CA1175026A

CA1175026A - System for controlling the heating of dispensable water to be stored in a storage tank

Info

Publication number: CA1175026A
Application number: CA000374897A
Authority: CA
Inventors: Ferdinand Hartmann
Original assignee: AUSTRIA EMAIL-EHT AG fur ELEKTRO- HEIZUNGSTECHNIK
Current assignee: AUSTRIA EMAIL-EHT AG fur ELEKTRO- HEIZUNGSTECHNIK
Priority date: 1980-04-10
Filing date: 1981-04-07
Publication date: 1984-09-25
Also published as: ES501206A0; DE3169328D1; ES8204135A1; EP0038318B2; EP0038318A1; EP0038318B1

Abstract

ABSTRACT OF THE DISCLOSURE

The system serves to control the heating of dispensable water, which is to be stored in a storage tank (1). A heat exchanger (9), which is separate from the storage tank, is connected in primary and secondary fluid circuits. The water to be heated is taken from the bottom of the storage tank (1) and is handled by a thermostat-controlled charging pump (10) to flow in the secondary fluid circuit through the heat exchanger (9) and to return as hot water to the top of the storage tank. The control system comprises a first thermostat (T1), which extends into the stor-age tank (1) preferably in the middle portion of its height, and a second thermostat (T2), which extends into the secondary fluid circuit between the cold water zone at the bottom of the storage tank (1) and the point where the secondary fluid circuit enters the heat exchanger (9), The first thermostat (T1) serves to start the charging pump (10) to initiate the charging operat-ion. The second thermostat (T2) serves to de-energize the charging pump to terminate the charging operation. The first thermostat (T1) closes an electric circuit when sensing a water temperature below a predetermined lower limit. The second thermostat opens an electric circuit when sensing a water temperature above a predetermined upper limit.

Description

~ S~26 SUMMARY OF INVENTION

A system for heating and storing dispensable water with a heat exchanger and a storage tank. The heat exchanger is separate from the storage tank. The secondary circuit of the heat eschangers comprises a changing pump and the storage tank. A control system is pro~ided.

~s~

This invention relates to a system for controlling the heating of dispensable water to be stored in a storage tank, in an arrangement in which the water is heated by means of a heat exchanger~ which is separate from the storage tank and connected in primary and secondary fluid circuits. The water to be heated is taken from the bottom of the storage tank and is handled by a thermostat~controlled charging pump to flow in the secondary fluid circuit through the heat exchanger and to return as hot water to the top of the storage tank.
In conventional systems for heating dispensable water, a heat exchanger is mounted in a storage tank, e.g. in the lower half of that tank, which may be an upright cylindrical tank and is usually provided with suitable heat insulation. The heat ex-changer is supplied with heat from a heating fluid circuit, which in most cases belongs to a central heating system. In such systems the hot fluid coming, e.g.~from the boiler of the central heating system delivers part Or its heat contentsto the water held in the storage tank so that said water is heated uniformly as a result of the convection. But the temperature of the used heating water returning from the heat exchanger will increase with the temperature of the water in the tank, so that the temperature difference between the contents of the storage tank and the hot water flowing, e,g. in the flow main from the heating system boiler as well as the rate at which heat is transferred to the dispensable water will gradually be reduced, As a result, the heat transfer rate and efficiency of the heat exchanger will decrease as the temperature of the dispensable water increases.

In systems for heating dispensable water with solar energy, the arrangement is substantially the same as described herein~
before but the heat exchanger is connected to the hot-fluid circuit of the system for recovering solar energy.
Where heat pumps are used to heat dispensable water, the above-described decrease of the efficiency of the heat exchanger as the temperature of the dispensable water increases gives rise to problems because the heat pump can operate only within a small heat output range and will shut down if its heat output is inadequate. As a result, an increase of the water temperature will reduce the duty cycle of the heat pump, so that the latter will operate in short intervals. As the water must cool until the heat pump is restarted, more time will be required to heat the entire water in the water heater to the highest possible temperature.

