Water heater

A water heater comprises an elongated vessel having a heat exchanger that occupies a substantial portion at one end and leaves a minimum volume storage and blending zone at the other end of the vessel. Water is recirculated through the heat exchanger, preferably continuously, through a recirculation conduit that takes water through its inlet essentially solely from the outlet end of the heat exchange zone and returns it to the inlet end, the intake of the recirculation conduit being located at the lower end of the volume storage and immediately above an apertured partition extending across the interior of the vessel immediately above the heat exchanger. The temperature sensor of the control thermostat is received within a tunnel secured to the partition. The tunnel has two side outlet openings away from the aperture so that the thermostat is always subject to the hottest water just above the heat exchanger for fast and accurate control of the heating fluid on failure of the circulating pump.

REFERENCE TO RELATED PATENT 
The present invention is an improvement of my invention as described in 
U.S. Pat. No. 4,046,189 issued on Sept. 6, 1977. 
BACKGROUND OF THE INVENTION 
Water heaters designed for commercial, industrial and institutional 
applications are generally classified in one of three categories, 
instantaneous, semi-instantaneous and storage. A water heater falling in 
the instantaneous category is characterized by the absence of any 
significant storage capacity and by the fact that the heat exchanger 
occupies substantially all of the vessel or jacket. Instantaneous water 
heaters present difficult temperature control problems, inasmuch as cold 
water is delivered to the water heater and flows essentially straight 
through and out with no opportunity for blending and thus no opportunity 
for smoothing temperature variants that result from variations in demand, 
and the heat is supplied at a high rate, relative to the rate of water 
flow, thus emphasizing the lags between changes in draw and changes in 
heat input. 
The semi-instantaneous category of water heaters is generally characterized 
by a relatively small tank which serves primarily as a mixing and blending 
zone in which water delivered from the heat exchanger is blended with 
water in the vessel. It is possible to obtain very close temperature 
control in semi-instantaneous water heaters, inasmuch as the blending 
principle tends to smooth out temperature gradients of water coming from 
the heat exchanger and thus makes the temperature of the delivered hot 
water less subject to variations in demand. The semi-instantaneous types 
often use heat anticipators or forward feed devices for further 
enhancement of temperature uniformity. 
The storage-type water heaters are characterized by large tanks and 
relatively small heat exchangers and rely upon slow rates of heating of 
large volumes of water in the tank to maintain the water at a desired 
temperature. Of the three types, the storage types have the slowest 
recovery, and present problems of temperature control that often cannot 
readily be solved particularly in applications involving frequent large 
draws of hot water. 
Each of the three types has various advantages and disadvantages, and the 
type selected is, of course, dependent upon the requirements of the 
particular installation. The present invention provides a solution to the 
problem, on the one hand, of the relatively high cost of presently known 
semi-instantaneous type water heaters, as compared to the instantaneous 
type, which results primarily from the large vessel required for presently 
known semi-instantaneous water heaters, and overcomes the disadvantage of 
relatively poor temperature control characteristic of instantaneous type 
water heaters. Accordingly, a water heater embodying the invention offers 
the size advantage of an instantaneous water heater and the temperature 
control advantage of a semi-instantaneous water heater. 
Further, the present invention improves upon the operation of the hot water 
heater described in my patent referred to above by locating the control 
thermostat immediately above an apertured partition immediately above the 
heat exchanger in a tunnel having two side outlets with the circulating 
inlet opening into the tunnel for accurate and prompt control of the 
heating fluid on failure of the circulating pump to prevent overheating 
water from leaving the heater. The apertured partition or plate and the 
tunnel with two side outlets acts to stratify the heated water in the 
storage volume and blend it preventing excessively hot water from 
appearing at the top of the storage volume. 
