Abstract:
A water heater system includes a boiler having a supply port, an output port and a recirculation input port. A heat exchanger includes first input and output ports, and second input and output ports. An averaging tank has an inlet and an outlet. A first fluid flow subsystem is for controllably directing water along a primary loop through the boiler and from the output port of the boiler to the input recirculation port via either a first path through the first ports of the heat exchanger or a second path bypassing the heat exchanger. A second fluid flow subsystem is for directing water along a secondary loop through the second ports of the heat exchanger, through the inlet and outlet of the averaging tank, and back to the heat exchanger, whereby water directed through the secondary loop is heated from water directed through the primary loop via the heat exchanger.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/640,752, filed on Dec. 30, 2004, the disclosure of which is herein incorporated by reference in its entirety. 

   TECHNICAL FIELD 
   The present invention relates to water heater systems, and more particularly relates to a semi-instantaneous water heater system that can maintain water temperature within a prescribed error band at any rate of flow whether varying or continuous between zero flow and the maximum capability of the energy source. 
   BACKGROUND 
   The commercial water heater industry has been served by storage tank water heaters that are sized to contain sufficient water at a specified temperature to satisfy demand during the highest expected usage. While this method has proven satisfactory for most applications, it requires large storage volumes, with associated losses, large footprint, and excessive set point temperatures to ensure performance. The commercial water heaters are typically operated in various ways. 
   One way is to select a maximum and minimum temperature set point relatively far apart from one another in order to minimize the frequency of power cycling of the water heater. For example, the maximum temperature set point might be twenty degrees higher than the desired water temperature. The water heater is cycled on until the actual water temperature reaches the maximum temperature set point. When the actual temperature of water in the heater drops to the minimum temperature set point at around the desired temperature, power to the water heater is cycled on again until the actual temperature reaches the maximum temperature set point. A drawback with this approach is that an inordinate amount of energy is required for heating the water in the water heater to a temperature well in excess of the desired temperature. Moreover, the excessive temperature can lead to scalding should water be drawn toward the end of an operating cycle. Further, employing a large water heater can typically leads to temperature striations along various levels of the water heater leading to high fluctuations in water temperature should a high load demand be suddenly imposed on the water heater. 
   A second way to operate a large water heater is to select a maximum and minimum temperature set point relatively close to one another in order to minimize energy consumption. For example, the maximum temperature set point might be only a few degrees higher than the desired water temperature. The water heater is cycled on until the actual water temperature reaches the maximum temperature set point. When the actual temperature of water in the heater drops to the minimum temperature set point at around the desired temperature, power to the water heater is cycled on again until the actual temperature reaches the maximum temperature set point. A drawback with this approach is that the close proximity between the maximum and minimum temperature set points results in frequent on and off power cycling which can shorten the operating life of the equipment for cycling power to the water heater. 
   Instantaneous heaters have also been applied with limited success. Their inability to respond to instantaneous flow changes and high cycling rates of the water heater due to recirculation loads has limited use by this method. 
   Accordingly, it is a general object of the present invention to overcome the drawbacks associated with prior water heater systems. 
   SUMMARY OF THE INVENTION 