~ erman Early Disclosure 25 o8 135 discloses a system comprising a storage tank and a separate heat exchanger, which is connected in a primary fluid circuit including a room-heating system and in a secondary fluid circuit, which incorporates a thermostat~controlled pump and in which the water to be heated is taken from the bottom part of the storage tank, passed through the heat exchanger and returned as hot water to the top of the storage tank. The heat exchanger contains an electric heater, which permits a supply of additional heat for a certain adaption to varying operating conditions. A combination of such heat exchanger with a heat pump would be uneconomical because it would invo]ve a consumption of` expensive electric energy '.

~75~g;2ti It is an object of the invention to provide a simple control system, which is of the kind described first hereinbefore and when used with a heat pump will prevent an intermittent operation of the heat pump.
This is accomplished in accordance with the invention in that a first thermostat extends into the storage tank, preferably in the middle portion of the height of the tank and a second thermostat extends into the secondary fluid circuit between the cold water zone at the bottom of the storage tank and the point where the secondary fluid circuit enters the heat exchanger~
the first thermostat serves to start the charging pump for the charging operation, the second thermostat serves to de-energi-ze the charging pump to terminate the charging operation, the first thermostat cIoses an electric circuit when sensing a water temperature below a predetermined lower limit, and the second thermostat opens an electric circuit when sensing a water temperature above a predetermined upper limit.
A heat pump is preferably incorporated in the primary fluid circuit and the heat transfer properties of the heat exchanger are so matched to the heat pump that heat at the maximum rate will be transferred at the highest temperature to the dispensable water. That secondary fluid circuit preferably incorporates a throttle valve for adjusting and controlling the flow rate, and a temperature-controlled check valve for preventing a natural circulation.
In conjunction with the matched rate at which heat i.s trans-ferred by the heat exchanger and the control of the flow rate ~L17SOZ~

in the secondary fluid circui.t by means of the throttle valve, the control system according to the invention can be used to ensure that the cold dispensable water taken at the lowest poss-ible point will be heated to the highest possible end temperature in a single pass of the dispensable water through the heat exchanger. The storage tank is charged from above and the transitional zone between warm and hot water will descend until the charging of the storage tank has been terrnlnated.
The undesired mixing of hot and cold water as a result of the turbulence which is due to the convection in conventional storage tanks will be avoided.
Further details and features of the invention will now be explained with reference to the drawing, in which Figure 1 is a diagrarnrnatic view showing an illustrati,ve embodiment of a control system according to the invention for use with a storage tank and a separate heat exchanger, which is connected in a primary fluid circuit including a heat purnp, Figure 2 shows a modification of the embodiment of Figure ?
and Figure 3 shows an embodiment similar to that of Figure 2 but associated with two storage tanks, The pressure-resistant storage tank 1 shown in Figure 1 consists preferably of an upright cylinder having a crowned bottom end 2 and a crowned top end 3.
A cold water supply pipe 4 enters through the bottom end 2. The inner end of the pipe 4 faces a baffle plate 5. The discharge ~l75~Z6 pipe 6 for the delivery of the dispensable water extends through the top end 3, Another baffle plate 7 is disposed in the upper portion of the storage tank 1 near its top end 3. The secondary fluid circuit comprises the pipes 8a~ 8b and 8c. The pipe 8a extends from the heat exchanger generally designated 9 and opens in the top end 3, The pipe 8b extends from the bottom end 2 to a charching pump 10, an adjustable throttle valve lland a temperature-controlled check valve (not shown), which prevents a natural circulation of water because it is closed unless the charging pump is in operation. The pipe 8c leads from the above-mentioned temperature-controlled check valve back to the heat exchanger 9. The control is effected by the thermostats T1 and T2, which extend into the secondary fluid circuit. The thermostat T1 extends into the storage tank 1 in the middle portion of its height. The thermostat T2 extends into the secondary fluid circuit where the pipe 8c enters the heat exchanger 9.
The primary fluid circuit comprises a heat pump 13, a circulation pump 14, a precedence control valve 15, which consists of a solenoid-controlled three-way valve and in the present case causes hot fluid to flow to the room--heating system 16 unless the storage tank 1 is being charged, and a thermostat T3, which extends into the primary fluid circuit where it enters the heat exchanger 9.
This system effects a control in the following manner. A
switching device 17 includes a relay, which comprises a winding 18 and normally opens contacts 19, 20a and 20b. The winding 18 is connected to a voltage source, e.g. to an electric power supply system , in series with the series-connected contacts of ~7SI~Z~