SUMMARY OF THE INVENTION 
More particularly, there is provided, in accordance with the present 
invention, a water heater having a relatively small vessel, as much as 80% 
smaller than the tanks of presently known semi-instantaneous type water 
heaters, but capable of providing uniform temperature water over a wide 
range of demand rates with rapid response to changes in draw. A 
substantial portion of the vessel, generally somewhat more than half, is a 
heat exchange zone and contains a heat exchanger, such as a tubing bundle 
through which a hot fluid, generally steam or high temperature boiler 
water or other heating fluid, is conducted. The remainder of the vessel 
constitutes a minimum volume storage and blending zone. Hot water is drawn 
from the storage and blending zone, preferably from the end of the vessel 
remote from the heat exchanger. Make-up water, generally cold water from a 
water supply, is supplied at the inlet end of the heat exchange zone, and 
water is recirculated, preferably continuously, by pumping it through a 
recirculation conduit, the intake of which is located very close to the 
outlet end of the heat exchanger so that it receives essentially solely 
water from the heat exchange zone, and the outlet of which leads into the 
inlet end of the heat exchanger. An important aspect of the invention 
involves locating the temperature sensor of the temperature control 
thermostat adjacent the recirculation conduit, perferably near the intake 
and in a tunnel, on an apertured partition immediately above the heat 
exchanger, so that it is responsive to a flow of water from the heat 
exchange zone upon failure of the circulating pump. 
Inasmuch as water coming from the heat exchange zone is a mixture of 
recirculated hot water and cold water that has been heated in the heat 
exchange zone, its temperature is indicative of the demand on the water 
heaters for hot water in that the amount of cold water entering the vessel 
is equal to the draw at any point in time. Thus, as hot water is withdrawn 
from the water heater, cold water enters at a rate equal to the draw, and 
the rate of temperature drop in leaving the heat exchange zone is 
indicative of the draw rate. The high velocity flow of such water over the 
temperature sensor will promptly detect the drop in temperature and will 
cause the temperature control to respond by increasing the heat input to 
the heat exchanger. In the meantime, the quantity of water leaving the 
heat exchange zone that is not recirculated is blended with water in the 
vessel in the storage and blending zone adjacent the hot water outlet, and 
therefore, a change in the temperature of the water coming from the heat 
exchanger tends to be smoothed out in a manner similar to known 
semi-instantaneous type water heaters. 
Upon failure of the circulating pump enough flow of heated water from the 
heat exchanger will pass through the aperture of the plate or partition 
and will flow over the temperature sensor to adjust the flow of the 
heating fluid and prevent the occurrence of excessively hot water at the 
discharge from the storage volume. 
The invention offers the advantages of having a small-sized vessel, a 
characteristic of an instantaneous type water heater, in combination with 
significantly improved temperature control, as compared to an 
instantaneous, a characteristic of semi-instantaneous type water heaters. 
It is efficient to operate, and the small size of the vessel reduces the 
cost of building, shipping and installing the water heater. The reduction 
in size of the vessel means, of course, that it occupies much less space, 
an important advantage to the building occupant. The water heater is not 
dependent upon thermal convection of the water for temperature control or 
other reasons and, therefore, can be mounted upright or horizontally, 
another contribution to flexibility of use and saving of space in many 
cases.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference numeral 10 designates generally an elongated vessel, the length 
of which is substantially greater than the dimensions of the transverse 
cross sections. The vessel is preferably a circular cylinder constructed 
of copper-silicon or some other durable, corrosion resistant material and 
is open at its lower end 11 and closed by an integral dome 12 at its upper 
end and has a circular, outwardly extending flange 14 at its lower end. A 
heat exchanger 16 occupies a substantial portion, generally somewhat more 
than one-half of the bottom of the vessel, and is constituted by a bundle 
of generally U-shaped tubes 18 mounted in a tube plate 20. The heat 
exchanger is mounted in the vessel by bolting the plate 20 between the 
flange 14 of the vessel and a fitting 22 that is formed to provide an 
inlet 24 and an outlet 26 for a hot fluid, usually either steam or hot 
boiler water, circulated through the tubes 18 between the inlet 24 and 
outlet 26. The water heater shown in FIG. 2 is set up for steam as a heat 
source, the steam being conducted to the inlet 24 through a pipe system 27 
having a temperature control valve 28 interposed therein. Condensate is 
removed through a condensate outlet 30. 
Hot water is drawn from the vessel 10 through a hot water outlet 32 
extending from the top 12 of the vessel and a pressure relief valve is 
provided at 32'. Make-up cold water is conducted into the vessel through a 
water inlet 34 at the lower, i.e., inlet, end of the heat exchanger 16. 
