GB2149071A - Thermal storage water heater - Google Patents

Abstract

An apparatus for heating water includes a thermal storage mass of magnesite bricks 12, 13 which are electrically heated during off-peak electric utility hours. The heat is transferred from the bricks to water to be conducted to the point of use by a natural circulation fluid circuit having a first heat exchanger 28 for transferring heat from the bricks to the fluid circuit, a second heat exchanger 44 for transferring heat from the fluid circuit to the water, and a valve 52 responsive to the demand for hot water to control the flow of fluid in the fluid circuit. The first exchanger includes a pair of spaced parallel plates, each plate having a plurality of projections defining passages therebetween. The projections of each plate extend 52 into the passages defined on the other plate of the pair. <IMAGE>

Description

SPECIFICATION Thermal storage water heater BACKGROUND OF THE INVENTION Hot water for domestic purposes such as bathing and cleaning, as well as for hydronic space heating, is an essential commodity and one which requires a large portion of the energy consumed by a home, commerical enterprise or institution. Thus, systems which will provide hot water cheaply and efficiently are constantly being sought.

One energy source for providing hot water which is clean and dependable, but sometimes considered to be too expensive, is electricity. However, systems have been developed to reduce the cost of employing electricity for heating purposed by using electrical energy during the off-peak hours of the electric generating plants, during which the cost of electricity is substantially lower than during peak hours. The electricity consumed during off-peak hours is used to produce and store heat at high temperature for use during peak hours. For example, space heating units which employ off-peak electricity to generate and store heat in ceramic bricks for use at later times on demand are commerically available from Stiebel Eltron North America of Boston, Massachusetts under the model designations ETS and ETT.In these space heating units, electric resistance heating elements are positioned among the bricks to transfer heat to the bricks during off-peak hours. Air is circulated over the bricks and into the space to be heated at any time heat is demanded. Although these devices are a clean, efficient source for space heating, they do not involve hot water, and cannot replace boilers in existing hydronic systems or provide hot water for domestic uses such as bathing and cleaning.

Devices have also been devised which use off-peak electricity to store heat in a fluid, such as water. In these devices, water is heated under pressure to a relatively high temperature, up to 280 degrees F., and withdrawn on demand and passed through one side of a heat exchanger to heat water passing through the opposite side of the heat exchanger and flowing to the point of use.

Heating systems of this type are disclosed in U.S. Patent No. 3,422,248 to A.A. Beaulieu et al. However, such units require a large pressurized tank for storing a relatively large quantity of fluid at high temperature, as well as requiring considerable piping and controls, which makes such units more efficient for larger installations.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a compact, efficient water heater which stores heat generated from electricity at off-peak hours.

It is a further object of the present invention to provide a thermal storage water heater employing a relatively small mass of material having the capacity of storing large quantities of heat and surrendering the heat on demand.

It is another object of the present invention to provide a thermal storage water heater which utilizes a fluid circuit responsive to the demand for hot water to remove heat from the thermal mass.

In order to fulfill these and other objects, the thermal storage hot waterheater according to the present invention includes a thermal storage mass comprising heat storage bricks or cast metal housed within a thermally-insulated cabinet, the thermal mass having extending therethrough a plurality of electric resistance heating elements for heating the thermal mass to a predetermined temperature during off-peak hours of the electric utility. A first heat exchanger in the form of spaced parallel plates is in contact with the thermal mass and forms part of a closed fluid circuit. The fluid circuit also includes a flow controller and a second heat exchanger for transferring heat from the fluid circuit to water which flows to the point of use.Water flows by natural circulation through the fluid circuit, being heated to steam by the first heat exchanger which is in contact with the thermal mass and transferring its heat in the second heat exchanger to the water to be used. The flow controller is responsive to the demand for hot water at the point of use to control the circulation of the fluid in the fluid circuit and the rate of withdrawal of heat from the thermal mass.

In achieving the thermal storage water heater according to the present invention, several additional problems were overcome.

For uniform heating of the water in the first heat exchanger, which is in contact with the thermal mass, it is important that the water be distributed evenly over the surface of the heat exchanger, so that the water is heated evenly -and heat is withdrawn equally from all of the thermal mass. Furthermore, as the steam in the fluid circuit flows toward the outlet of the first heat exchanger, it is subject to further heating which tends to cause superheating of the steam. Superheated steam is undesirable because it prevents precise control of the heat transfer through the second heat exchanger and of the temperature of the water demanded for use.