the thermostats Tl and T2, When the dispensable water in the storage tank 1 is at a sufficiently high temperature, the contact of the thermostat Tl will be open and the contact of the thermostat T2 will be closed because it senses the temperature of cold dispensable water which has entered the secondary fluid circuit from the bottom end of the storage tank, When hot d~pensable water is taken from the discharge pipe 6, additional cold water will be supplied through the supply pipe 4, As soon as the temperature sensed by the thermostat Tl falls below a predetermined limit, the contact of the thermostat Tl will be closed so that the winding 18 will be energized and the contacts 19, 20a and 20b will close. The contact 19 serves as a holding contact which when closed shunts the contact of the thermostat Tl. When the contacts 20a, and 20b are closed, the solenoid of the precedence control valve 15 will be directly connected to the voltage !sour~e and the charging pump 10 will be connected to the voltage source by means of the contact of the thermostat T3, When the temperature of the fluid in the primary fluid circuit is below a predetermined lower limit~ the contact of the thermostat T3 will be open so that the charging pump will not start until the fluid in the flow main at the thermostat T3 is at the required temperature.
The heat ~hanger 9 has such heat transfer properties and the throttle valve 11 is adjusted to contro~ the flow in the secondary fluid circuit at such a rate that the cold dispensable water taken from the storage tank 1 at the lowe~t possible point will be heated to the highest possible temperature in a single ~ 7 -` ~3l'75;~;Z6 pass through the heat exchanger 9.r~e storage tank 1 is chargedfrom above. The transitional ~one between hot and cold dispens-able water descends until charging of the storage tank 1 has been terminated in that the thermostat T2 has opened the e~tric circuit of the winding 18 so that the contact 19, 20a and 20b have been opened and the precedence~control valve 15 has been shifted to cause the room~heating operation to be resumed.
When the roorm-heating system 16 is inoperative, the thermostat T1 in addition to effecting the switching ~perations described hereinbefore will be used to start the heat pump 13 and to initiate the operation of the primary fluid circuit. The thermo-stat T3 does not start the charging pump 10 until the maximum flow temperature in the flow main has been reached. This will ensure that the secondary circuit will not be started until heat at the desired rate can be delivered by the primary fluid circuit. The thermostat T1 can be mounted on the storage tank on any desired level, which will usually be determined by the extent to which the storage tank is to be discharged until its reheating is initiated.
It will be understood that other embodiments may be adopted.
For instance, the heat pump may be used only to heat dispensable water or the prirnary fluid circuit may be flown through by an intermediate fluid or a coolant. The components of the control system may also be replaced by functionally equivalent ones. For instance~ the thermostat T3 may be replaced by a time-limit relay so that the charging pump will be started with a predetermined delay after the closing of the contacts 20a, 20b.

~ 75C~

It will also be underStood that the means for supplying thermal energy may consist, e.g. of a boller of a central heat-ing system or a systern for recovering solar energy rather than a heat pump. In such heating systems too, the fact that the heat transfer rate is constant throughout the charging operation will ensure that the system will alway operàte at a constant, high efficiency.
In accordance with a preferred further feature of the inventi-on, the flow rate in the secondary fluid circuit connected to the heat exchanger is controlled in dependence on the temperature of the ~ater leaving the heat exchanger in said circuit.
In a system embodying that feature, the secondary fluid circuit incorporates a flow control valve, which is controlled by an additional temperature sensor in dependence on the temperature of the dispensable water leaving the heat exchanger in the secondary circuit (Figures 2 and 3) The flow rate maD be continously or intermittently controlled, e.g., by a sample--and-hold controller.
In Figures 2 and 3, elements which are shown also in Figure 1 are designated with the same reference characters and their description will not be r~ ~ted. As in the embodiment of Figure 1, the pipe 8b extends from the bottom end 2 to a charging pump 10 and an adjustable throttle valve 11. A series connection of a solenoid valve 21 and another throttle valve 22 is connected in parallel to the throttle valve 11. The pipe 8c extends from the throttle valve 11 back to the heat exchanger 9.
An addit~ional thermostat T4 extends into the secondary fluid circuit where it leaves the heat exchanger 9.

