GB2143343A

GB2143343A - Thermostatically controlled mixer

Info

Publication number: GB2143343A
Application number: GB08318899A
Authority: GB
Inventors: Kenneth Leopold Ellington; Horst Heimel; Andrew Stephen Lefley; Harro Kiendl
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-07-13
Filing date: 1983-07-13
Publication date: 1985-02-06
Also published as: GB8318899D0; DE3425445A1; DE3425445C2

Abstract

In a thermostatically controlled mixer valve the temperature TM of the valve (4) outlet is sensed (5) and compared with a pre-set value TSET (6). The difference signal e is applied via a temperature controller (1) to a motor (2) driving the valve (4) through a gearbox (3). The arrangement includes a secondary feedback loop wherein additional control signals are derived to compensate for mechanical or electromechanical changes affecting the control loop, such as gearbox hysterisis, motor speed characteristics, valve calcification etc. Typically an additional control signal is derived (7) to indicate valve position and is fed back via a position controller (8). Various parameters can be used for the additional control signal. <IMAGE>

Description

SPECIFICATION Device and means for controlling air conditioning plants in buildings The invention relates to a device and means for controlling air-conditioning plants in buildings.

In order to air-condition buildings or environments, plants are normally designed so that there is a centralised unit consisting of a refrigerating group which cools a refrigerating mixture which is then sent, via an appropriate circuit, to local airconditioning units situated in the individual rooms.

Air-conditioning of the rooms is controlled separately by means of the local units and, if necessary, by means of the central refrigerating unit.

According to several designs, the refrigerating group is practically always connected and the liquid is sent to the various local air-conditioning units at a temperature which is practically independent of the different heat loads in the various rooms.

Plants designed in this manner, even if they are equipped with devices for adjusting the temperature of the mixture which is sent to the local units, can never operate under optimum conditions in view of the variability of the heat load.

The object of the present invention is to provide a controlled plant which is able to adapt itself to the variations in heat load in the different rooms.

According to the present invention, there is provided a device and means for controlling an airconditioning plant in buildings, of the type comprising mainly: - a refrigerating group of variable power, - a circuit for distributing the refrigerated mixture, - a series of ventilated air-conditioning units which use cooling water and are equipped with a motorized three-way valve for distributing the flow of the water through the heat exchanger, characterized in that each local unit is controlled by a peripheral control system which pilots the three-way valve and which also sends signals to a central control group according to the ambient temperature, in such a way that the peripheral controller on the local unit which is in the room where there are the heaviest load conditions is temporarily the "pilot" which controls as a result of this fact, by means of the central control group, connection of refrigerating stages in a manner dependent on the divergence between the ambient temperature and the desired temperature, in such a way that the plant can operate with a refrigerated mixture at the highest temperature compatible with the least favoured room and hence with the smallest possible power consumed by the refrigerating group.

As a result, there is provided a control device which progressively takes as a reference the condition of the room which is least favoured from the point of view of heat or of the room in which the ambient temperature is furthermost removed from the nominal pre-chosen value on account of the heat load at that particular moment.

There is also provided a control device by means of which it is possible to connect power stages which vary and progressively increase or decrease in accordance with the condition of the least favoured room and the general condition of the entire plant.

The plant is controlled both locally and in the refrigerating group in such a way that the refrigerated mixture is sent in any case at the highest temperature compatible with the condition of the least favoured room.

Other characteristics and other advantages of the invention will become more apparent from the detailed description of a preferred embodiment given by way of an indicative but non-limiting example and illustrated in the attached drawing plates in which: Figure 1 shows the diagram of a local airconditioning unit with its main components, Figure 2 shows a plant consisting of several local units and a refrigerating group, Figure 3 shows the basic characteristic curve for connection of the stages, via the central controller, obtained for the refrigerating group as a function of the temperature of the pilot room, Figure 4 shows the basic characteristic curve obtained for regulation of the three-way valve as a function of the ambient temperature, Figure 5shows a graph of the changes in temperature in three hypothetical rooms, Figure 6 shows the progression with time of the difference between the actual temperature and the nominal temperature, the degree of opening of the three-way valves, and the number of refrigerating power stages connected, for the same rooms as in Figure 5, Figures 7and 8 show the special case of a plant for a single room to be air-conditioned.