Baffles 36 extend in from opposite sides of the vessel part way across the 
heat exchange zone in spaced-apart, staggered relation, thereby promoting 
good heat transfer conditions by increasing the velocity of the water and 
causing it to flow along a tortuous path represented by the arrowed lines 
in the drawing. The uppermost baffle 36a is positioned to direct the flow 
of water from the heat exchange zone toward the aperture 56 in plate 55 
which is described immediately below. 
Hot water is recirculated, preferably continuously, back through the heat 
exchanger by a recirculation system composed of a recirculation conduit 38 
and a pump 40. An inlet portion 38b of the recirculation conduit opens 
horizontally into the vessel, the intake 38a thereof being positioned very 
close to the heat exchanger above a partition to be described hereinafter. 
Accordingly, the water drawn into the recirculation conduit is essentially 
exclusively water that comes from the outlet end of the heat exchanger 
through an aperture in the partition. The outlet 38b from the 
recirculation conduit 38 leads into the inlet end of the heat exchange 
zone at the bottom of the vessel; preferably, but not necessarily, the 
cold water inlet 34 and the recirculation outlet 38b are coincident. 
The temperature of the hot water produced in the water heater is controlled 
by a temperature control thermostat 42 which includes a temperature 
sensing probe 44 located adjacent the recirculating inlet 38a. In the 
embodiment shown in the drawing, the temperature sensing probe 44 is 
located spaced from and above partition or plate 55 having an aperture 56 
both immediately above the heat exchanger 18 and in tunnel 57 having side 
outlet openings 58 and 59 adjacent recirculating inlet 38a, partition or 
plate 
It will be seen that aperture 56 is adjacent one end of a diameter of plate 
55 while openings 58 and 59 are adjacent the other end of the diameter. 
It has been determined that the area of orifice 56 should be approximately 
10% of the total area of the plate 50. The cross sectional area of tunnel 
57 should be approximately between 15 and 20% of the total area of plate 
55. The combined area of the two side outlet openings 58 and 59 in tunnel 
57 should be approximately 10% of the total area of plate 55. 
For example, for a 6" nominal diameter heater 10, the inside diameter 
thereof is approximately 61/8". The plate 55 is then 6" diameter, the 
rectangular orifice 54 in plate 55 is 2" long by 11/2" wide. The height of 
tunnel 57 is 2" and the width is 2". The two side outlets 58 and 59 in the 
tunnel each measure 1" in height by 11/2" long. 
The 6" diameter unit is the smallest size tested and tests on 8", 10" and 
12" nominal diameter heaters have been conducted. It has been found that 
the orifice and tunnel dimensions should be increased proportionately to 
maintain the same percentage ratios as indicated above for the larger 
sizes, 8" through 12". Larger than 12" diameter units would require the 
same proportions. This is logical since the heating capacity of the units 
increases almost in direct ratio to the cross-sectional area of the heater 
shell, which in turn, determines the diameter of the orifice plate. 
The disposition and areas of openings 56, 58 and 59 cause circulation of 
the heated water from the heat exchanger 38 to circulate in the direction 
of the arrows in FIG. 2, the heated water leaving openings 58 and 59 moves 
in flat spirals into the volume 50 for stratification and blending thereof 
with the stored heated water for a more uniform gradient of temperature to 
outlet 32 preventing any sudden delivery of overheated water. The 
thermostat is responsive to temperature changes of the recirculated hot 
water with a minimum of delay particularly upon failure of the circulating 
pump 40. The temperature control thermostat and the sensor may be of any 
suitable type, many of which are well known to those skilled in the art, 
the one illustrated being of a pneumatic type. The temperature control 
thermostat 42 is connected by a line 46 to the temperature control valve 
28 and controls the valve 28 to supply steam from the inlet of the heat 
exchanger at a rate such that the temperature of the water leaving the 
heat exchange zone is maintained substantially constant. The water heater 
also includes a thermometer 48 adjacent the hot water outlet, the 
thermometer merely permitting the temperature of the hot water to be 
determined but playing no role in the control of the water heater. 