.~

31 1~5~;26 The system shown in Figure 2 operates as foilows:
When the temperature of the dispensable water in the storage tank 1 is sufficient~y ~igh, the contact of the thermostat T1 will be open~ whereas the contact of the thermostat T2 will be closed because it senses the temperature of cold dispensable water coming from the bottom of the storage tank~ When hot idspensable water is taken through the dischaFge pipe 6~ cold water will be supplied by the supply pipe 4, As soon as the temperature at the therrnostat Tl falls below a predetermined ~alue~ the thermostat T1 will close its contact to energize the winding 18 so that the contacts 19~ 20a and 20b ~ill be closed, The contact 19 ser~es as a holdlng co~tact~,which shunts the contact of the thermostat T1, When the contacts 20a and 20b are closed ,, the solenoid which controls the precedence control ~al~e 15 is directly connected to the voltage source, the charg-ing pump is connected to the ~oltage source by the contact of the thermostat T3 and the solenoid of val~e 21 is connected to the ~oltage source by the contact of the thermostat T4,The contactof the thermostat T~ is open when the temperature of the fluid in the primary fluid circuit is below a predetermined value.
As a result, the operation of the charging pump will not be initiated until the desired temperature in the flow main is sensed by the thermostat T3. The contact of the thermostat T4 is open when the thermostat T4 senses a predetermined tempera-ture of preferably 50C so that the solenoid ~alve 21 will be ~ 10 5~2~;

open and the flow rate will be a maximum when the thermostat T4 senses a temperature above said threshold. Owing to the hysteresis of the thermostat T4, the latter will open its contact when sensing a temperature of about 45C and the solenoid valve 21 will then close to permit only a minimum flow rate. The maximum and minimum flow rates are determined by the throttle valves 11 and 22, respectively. In this way it is ensured that the cold dispensable water taken at the lowest possible point will be heated to the maximum end temperature in a single pass through the heat exchanger. The storage tank 1 is charged from above and the transitional æone between the warm and cold dispensable water will descend until the charging of the storage 1 has been terminated because the thermostat T2 has de-energized the winding 18 so that the contacts 19, 20a and 20b have been opened and the precedence control valve 15 has been shifted to the position for room heating.
Figure 2 shows the application of the invention in a system comprising a heat pump 13, a storage tank 1 for heating dis-pensable water and a storage tank 1' for heating room-heating water. The elements associated with the storage tank 1' are provided with the same reference numerals as those associated with the storage tank 1 but provided with a prime ('). In that case the relay of the switching device 17 comprises an additional contact 20c, which opens the circuit for energizing the charging pump 10' as soon as the relay winding 18 is energized. This will ensure the precedence of the system for heating dispensable water. In other respects~ this system has the same mode of ~ ~7'~326 operation as the embodiment described last hereinbefore and for this reason said operation need not be described once more.
In the embodiments described with reference to Figures 2 and 3 the bypassing of the first throttle valve 11 or 11' in the secondary fluid circuit by the solenoid valve 21 or 21' or by the combination of a solenoid valve 21 or 21' and a second throttle valve 22 or 22' will result in an increase of the rate at which water flow,s through the heat exchanger 9 or 9' so that an increase of the rate at which heat is available from the heat pump will cause the thermostat T4 or T4' to close its contact and can be utilized for a more rapid filling of the storage tank.
Within the scope of the invention, the valve for controlling the flow rate may consist of an infinitely adjustable valve, which is adjustable, e.g. by a positioning motor also in dependence on the temperature of the water leaving the heat exchanger in the secondary fluid circui~ A voltage signal representing the sensed temperature (actual value) is amplified and compared with a signal representing a desired value, which can be selected. The voltage representing the difference between the desired and actual values is applied by a power amplifier to the positioning motor. Such feedback control system can also be used as a final control element for controlling the motor driving the charging pump 10,10' so that the speed of the motor is varied in dependence on the differential voltage obtained as described hereinbefore and the flow rate is thus adapted to the rate at which heat is currently available. In that case the drive motor is connected to the output of the power amplifier.