In the thermal mass water heater according to the present invention, the problems associated with the heat exchanger in contact with the thermal mass have been overcome by the heat exchanger of the present invention in which the parallel spaced plates are provided with a pattern of spaced projections, which greatly increase the heat transfer surface area of the plates. The projections of each plate of a pair of spaced plates face and project to ward the other plate of the pair, and the pattern of projections on each plate is complementary to the pattern of the other plate, so that the projections of each plate of a pair extend partly into the spaces between the projections on the other plate. As a result, a large number of interconnected restricted flow passages are formed to define tortuous paths between each pair of spaced parallel plates.

Thus, condensate flowing down from the top edges of the plates is subjected to repeated deflections and changes in direction by the projections. This action assures the widespread, even distribution of the condensate across the plates. Furthermore, the restricted flow passages defining the tortuous paths slow the flow of condensate across the plates, thus allowing more time for the water to absorb heat, and cause thorough mixing of the rising steam with the falling condensate.

thereby keeping the steam saturated, preventing superheating and eliminating the problems associated with superheating.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partially exploded perspective view of the thermal mass water heater according to the present invention; Figure 2 is an elevation of.the plates of a heat exchanger in the thermal mass water heater according to the present invention; Figure 3 is an enlarged cross section of a water distribution conduit, a steam collecting conduit and part of a heat exchanger in the thermal mass water heater; Figure 4 is an enlarged cross section of the vacuum-responsive valve in the fluid circuit of Fig. 1; and Figure 5 is a cross section of a thermallyresponsive valve of an alternative embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PRE FERRED EMBODIMENT As is best illustrated in Fig. 1, the thermal mass water heater according to the present invention, which is generally designated by the reference numeral 10, comprises a thermal storage mass of cast metal or, as illustrated, a plurality of bricks 1 2 and 1 3 made of a material which is capable of storing large quantities of heat and surrendering the heat on demand. Magnesite is especially well suited for use in such thermal storing bricks.

The bricks 1 2 and 1 3 are stacked to define the thermal mass and at least some of the bricks 1 2 are provided with grooves 14 or similar indentation to accommodate electrical resistance heating elements 16, which are connected to a source of electrical power through a control panel 1 7 and are operated at off-peak hours of the electrical power generating station to take advantage of the lower rates which are charged during such hours.

The bricks 1 3 are shaped so that they can be stacked to cooperate with the bricks 1 2 to define a compact thermal mass having high thermal storage and a configuration well suited for receiving heat exchangers to be described hereinafter.

The bricks 1 2 and 1 3 are enclosed within a cabinet comprised of side panels 1 8 and 20, a front panel 22, a rear panel 24 and a top panel 25. Thermal insulation 26 is interposed between the bricks 1 2 and 1 3 and the interior surfaces of the panels, and insulating bricks 27 provide a support surface for the heatstoring bricks. Embedded in the thermal mass.

in contact with the bricks 1 2 and 1 3, are a plurality of heat exchangers 28, one of which is shown. As can better be seen in Figs. 2 and 3, each heat exchanger 28 comprises a pair of spaced parallel plates 30 vertically oriented in surface contact with the thermal mass and sealed to one another along their edges. Each plate includes a pattern of spaced surface projections 31 on the surface thereof opposite the surface in contact with the bricks 1 2 or 13. Although the projections 31 iliustrated are generally rectangular, projections of other shapes, such as diamond-shaped, are also suitable.The plates 30 are spaced apart less than twice the height of the projections 31, and the projections 31 of each plate 30 face the opposite plate 30 of the pair and extend into the spaces between the projections of the opposite plate. Thus, the plates 30 together define a network of restricted tortuous passages.

Each pair of plates 30 depends from a steam collecting conduit 32 which defines a portion of a closed, natural circulation fluid circuit. generally designated by the reference numeral 34, the plates 30 being in fluid comminication with the steam collecting conduit 32 through a slot 36 defined at the bottom of the steam collecting conduit 32.