   The present invention resides in a water heater system comprising a boiler including a supply port, an output port and a recirculation input port. A heat exchanger has a first input port, a first output port, a second input port and a second output port. An averaging tank has an inlet and an outlet. A first fluid flow subsystem is for controllably directing water along a primary loop through the boiler and from the output port of the boiler to the input recirculation port via either a first path through the first ports of the heat exchanger or a second path bypassing the heat exchanger. A second fluid flow subsystem is for directing water along a secondary loop through the second ports of the heat exchanger, through the inlet and outlet of the averaging tank, and back to the heat exchanger, whereby water directed through the secondary loop is heated from water directed through the primary loop via the heat exchanger. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a water heater system embodying the present invention. 
       FIG. 2  is a schematic diagram of a water heater system in accordance with a second embodiment of the present invention. 
       FIG. 3  are graphs illustrating various operating parameters of the water heater system in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A water heater system embodying the present invention is indicated generally by the reference number  10 . The system  10  comprises a boiler  12 , a controller  14 , a heat exchanger  16 , and an averaging tank  18 . The controller  14  is shown as being separate from the boiler  12 , but it should be understood that the controller can be part of the boiler circuitry without departing from the scope of the present invention. 
   The boiler  12  includes an input port  20 , an output port  22  and a recirculation input port  24 . A first pump  26  has a control terminal  28  for receiving a control signal from the controller  14 , an input  30  coupled to the output port  22  of the boiler  12 , and an output  32  coupled to the recirculation input port  24  of the boiler. A second pump  34  has a control terminal  36  for receiving a control signal from the controller  14 , an input  38  coupled to the output  32  of the first pump  26 , and an output  40  coupled to a first input port  42  of the heat exchanger  16 . 
   As mentioned above, the heat exchanger  16  includes a first input port  42  coupled to the output  40  of the second pump  34 . A first output port  44  of the heat exchanger  16  is coupled to the recirculation input port  24  of the boiler  12 . When the first pump  26  is on and the second pump  34  is on, water flows around a primary loop through the boiler  12 , through the first pump  26 , through the second pump  34 , through the first input and output ports  42 ,  44  of the heat exchanger  16  and back to the boiler. When the first pump  26  is on and the second pump  34  is off, water leaving the output port  22  of the boiler  12  flows through the first pump  26  and returns to the recirculation input port  24  of the boiler so as to bypass the heat exchanger  16  for the reason to be explained more fully below. 
   The averaging tank  18  includes an inlet  46  coupled to a second output port  48  of the heat exchanger  16 , and an outlet  50  for allowing water to be channeled either back to the averaging tank  18  and to remote locations for end use. A third pump  52  for moving water to the averaging tank  18  has a control terminal  54  for receiving a control signal from the controller  14 , an input  56  coupled to a supply line  58  and to the outlet  50  of the averaging tank, and an output  60  coupled to a second input port  62  of the heat exchanger  16 . When the third pump  52  is on, water flows from the supply line  58 , through the heat exchanger  16  via the second input and output ports  62 ,  48 , and through the averaging tank  18  via the inlet  46  and the outlet  50  thereof. Water exiting the averaging tank  18  can then flow via exit line  64  to remote locations for end use. A portion of the water leaving the averaging tank  18  is recirculated by flowing through a return line  66  to the input  56  of the third pump  52 . 
   The system  10  further includes a plurality of sensors communicating with the controller  14  for transmitting to the controller signals indicative of the water temperature at various locations in the system. As shown in  FIG. 1 , a first sensor  68  is located along the primary loop between the output port  22  of the boiler  12  and the input  30  of the first pump  26  to detect the water temperature of the boiler  12  (Tblr) adjacent to the output port of the boiler. A second sensor  70  is located along the secondary loop adjacent to the outlet  50  of the averaging tank  18  so as to detect the set point water temperature (Tsp) of the averaging tank. A third sensor  72  is located along the supply line  58  to the secondary loop so as to detect water supply temperature (Tc) to the secondary loop. A fourth sensor  76  is located along the secondary loop downstream in the direction of water flow of a junction  78  of the supply line  58  and the secondary loop and upstream of the heat exchanger  16  so as to detect water temperature (Tmix) of a mixture of supply water and water leaving the averaging tank  18 . 
