain the temperature of each heating element within a range of +3.0 to +4.5 C. The manner in which this can be achieved will now be described The system shown in Figure 3 incorporates three separate heating elements 1 (or three groups of such elements).These are a balanced compromise between the mechanical strength required to withstand the abuse they receive during their installation and integration into the base of the relevant structure, the electrical resistance required to match to a voltage below 55 volts to earth (even 42.5 volts on an E L V safe system as defined by VDE 0110) with the power dissipation per metre to give a heat dissipation per unit of area required to limit the 00C isotherm to a distance just below the point for the lowest operating temperature of the application (e.g. -18 C for a coldroom to -200 C for liquid gases) and just above the heating element itself. The weight and handling ability are also a factor in determining the length of the element.Taking all the above factors into consideration a compromise solution is found by making the heating elements from stainless steel wire of a diameter of over 2.lmm and length appropriate to the required voltage. This is covered in an inert plastics sleeve which, together with the anti-corrosion properties of the stainless steel wire, eliminate any long-term degradation of the element due to corrosion and erosion.

These heating elements can be divided or multiplied into convenient sections of three parts in order that they can be matched to the heatingjcooling ratio, of more than 1:3, of the mass of the structure incorporating these heating elements.

It is required that the temperature of the area surrounding the heating element or the actual temperature of the element itself can be measured with accuracy to better than ±0C, that the system is reliable with a long life, and that it is not affected by M.F.I. (Magnetic Field Interference) and E.R.F.I. (Electric and Radiated Field Interference). This can be achieved by using a thermocouple, where space is a premium but which has to have the added circuitry of "cold couple" stabilising to be accurate, or a thermistor or semiconductor sensor which has a higher output and which is used more generally. Alternatively the determination of the actual heating element temperature can be achieved by an extension of the timing sequencing circuitry whereby the Ohmic resistance of the heating element is sensed during the power-off part of the timing cycle and its temperature is calculated by the measure of its "change of resistance" which is then used to control the feed of the next "poweron/power-off" part of the timing sequence to the matching segment of the heating element represented by that resistance change.

This method of temperature control can also be used for smaller scale heating applications where there are less than three heating elements employed, such as coldstore door heating, pipe trace heating, drain heating and small soil heating systems.

This system can incorporate a solid state high current switch which, only by its recent availability, enables a very long life to be achieved if sensible guide lines are followed. The present availability of devices to carry out the function of switching is limited to a choice of a "Back to Back" or Inverse Parallel SCR (Silicon controlled rectifier), a Programmable Unijunctfon Transistor (P.U.T.) or a TRIAC (TRIode ACsemiconductor).

The circuit shown in Figure 3 includes solid state electronic devices 4 which have the capability of detecting the information output of temperature sensors 2 and which can compare that information to a preset voltage representing the desired temperature control points and which produce switching signals to matching high current switches 3. The devices 4 are also controlled by a system timer 5 which energises each sensor 4 in turn for a predetermined ON/OFF time to match the temperature operating ratio as shown in Figure 2. The timing cycle ensures that each temperature sensor 2 is monitored and the appropriate switch 3 is operated as required within an ON to OFF ratio of less than 1:3. The system causes the selective operation of the three heating elements 1 (or multiples thereof) to be operated by one low voltage/high current power source (in general an electrical transformer 7 operating from the general mains supply 8 as required), to maintain the heated area within close temperature limits, as chosen by the user. A power supply 6 provides the required AC and DC voltages and current to operate the solid state electronic circuits. The whole system can be multiplied or divided up to suit any desired area to be heated and still utilise the savings made by the 1:3 operating characteristic of the system.

Claims (5)

1. A temperature control system for a barrier layer associated with a cold area, the system comprising a number (N) of individual heating elements or linked multiples thereof, a single energy source for supplying energy to heat any one of the heating elements or multiples, sensing units for monitoring the temperaturs in the vicinity of each heating element or multiple, and a switching device for selectively connecting the energy source sequentially over time intervals less than 1/N to each heating element or multiple for heating thereof if the related sensing unit registers a temperature below a predetermined minimum.

2. A system according to Claim 1, wherein the number of heating elements or multiples thereof is three.

3. A system according to Claim 1 or Claim 2 which incorporates solid state semiconductor technology enabling the low voltage/high current to be switched to each heating element or multiple in turn to control the temperature.

4. A method of controlling the temperature of a barrier layer associated with a cold area wherein a number (N) of individual heating elements or linked multiples thereof are located just above the barrier layer, a single energy source supplies energy to heat any one of the heating elements or multiples, sensing units monitor the temperature in the vicinity of each heating element or multiple, and a switching device selectively connects the energy source sequentially over time intervals less than 1/N to each heating element or multiple for heating thereof if the related sensing unit registers a temperature below a predetermined minimum.

5. A temperature control system or a method of controlling the temperature of a barrier layer associated with a cold area and substantially as herein described with reference to the accompanying drawings.