The steam collecting conduits 32 are connected by a manifold fitting 38 to a riser pipe 40, which in turn is connected to an inlet 42 on the fluid circuit side of a heat exchanger 44. The heat exchanger 44 also has an opposite inlet 46 and an outlet 48 for carrying the heated water to its point of use. An outlet 50 on the fluid circuit side of the heat exchanger44 is connected through a pressure-responsive control valve 52 to a manifold fitting 54.

which directs the return flow to a plurality of condensate distribution conduits 56. Although Fig. 1 depicts the heat exchanger 44 as a shell-and-tube structure in which the fluid circuit communicates with the tube and the water to be heated communicates with the shell, it is understood that the fluid circuit can communicate with the tube. Moreover, other suitable heat exchanger structures can be employed.

The condensate distribution conduits 56 are concentrically disposed within the steam collecting conduits 32 substantially throughout the length of the steam collecting conduits.

Each condensate distribution conduit 56 contains a plurality of regularly spaced openings 58 along its length for placing the condensate distribution conduits 56 in fluid communication with the steam collecting conduits 32 and ultimately with the heat exchangers 28.

In operation, the electrical resistance heating elements 1 6 are operated during off-peak hours until the thermal mass defined by the bricks 1 2 and 1 3 attains a predetermined high temperature. Then the electrical resistance heating elements 1 6 are turned off, and the bricks 1 2 and 1 3 retain the heat until hot water is demanded at the point of use. Condensate issuing from the openings 58 in the condensate distribution pipes 56 travels through the steam collecting pipes 32 and through the slots 36 onto the plates 30, which define large heat transfer surfaces in the heat exchangers 28.The condensate contacting the plates 30 turns into steam and rises between the plates 30, up through the slots 36 and into the steam collecting conduits 32.

The projections 31 substantially increase the surface area of the plates 30 and the transfer of heat to the falling condensate. In addition, the netowrk of tortuous passages defined by the interposition of the projections 31 of each plate 30 of a pair in the spaces between the projections 31 of the other plate slows the flow of condensate down the plates, thereby additionally increasing the heat transfer, and increases the turbulence of the condensate, thereby increasing the mixing of the falling condensate with the rising steam. The mixing keeps the steam saturated and eliminates superheating, which can prevent proper control of the water heater 1 0.

From the steam collecting conduits 32, the steam travels through the manifold fitting 38 and up through the riser pipe 40 to the inlet 42 on the fluid circuit side of the heat exchanger 44. In passing through the heat exchanger 44, the steam gives up its heat to the water traveling through the opposite side of the heat exchanger from the inlet 46 to the outlet 48, and condenses, leaving the heat exchanger 44 as water flowing through the return conduit 53. The condensing of the steam causes a slight vacuum to exist in the fluid circuit. As hot water continues to be demanded at the point of use, the condensate and steam continue to circulate in the fluid circuit 34.This is due to the fact that the pressure-responsive valve 52 interposed in the return conduit 53 is maintained in an open position by the vacuum condition existing in the fluid circuit 34 as a result of the condensation of the steam.

As can be seen more clearly from the cross section of Fig. 4, the pressure-responsive valve 52 comprises a valve element 60 biased against a valve seat 62 by a seating spring 64. The valve element 60 is engaged on the side opposite the slating spring 64 by a stem 66 which is connected to a flexible diaphragm 68 defining with the casing of the valve an upper chamber 70 and a lower chamber 72.

The diaphragm is biased toward the lower chamber 72 by a diaphragm spring 74, mounted in the upper chamber 70, which exerts more force than the seating spring 64.

The lower chamber 72 is in communication with the fluid circuit side of the heat exchanger 44. As long as the vacuum condition is maintained in the fluid circuit, a vacuum exists in the lower chamber 72, so that the diaphragm spring 74 forces the diaphragm 68, the stem 66 and the valve element 60 down against the bias of the weaker seating spring 64. Thus, the valve element 60 is moved away from the seat 62 so that the flow of condensate through the return conduit 53 continues, and additional condensate flows to the heat exchangers 28 to produce more steam. However, when the demand at the point of use decreases, the steam in the fluid circuit transfers its heat more slowly and condenses more slowly. As a result, a positive pressure builds in the fluid circuit, and, thus, in the lower chamber 72 of the valve 52.This pressure forces the diaphragm 68 upward against the bias of the diaphragm spring 74 and allows the seating spring 64 to move the valve element 60 toward its closed position, so that the flow of condensate through the return conduit 53 to the heat exchangers 28 is reduced and less steam is produced. When demand for hot water ceases, the valve 52 closes and no more steam is produced. Steam will again be produced when water is demanded at the point of use and the steam on the fluid circuit side of the heat exchanger 44 condenses.