   A water heater system in accordance with a second embodiment of the present invention is indicated generally by the reference number  110 . Like elements with the system  10  are indicated by like reference numbers preceded by “1”. The system  110  comprises a boiler  112 , a controller  114 , a heat exchanger  116 , and an averaging tank  118 . The controller  114  is shown as being separate from the boiler  112 , but it should be understood that the controller can be part of the boiler circuitry without departing from the scope of the present invention. 
   The boiler  112  includes an input port  120 , an output port  122  and a recirculation input port  124 . A first pump  126  has a control terminal  128  for receiving a control signal from the controller  114 , an input  130  coupled to the output port  122  of the boiler  112 , and an output  132  coupled to an input  133  of a three-way control valve  135 . The three-way valve  135  further has a control terminal  137  for receiving a control signal from the controller  114 , a first output  139  coupled to a first input port  142  of the heat exchanger  116 , and a second output  141  coupled to the recirculation input port  124  of the boiler  112 . 
   As mentioned above, the heat exchanger  116  includes a first input port  142  coupled to the first output  139  of the three-way valve  135 . A first output port  144  of the heat exchanger  116  is coupled to the recirculation input port  124  of the boiler  112 . When the first pump  126  is on, the first output  139  of the three-way valve  135  is open, and the second output  141  of the three-way valve is closed, water flows around a primary loop through the boiler  112 , through the first pump  126 , directed by the three-way valve through the first input and output ports  142 ,  144  of the heat exchanger  116  and back to the boiler. When the first pump  126  is on, the first output  139  of the three-way valve  135  is closed, and the second output  141  of the three-way valve is open, water leaving the output port  122  of the boiler  112  flows through the first pump  126  and is directed by the three-way valve back to the recirculation input port  124  of the boiler so as to bypass the heat exchanger  116  for the reason to be explained more fully below. 
   The averaging tank  118  includes an inlet  146  coupled to a second output port  148  of the heat exchanger  116 , and an outlet  150  for allowing water to be channeled either back to the averaging tank  118  or to remote locations for end use. A second pump  152  for moving water to the averaging tank  118  has a control terminal  154  for receiving a control signal from the controller  114 , an input  156  coupled to a supply line  158  and to the outlet  150  of the averaging tank, and an output  160  coupled to a second input port  162  of the heat exchanger  116 . When the second pump  152  is on, water flows from the supply line  158 , through the heat exchanger  116  via the second input and output ports  162 ,  148 , and through the averaging tank  118  via the inlet  146  and the outlet  150  thereof. Water exiting the averaging tank  118  can then flow via exit line  164  to remote locations for end use. A portion of the water leaving the averaging tank  118  is recirculated by flowing through a return line  166  to the input  156  of the second pump  152 . 
   The system  110  further includes a plurality of sensors communicating with the controller  114  for transmitting to the controller signals indicative of the water temperature at various locations in the system. As shown in  FIG. 2 , a first sensor  168  is located along the primary loop between the output port  122  of the boiler  112  and the input  133  of the three-way valve  135  to detect the water temperature of the boiler  112  (Tblr) adjacent to the output port of the boiler. A second sensor  170  is located along the secondary loop adjacent to the outlet  150  of the averaging tank  118  so as to detect the set point water temperature (Tsp) of the averaging tank. A third sensor  172  is located along the supply line  158  to the secondary loop so as to detect water supply temperature (Tc) to the secondary loop. A fourth sensor  176  is located along the secondary loop downstream in the direction of water flow of a junction  178  of the supply line  158  and the secondary loop and upstream of the heat exchanger  116  so as to detect water temperature (Tmix) of a mixture of supply water and water leaving the averaging tank  118 . 
   The present invention embodied in the systems of  FIGS. 1 and 2  uses the energy stored in an iron boiler to reduce boiler cycling, the low heat capacity of a plate heat exchanger combined with an averaging tank to maintain temperature accuracy during load changes. 
   The Advantageous of this Type of System Are: 
   