A description follows, with reference to the Figures mentioned, of a plant which concerns a plurality of rooms, each of which is provided with a local air-conditioning unit, in accordance with the diagram shown in Figure 1 which illustrates the air-conditioner 1 complete with the fan 2 and a heat exchanger 3 through which passes a flow of refrigerated mixture distributed by the system of delivery tubes 4 and return tubes 5; the flow of the refrigerated mixture through the heat exchanger is regulated by the actuation of a three-way valve 6 driven by a servomotor 7 which is piloted by a control system 8, which can be defined as a peripheral unit, according to the ambient temperature detected by the sensing element 9.

The peripheral control system 8 also transmits to the central controller information concerning the desired temperature, the actual ambient temperature and, in particular, the difference between these two temperatures.

Figure 2 shows the schematic structure of the control device for a hypothetical plant with four rooms, each of which is provided with a local air-conditioning unit.

Each of these four rooms, which are denoted by the letters A, B, C and D, has an air-conditioning unit of the type shown in Figure 1 with a sensor which is used to detect the ambient heat conditions and which is connected to a peripheral control system, denoted by 10,11, 12 and 13 for the various rooms, which system actuates the three-way valve and transmits the information concerning the ambient conditions to the central controller 14 which, in turn, actuates the refrigerating group 15.

In the example shown here, this refrigerating group 15 consists of four refrigerating machines which can be used in accordance with the power required, as will be described below.

The temperature which is established in each of the rooms is the result of the balance between the heat load to be removed and the refrigerating power of the local air-conditioning unit, this power being affected by the temperature of the refrigerated mixture.

Thus, if with the three-way valve completely open and hence a maximum flow of regrigerated mixture through the exchanger and therefore maximum heat-exchange efficiency, for at least one of the local units, the head load to be removed exceeds the refrigerating power, it is necessary for the temperature of the refrigerated mixture to be lowered until the thermal equilibrium is re-established at the required ambient temperature.

The graph in Figure 3 shows the basic characteristic curve for connection of the power of the refrigerating group as a function of the temperature of the pilot room or of the room in which, at that particular moment, the temperature is furthest removed from the pre-chosen value " tSET "; in this graph the following quantities also appear : l'tMAX I ,which corresponds to the maximum permissible ambient temperature, "At refrigerating plant ,which corresponds to the proportional band of the characteristic curve for regulation.

As can be seen, when the temperature of the room which has been chosen as the " pilot " of the central control group ( denoted by 14 in Figure 2 ) varies below the required temperature "t5ET", since the requirement of the room has been satisfied, the refrigerating group is not in operation.

When the temperature of the pilot room tends to rise, it can be seen in the case shown that the four stages, denoted respectively by 16, 17, 18 and 19, are connected, which in fact corresponds to the connection of four separate refrigerating machines or to the distribution of the power of a single refrigerating machine.

The variation in stages of the refrigerating power can be replaced by a linear variation as a function of the characteristic curve indicated by 20, which can be obtained by means of a special structure of a refrigerating group with variable power.

The function of the three-way valve in each local unit can be seen when, as a result of an excess of refrigerating power with respect to the requirements of the room, the ambient temperature falls below the desired value, thereby making it necessary to reduce the refrigerating effect by distribution of the flow through the exchanger.

In Figure 4, which shows the basic characteristic curveforthe opening ofthethree-wayvalvewhich is located in each of the local units, obtained from the peripheral control system as a function of the ambient temperature, the following parameters appear: "tNOM "is equal to the value of the ambient temperature above which it is not necessary to distribute the flow of the refrigerated mixture, "tMIN " corresponds to the minimum permissible temperature, "At valve" corresponds to the proportional band of the characteristic curve for regulation.