The rate of supply of steam to the heat exchanger varies according to the 
draw of hot water from the outlet 32 which, of course, directly affects 
the rate of intake of make-up cold water and, thus, the requirement for 
heat input to the heat exchanger. The ability of the water heater to 
produce hot water of substantially constant temperature at the hot water 
outlet, despite the absence of the large storage and blending zone 
characteristic of both storage and semi-instantaneous type water heaters, 
results from the construction of the water heater in a way that minimizes 
the time of response between a change in the temperature of water leaving 
the heat exchange zone and adjustment of the supply of steam to the heat 
exchanger and from the provision for recirculation of a substantial part 
of the hot water coming from the heat exchanger at all times. 
The response time of the thermostat is reduced by reason of the location of 
the temperature sensing probe above partition or plate 55 where there is a 
velocity flow of water over the probe even on failure of the circulating 
pump 40, thus maintaining the rate of heat transfer from the water to the 
probe, as compared to the rate of heat transfer that would be obtained if 
the probe were located elsewhere in the water heater vessel. In addition, 
the temperature sensing probe senses the temperature of water that comes 
substantially exclusively from the outlet end of the heat exchanger, which 
is necessarily water that will tend to vary in temperature relatively 
widely in response to changes in hot water draw from the water heater, and 
that indeed has the greatest variation in temperature of any water in the 
vessel. 
The recirculation of hot water through the heat exchanger by way of the 
recirculation conduit 38 provides a result very similar to that provided 
by a relatively high volume blending and storage zone in that a 
substantial part of the water coming from the heat exchanger is kept in 
closed circuit in the recirculation system, thus reducing the amount of 
water coming from the heat exchanger that goes to the hot water outlet. In 
general, it is desirable for good temperature control to recirculate hot 
water at a rate of about 20% or more of the normal draw for which the 
water heater is designed. 
At times when the water heater is operating at a relatively high demand or 
draw rate, the rate of steam supply to the heat exchanger will be 
relatively high, inasmuch as a substantial part of the water coming 
through the inlet 38b will be cold, make-up water. Assuming a steady-state 
operation at a high draw, the temperature control valve will be set in 
response to the temperature control thermostat to provide delivery of 
water from the heat exchanger at a temperature close to the temperature of 
the water discharged from the outlet. Although a substantial part, perhaps 
20% or more, of the water coming from the outlet of the heat exchanger is 
continuously recirculated through the conduit 38, a substantial part, say 
the other 80%, is drawn off relatively quickly through the hot water 
outlet. Upon a drop in demand, which will often be a relatively large 
fraction of the then existing high total demand, the temperature of water 
coming from the heat exchanger will increase due to a decrease in the 
amount of cold water coming in the inlet 34. The temperature probe is, as 
described above, highly sensitive to temperature changes of the water 
coming from the heat exchanger by reason of its location above the 
apertured partition or plate 55 and within the tunnel 57 and will produce 
a relatively rapid response of the thermostat and the control valve to 
reduce the supply of steam to the heat exchanger particularly upon failure 
of pump 40. Inasmuch as it is probable that at periods of high demand, 
percentage changes in draw can be relatively high, the water heater can 
readily maintain close tolerance to a desired temperature in the hot water 
delivered inasmuch as a percentage of the higher temperature water leaving 
the heat exchanger is recirculated and the remaining part is blended with 
water already in the blending zone 50, and the temperature of water 
leaving the heat exchanger is not greatly increased. 
In periods of low demand, the percentage change in draw is likely to be 
somewhat greater, and the recirculation system plays a more important role 
in temperature control in that most of the water coming from the heat 
exchanger circulates in closed circuit back through the heat exchanger, 
with only a relatively small fraction being discharged to the storage and 
blending area 50. Thus, even though there may be a relatively large 
percentage change in demand during periods of low overall demand, which 
will, in turn, produce a relatively large and rapid change in the 
temperature of the water discharged from the heat exchanger, only a small 
fraction of the water coming from the heat exchanger goes to the storage 
and blending area 50 where it is blended with water of the desired 
temperature. Accordingly, the relatively high percentage change in demand 
at low overall demand rates produces little change in the temperature of 
the water delivered at the hot water outlet. Moreover, the storage and 
blending zone 50 becomes more significant in low draw situations, in that 
it provides a reservoir for water of the desired temperature from which 
demand may be satisfied during the short times when transient conditions 
prevail in the heat exchange zone. 
Under all conditions, the high rate of recirculation smoothes out the 
temperature variations that would exist in water leaving the heat 
exchanger if only cold water were being heated.