       
       1. Accurate temperature delivery over the entire flow range allowing the reduction of set point temperature with the associated reduction in scalding potential and recirculation losses. 
       2. Small footprint 
       3. Low cycling rates with a modulating iron boiler of moderate turndown (4:1). (The turndown is the continuous change in BTU/hr of which the boiler is capable.) 
       4. A low time constant (the speed with which the system can be readjusted to a new set point) allows variation of set point to meet changing water temperature requirements throughout the day. 
     
  
   The averaging tank acts as a “flywheel” to store sufficient energy to maintain temperature during the boiler start delay and rapid changes in load. 
   The second pump  34  (see  FIG. 1 ) or the three-way control valve  135  (see  FIG. 2 ) in the primary loop moves or directs boiler energy to the secondary loop via the heat exchanger or bypasses the heat exchanger back to the recirculation input port of the boiler. 
   The controller derives the necessary information for the specified performance from the four temperature sensors shown in the embodiments of  FIGS. 1 and 2 . A set point (the operating temperature of the water heater system) and a bandwidth (BW—the total temperature error allowed. IE, To max to To min)) are entered in the controller. A maximum boiler temperature (Tblr max) is also entered into the controller. 
   With respect to the system  10  shown in  FIG. 1 , the first pump  26  and the third pump  52  operate continuously. The first pump  26  maintains flow through the boiler  12  while the third pump  52  continuously mixes the water in the averaging tank  18 . The second pump  34  is turned on by the controller  14  at the minimum water temperature (Tsp−BW/2) to transfer the energy in the primary loop into the water. The second pump  34  is turned off by the controller  14  at the maximum water temperature (Tsp+BW/2) to stop additional energy transfer to the water. The fast response of the second pump  34  and the low heat capacity (WCp) of the plate heat exchanger  16  ensure a rapid system response to the cycling of the second pump  34 . 
   The boiler is started by the controller when either of two conditions is met as will be now explained with respect to the following equations.
 
 DT available=( Tblr−T min)× WCp ( blr )/ WCp (tank)  Equation 1
     Where: DT available is the amount of temperature that the averaging tank can be increased from the energy stored in the primary source (in this case, the KN boiler).
 
 FF %=( T setpoint− T mix)/ T ref  Equation 2
   Where: Tref=qmax/(500.4×Qmix(pump in secondary loop)   

   FF % is the percent of load created by the amount of water drawn from the system. The maximum (100%) load is when the boiler must run at its full output to meet the demand. This signal tells the boiler what energy is needed to meet the instantaneous demand. 
   Terms: 
   
       
       Tblr—the temperature of the boiler water 
       Tmin—the minimum allowed temperature of the potable water 
       WCp(blr)—the energy storage capacity of the primary loop (the boiler) 
       WCp(tank)—the energy storage capacity of the averaging tank 
       Tsetpoint—the desired potable water temperature 
       Tmix—the temperature of the water resulting from the mixture of cold water and averaging tank water being drawn into the system by pump in the secondary loop 
       Tref—the maximum temperature difference that could exist at 100% demand 
       qmax—the maximum net energy available to the system from the boiler 
     
  
   The boiler input energy required (FF) is calculated from Eq. [2]. This is the energy at which the boiler operates when it is running. If the value of (FF) is greater than the minimum input capable by the boiler, the controller immediately starts the boiler. If the value of (FF) is less than the minimum boiler input, then: when the second pump  34  (see  FIG. 1 ) turns on or the control valve  126  is activated to direct water to the heat exchanger (see  FIG. 2 ), the controller determines if there is enough stored energy in the primary loop to raise the temperature of the averaging tank equal to or greater than the bandwidth. If there is, a boiler start is suppressed. If not, the boiler is started by the controller and operates at its minimum input. Once started, the boiler operates until it reaches Tblr max. 
     FIG. 3  illustrates by way of example graphs of various operating parameters of a water heater system in accordance with the present invention. The system being illustrated is operating at about 5% load, has a set point of about 120° F., and has a bandwidth of 6° F. 
   A graph  310  illustrates the water temperature at the output of the boiler (Tboil out) over time. A graph  312  illustrates the water temperature of the averaging tank over time. A graph  314  is indicative of when the boiler is turned on and turned off over time. A graph  316  is indicative of when water flow in the primary loop bypasses the heat exchanger over time. A graph  318  is indicative of water supply demand over time. As can be seen by the graph  312 , a water heater system in accordance with the present invention maintains the temperature of the averaging tank at a generally constant temperature of about 120° F. during the cycling of the boiler and over varying water supply demand conditions. 
   Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.