Below the temperature "tNOM "distribution of the flow commences until, following a further decrease in the ambient temperature to " tMXN ",the valve completely excludes the exchanger.

In optimum operating conditions, the three-way valve of the pilot room should be completely open before connection of the refrigerating group stages commences ; at the same time, it is generally appropriate that the temperature interval between " tMIN "and " tMAX " should be as small as possible.

As a result, if the characteristic curves shown in Figures 3 and 4 are considered, the value "tsET"Of Figure 3 should theoretically coincide with the value "tNOM " of Figure 4.

In any case, it can prove advantageous to give " tSET " a different value from "tNOM "and, more particularly, a value less than that of" tNOM will be chosen for" t5ET" if one wishes to bring forward the moment at which the refrigerating group intervenes, with respect to a temperature change which is occurring, or it is possible to choose a value of "tSET " which is greater than the value of" tNOM "with respect to a required delay time which is capable of reducing the effects of momentary variations which subsequently do not last with time.

It is important to point out that, although, for the sake of simplicity, the characteristic curves for regulation as a function of the ambient temperature, as shown in Figures 3 and 4, are linear and of the type P ( purely proportional ) ,they could, if it were more convenient, be of the type P + I ( proportional + integral ), P + I + D ( proportional + integral + derivative ) or of any other type known and currently applied in the field of regulation.

Furthermore, connection and disconnection of the refrigerating stages, in addition to conforming to the characteristic curve for regulation shown in Figure 3, can be subjected to timing so as to avoid, in the case of cyclical thermal variations which occur too suddenly in the air-conditioned rooms, connection and disconnection phenomena at intervals which are dangerous for the refrigeration compressors.

Figure 5 shows the hypothetical progression of the temperature in three rooms ; it should be noted that the progression of the temperature is linked, for each of the rooms, both to the state of variation of the heat loads which occur in each room and to the function of the temperature of the refrigerating mixture which circulates in the plant. Furthermore, it has been assumed in this diagram that the temperature values " tSET " and "tNOM "ofthe individual peripheral control systems ( using the notation of Figures 3 and 4) have been adjusted so as to coincide. In each of the rooms a different value has been chosen for " tSET " = " tNoM ". Finally, although the values of " art valve "and" At refrigerating plant "can be different in order to satisfy more effectively the requirements of the regulation process, it has been assumed in Figure 5, for simplicity of explanation and illustration, that these values are the same in the various rooms.

Figure 6 (top graph) shows the progression not of the actual temperature but of the difference in the temperature in each room with respect to its own value "tNoM" these differences are also indicated for three moments chosen at random, in Figure 5, by thesymbols"At,At6,Atc".

Figure 6 (middle graph) shows the progression of the degree of opening of the three-way valves for the three local units resulting from the progression of the ambient heat profile, and (bottom graph) the number of refrigerating stages connected, as a function of time ; these four diagrams are based on the same time-scale as the progression of the temperature differences.

The device for controlling the plant operates in the following manner: Up to the moment Si, the three rooms have a temperature which is less than their appropriate value "tNoM "; II ; as a result, the peripheral controller will pilot the three-way valve of each local unit in a totally closed position (as in room A) or in a partially closed position (as in rooms B and C ).

At the moment Si the temperature of room B exceeds the pre-chosen nominal value and at this moment the central control device chooses this room as the pilot room and, in accordance with the temperature which gradually rises, it inserts refrigerating stages which in fact increasingly lower the temperature of the mixture circulating in the plant.

Likewise, at the moment S2, the temperature of room A exceeds its own nominal value, but for as long as the difference between the momentary temperature and the appropriate nominal value of room A does not exceed the corresponding temperature difference in room B, no change occurs and room B is still the pilot room.