As an alternative to the pressure-responsive valve 52, a temperature-responsive valve 52' can be employed in the fluid circuit. As can best be seen from the cross section of Fig. 5 the temperature-responsive valve 52' includes a sensor 76 which is connected to the outlet line 48 of the water to be heated. The sensor comprises a fluid-filled bulb which is in communication with a chamber 38 through a conduit 80. The chamber 78 contains a bellows 82 carrying a control member 84 which moves a valve stem 86 and a valve element 88 carried by the valve stem 86. The valve element 88 is biased in the open direction by a spring 90 acting on the valve stem 86. The region in the chamber 78 which is outside the bellows 82 is filled with liquid, as is the conduit 80 which connects the bellows 82 to the sensor 76.When water is being drawn at the point of use, the temperature in the outlet line 48 is relatively cool. Therefore, the spring 90 biases the valve element 88 open, and condensate flows in the fluid circuit 34, thereby producing steam. As the demand for water decreases, the temperature in the outlet line rises, heating the liquid in the sensor 76 and transmitting heat through the liquid in the conduit 80 to the liquid in the chamber 78.

The heat causes the liquid to expand, which collapses the bellows 82 and moves the valve element 88 toward its closed position. Thus.

the flow of liquid in the fluid circuit is decreased, as is the production of steam.

As another alternative to the pressure responsive valve 52, a solenoid valve can be used in which a switch for actuating the solenoid is operated by a diaphragm responsive to the pressure in the fluid circuit 34.

Other desirable items which are provided in connection with the thermal mass water heater 10 are a pressure relief valve 92 and a manual valve 94 in the fluid circuit 34, a mixing valve 96 in the domestic water lines to control the temperature of the water at the point of use, and suitable temperature and pressure gauges.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordinly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

Claims (14)

1. Apparatus for heating a fluid medium comprising: a thermal storage mass; means for heating the thermal storage mass; and a fluid circuit including first means for transferring heat between the thermal storage mass and the fluid circuit, second means for transferring heat between the fluid circuit and said fluid medium, and control means responsive to the rate of heat transfer between the fluid circuit and said fluid medium to control the flow of fluid through the fluid circuit.

2. The apparatus of claim 1 wherein the thermal storage mass comprises magnesite.

3. The apparatus of claim 2 wherein the magnesite is in the form of bricks.

4. The apparatus of claim 3 wherein the bricks define recesses to accommodate the heating means.

5. The apparatus of claim 1 or 4 wherein the heating means comprises electrical resistance heating elements.

6. The apparatus of claim 1 wherein the first heat transfer means comprises at least one pair of spaced, parallel, heat-conducting plates. at least one of the plates of the pair being in contact with the thermal storage mass.

7. The apparatus of claim 1 wherein the first heat transfer means comprises at least one pair of spaced, parallel, heat-conducting plates, each plate having a plurality of spaced projections.

8. The apparatus of claim 7 wherein the projections of each plate extend into the spaces between the projections of the opposite plate of the pair.

9. The apparatus of claim 8 wherein a pattern of said projections extends over an entire surface of each plate.

1 0. The apparatus of claim 7 wherein at least one pair of plates depends from a first conduit defining a portion of the fluid circuit, the first conduit including an opening to provide communication between the first conduit and the plates.

11. The apparatus of claim 10 wherein the fluid circuit comprises a second conduit positioned within the first conduit and having at least one opening to provide communication between the second conduit and the first conduit.

1 2. The apparatus of claim 11 wherein the second conduit has substantially the same length as the first conduit and said at least one opening comprises a plurality of spaced openings

1 3. The apparatus of claim 1 wherein the control means comprises a valve mounted in the fluid circuit downsteam of the second heat transfer means.

14. The apparatus of claim 1 2 wherein the fluid circuit contains water.

1 5. Apparatus for heating a fluid medium substantially as hereinbefore described with reference to the accompanying drawings.