Claims

The embodiments of the invention in which an exclusive property or privilege are claimed are defined as follows:

1. A system for controlling the heating of dispensable water to be stored in a storage tank, which is connected to a heat exchanger, which is separate from the storage tank and connected in primary and secondary fluid circuits, in an arrangement wherein water to be heated is taken from the bottom portion of the storage tank and is handled by a thermostat-controlled charging pump to flow in the secondary fluid circuit through the heat exchanger and to return as hot water to the top portion of the storage tank, characterized in that a first thermostat extends into the storage tank, preferably in the middle portion of its height, a second thermostat extends into the secondary fluid circuit between the cold water zone at the bottom of the storage tank and the point, where the secondary fluid circuit enters the heat exchanger, the first thermostat serves to start the charging pump for the charging operation, the second thermo-stat serves to de-energize the charging pump to terminate the charging operation, the first thermostat closes an electric circuit when sensing a water temperature below a predetermined lower limit, and the second thermostat opens an electric circuit when sensing a water temperature above a predetermined upper limit

2. A control system according to claim 1, characterized in that a third thermostat extends into the primary fluid circuit where it enters the heat exchanger and the third thermostat senses the temperature of the fluid in the primary fluid circuit and when said temperature exceeds a predetermined upper limit closes a contact which is electrically connected in series with the charging pump so that the first thermostat is then enabled to start the charging pump.

3. A control system according to claim 1, characterized in that the primary fluid circuit connected to the heat exchanger included in known manner a three-way valve, which is operated to open the primary fluid circuit in response to the operation by which the first thermostat initiates the charging operation and is operated to close the primary fluid circuit in response to the operation by which the second thermostat terminates the charging operation.

4. A control system according to claim 1, characterized in that an adjustable throttle valve is connected in the secondary fluid circuit in series with the charging pump and is adapted to control the flow rate in the secondary fluid circuit in dependence on the heat transfer rate of the heat exchanger so as to ensure a heating of the water in the secondary fluid circuit to the highest possible temperature in a single pass through the heat exchanger.

5. A control system according to claim 1, characterized in that the secondary fluid circuit includes an adjustable flow control valve, which is adjusted in dependence on the temperature of the water leaving the heat exchanger in the secondary fluid circuit, which temperature is sensed by a temperature sensor.

6. A control system according to claim 5, characterized in that the adjustable flow control valve consists of a solenoid valve, which is connected in the secondary fluid circuit in parallel to a first throttle valve.

7. A control system according to claim 6, characterized by a second throttle valve connected in series with the solenoid valve.

8. A control system according to claim 5, characterized in that the temperature sensor consists of a thermostat, which extends into the secondary fluid circuit, where it leaves the heat exchanger and which when sensing a predetermined water temperature of preferably 50°C closes a contact to open the solenoid valve, provided that the charging pump has been started.

9. A system according to claim 5, characterized in that the flow rate control valve consists of an infinitely adjustable valve.

10. A system according to claim 9, characterized in that the infinitely adjustable valve is adjustable by a positioning motor, the temperature sensor consists of a thermocouple, the output voltage of the thermocouple is amplified to generate an actual-value signal, which is compared with a signal representing a selectable desired value, and the signal representing the difference between the desired and actual values is amplified by a power amplifier, the output of which is applied to the positioning motor for adjusting the infinitely adjustable valve.

11. A system according to claim 10, characterized in that the charging pump constitutes a final control element and the motor for driving the charging pump is connected to the output of the power amplifier.