Then, from moment S3, room A becomes the pilot room since, as can be seen, it is in the least favourable thermal condition so great is the difference between the actual temperature and the prechosen nominal temperature.

Likewise, at the moment S4, the temperature of room C exceeds the value of its own nominal temperature, but this room C will only become the pilot room later on, at moment S5, when the control device intervenes.

At the moment S6, again, the temperature of room B exceeds its own nominal value until moment S7 is reached, but since this difference is less than that which exists during the same period in room C, it is room C again which is considered to be the " pilot ".

At the moment S8, all of the rooms are at an effective temperature which is lower than their own nominal temperatures, thereby excluding the refrigerating group.

According to what has been described and illustrated, it will be noted that a basic feature of the device is that it comprises a controller which determines, by means of comparison, the room which gradually assumes a temperature furthermost removed from its own nominal temperature.

This room is thus chosen as the " pilot " for the plant and it is precisely its temperature variation which determines the gradual connection or discon nection of the various refrigerating machines which constitute the refrigerating group used to cool the mixture.

By means of the three-way valve which is located in each local unit, it is further possible to modulate the flow of cold mixture in the exchanger of the air-conditioner in such a way that its refrigerating power can be adapted to the requirements of the rooms in those cases where the temperature of the mixture is colder than is necessary for this room.

By using the signal from the room which gradually assumes the least favourable thermal conditions as the signal for controlling and actuating the refrigerating group, it is possible, on the one hand, to air-condition all of the rooms and keep them in the required temperature conditions and, on the other hand, to use always a minimum of refrigerating power which corresponds to supplying the plant with a refrigerated mixture at the maximum admissible temperature for the conditions existing in the rooms to be air-conditioned.

It should be noted that a!though a description has been given of a plant consisting of local units each located in a different room, the principle remains equally valid for local units intended to air-condition different areas of one large room, each being independent of the others ; furthermore, the principle remains equally valid in the cases of rooms where at least one of them is provided with two or several local units subject to the same peripheral controller and operating in parallel.

If one also wishes to control the value of the moisture content in each of the rooms by means of local air-conditioning units, the latter must have means for creating vapour ( humidification ) and for condensing the water vapour (dehumidification) in order to obtain this latter phenomenon, the temperature of the refrigerated mixture is normally lowered, thus also lowering the surface temperature of the heat exchanger below the dew point of the ambient air, thereby causing the desired condensation of the existing vapour in the form of moisture.

In the case where at least one of the rooms requires dehumidification, the peripheral controller of the corresponding local unit, which has a humidity sensor, sends the information to the central controller which, ignoring all of the functions described above, brings the temperature of the refrigerated mixture to a predetermined value (which is able to guarantee an adequate water vapour condensation rate ) for the time required to bring the relative humidity within the required limits.

As the temperature of the mixture in this case may probably be too cold, the thermal equilibrium of the rooms which are not affected by the request for dehumidification is guaranteed by the local controller with the partial closure of the three-way valve of the air-conditioning unit, whereas in the rooms subject to dehumidification, in which the three-way valve must remain completely open in order to obtain the maximum possible flow of refrigerated mixture and hence the lowest possible surface temperature of the exchanger, the heat balance can be obtained by means of further heating or by a decrease in the heat-exchange capacity, for example by reducing the air flow or by reducing, as a result of distribution, the area affected by the heat exchange.

The control system described above is used in particular in the case of an air-conditioning plant intended for a single room and thus comprising a single local unit or a single group of local units operating in parallel.

It is obvious that the local air-conditioning units are not provided with a three-way valve, which has the function of distributing the flow of refrigerated mixture in the case where its temperature is less than that required, since the control system will adapt the refrigerating power of the plant to the needs of the room in accordance with a characteristic curve similar to the one shown in Figure 3.

The skeleton diagram of the plant thus becomes the diagram shown in Figure 7, or the diagram shown in Figure 8 in the case where, for the sake of simplicity, the central control system also includes the peripheral controller, the same symbols and the same reference numbers being used as those in Figures 1 and 2.

In this application, in the case of a request for dehumidification, the temperature of the refrigerated mixture is lowered to the predetermined value for the time required and, if necessary, further heating is performed or the heat-exchange capacity is reduced in order to keep the ambient temperature within the permissible limits.

According to what has been described and illustrated, it can be seen that all of the objects which were proposed are achieved and that the control devices and means can be used easily and advantageously in plants having different characteristics.

Starting from the same inventive idea, it is obvious that the same control devices and means can also be installed in heating plants with the aim of introducing into the circuit a liquid at the lowest possible temperature, compatible with the condition of the room which from time to time assumes the least favourable thermal conditions.

It goes without saying that the component parts can be chosen, as required, to suit needs and circumstances.

Claims

1. Device and means for controlling an airconditioning plant in buildings, of the type comprising mainly: A- a refrigerating group of variable power, B- a circuit for distributing the refrigerated mixture, C-a series of ventilated air-conditioning units which use cooling water and are equipped with a motorized three-way valve for distributing the flow of the water through the heat exchanger, characterized in that each local unit is controlled by a peripheral control system which pilots the three-way valve and which also sends signals to a central control group according to the ambient temperature, in such a way that the peripheral controller on the local unit which is in the room where there are the heaviest load conditions is temporarily the " pilot " which controls as a result of this fact, by means of the central control group, connection of refrigerating stages in a manner dependent on the divergence between the ambient temperature and the desired temperature, in such a way that the plant can operate with a refrigerated mixture at the highest temperature compatible with the least favoured room and hence with the smallest possible power consumed by the refrigerating group.

2. Control device and means according to claim 1, characterised in that the said central control group pilots a refrigerating (or heating ) group of variable power, in a continuous manner or variably in stages.

3. Control device and means according to one of claims 1 or 2, characterised in that the control group choses from the signals coming from the various sensors of the rooms that signal which indicates an ambient temperature which is furthermost removed from the chosen nominal temperature, the said signal and hence the said room thus becoming " pilots " for connection of the refrigerating (or thermal ) power.

4. Control device and means according to claim 1, characterised in that the said control group keeps a room as the " pilot " until another room assumes less favourable conditions or until the temperature of another room has a greater divergence with respect to its own reference temperature.

5. Control device and means according to claim 1, characterised in that the three-way valve of each local unit is piloted by the said sensor of its own room and operates as a modulator of the flow of refrigerating mixture through the exchanger of the local unit.

6. Control device and means according to claim 1, characterised in that, when applied to a heating plant, the heating group is of the type having power which can be varied continuously or in stages.

7. Control device and means according to any one of the preceding claim, characterised in that in the case of a heating plant the hot liquid is progressively controlled by the room which assumes the least favourable thermal conditions, allowing its temperature, nevertheless, to be as low as possible.

8. Control device and means according to any one of the preceding claims, characterised in that in the case of an air-conditioning plant for a single room the three-way valve in the local units is excluded and the control system adapts the power of the plant to the requirements of the room.

9. Apparatus for controlling an air-conditioning plant comprising: refrigerating or heating means of variable power; a circuit for distributing a circulating liquid from the refrigerating or heating means; a series of ventilated air-conditioning units each having a heat exchanger connected in the circuit and a three-way valve for distributing the flow of the circulating liquid either through the heat exchanger or through a by-bass circuit or both; and characterized in that each air-conditioning unit is controlled by a peripheral controller which pilots the three-way valve and which also sends signals indicative of the ambient temperature to a central control group, the centrol control group being adapted to determine which one of the peripheral controls signals an ambient temperature differing the most from a predetermined nominal temperature unique to that controller, and to vary the power of the refrigerating or heating means as a predetermined function of the said temperature difference.

10. Control device substantially as hereinbefore described with reference to the accompanying drawings.