Abstract:
Apparatus for electrical water heater air entrapment detection includes two electrodes residing at opposite ends of a heater housing. The electrodes are part of a circuit and if either electrode is surrounded by air, the circuit is opened and entrapped air is sensed. The temperature sensors are located at opposite ends of the heater housing, the electrodes may be integrated into the temperature sensors. When entrapped air is sensed, a heater relay is opened preventing powering a heater element(s). A spa pump is operated to clear the entrapped air, and when a variable speed pump is present, the pump is turned to high speed to clear the entrapped air.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electric water heaters and in particular to detecting the presence of entrapped air which may cause a dry fire. 
     Portable spas often use electric spa water heaters. Such heaters include a heater element(s) intended to be completely immersed in water while the heater is operating. Unfortunately, in some instances air may become entrapped in the heater housing, and energizing the heater while air is entrapped in the heater housing results in overheating and possibly destroying the heater element. Because air has a much lower density then water, when the heater is energized while a portion of the heater element is exposed to air, air temperature will be increased rapidly in the heater housing at a rate of 1000 degrees per seconds. Such overheating is the most frequent cause of heater failure in hot tub applications. 
     The heater element comprises an inner wire separated from an outer tubular wall by insulating material, for example, Magnesium Oxide (MgO) insulating material. The outer tubular wall is generally connected to ground. A rapid increase of temperature of even a portion of the heater element destroys or damages the insulating material causing current leakage from the inner wire to the outer wall of the heater element and shorting to ground. The resulting failure is referred to as heater dry fire. 
     Many devices have been made trying to prevent heater dry fire by detecting to the present of fluid inside the heater housing. Examples of such devices include vacuum switch, pressure switch, flow switch, and temperature sensors which monitor the rapid increase of temperature in the heater housing. None of these devises have provided adequate protection to the heater element from dry fire. 
     Unfortunately, operation of a low speed pump may not provide sufficient water flow rate to push an air pocket through the heater housing and may only provide a flow below the air pocket. As a result, the air pocket may be entrapped against the highest interior surface in a heater housing. Vacuum and pressure sensors may fail to detect the air pocket because the fluid (a combination of water and air) inside the heater housing creates enough pressure or vacuum to close the pressure or vacuum sensors circuitry. 
     For example, as the low speed pump pumps water through the heater housing, the pump would create pressure inside the heater housing, even when air is entrapped inside the housing, where a pressure switch is mounted. Known pressure switches, at as low as 1.5 PSI, send a signal to a heater control circuit to activate the heater. The heater control circuit turns the heater “on”, heating the water circulating through the heater. Entrapped air inside the heater housing is compressed resulting in uniform pressure in both the water and entrapped air. If the heater comes “on” when air is present inside the heater housing, the heater (i.e., the pressure switch) cannot distinguish between water pressure and air pressure, resulting in heater damage almost immediately, for example in less than 10 seconds, depending on heater wattage. 
     U.S. Pat. No. 8,406,932 to Hollaway discloses a heater including a sensor which monitors an abnormal rise in temperature in the heater housing. Unfortunately, this method is not be effective at all for many reasons: 
     1—The sensor(s) can only detect the rise in temperature after the heater is turned on. When the heater comes on with air trapped in the heater housing, the heater internal insulation is destroyed or damaged by the time the sensors detect the rapid rise in temperature and turn off the heater.
 
2.—The temperature sensors cannot distinguish between air and water temperature in the heater housing. They can only turn the heater “On” or “OFF”. The electric heater can be heating air, water or a combination of air and water
 
     U.S. Pat. No. 7,791,004 to Reusche discloses temperature sensors and a fluid sensor attached to a water heating element insertable into a bucket of water. Unfortunately, the fluid sensor only detects the absence of water in a particular location and would fail to detect entrapped air residing against the ceiling of a water heater housing. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing apparatus for electrical water heater air entrapment detection which includes two electrodes residing at opposite ends of a heater housing. The electrodes are part of a circuit and if either electrode is surrounded by air, the circuit is opened and entrapped air is sensed. The temperature sensors are located at opposite ends of the heater housing, the electrodes may be integrated into the temperature sensors. When entrapped air is sensed, a heater relay is opened preventing powering a heater element(s). A spa pump is operated to clear the entrapped air, and when a variable speed pump is present, the pump is turned to high speed to clear the entrapped air. 
     In accordance with one aspect of the invention, there is provided an entrapped air detector which distinguishes between air and water. Air is an electric insulator however water is a conductor. The present invention is capable to distinguish between the two fluids. This new invention will protect the heater by not turning the heater “on” when air is present in the heater housing. The new invention will only turn the heater “on” if water is present in the heater housing. The current invention consists of measuring the presents of conductivity between the inlet and the outlet of the heater housing also known as heater manifold. 
     In accordance with another aspect of the invention, there is provided an entrapped air detector including a spa pump control circuit. When entrapped air is detected in the heater housing, the spa pump control circuit delays power to the heater element and turns a variable speed spa pump to high speed to clear the entrapped air. 
     In accordance with yet another aspect of the invention, there is provided a method for preventing damage to a spa heater due to entrapped air. The method includes monitoring a circuit including two electrodes residing at opposite ends of a heater interior, measuring the electrical resistance between the electrodes, and delaying providing power to a spa heater element if the measured resistance is above a threshold. The method may further include turning a spa pump to high speed if the measured resistance is above the threshold, and if the measured resistance is below the threshold for a period of time: providing power to the spa heater element; and turning the spa pump to low speed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  is a spa including a spa heater unit according to the present invention. 
         FIG. 2  depicts a side view of a first spa heater unit according to the present invention. 
         FIG. 3  is a cut-away view of a first spa heater of the spa heater unit according to the present invention. 
         FIG. 4  depicts a side view of a second spa heater unit according to the present invention. 
         FIG. 5  is a cut-away view of a second spa heater of the spa heater unit according to the present invention. 
         FIG. 6  is a heater element of the spa heater according to the present invention. 
         FIG. 6A  is a cross-sectional view of the heater element taken along line  6 A- 6 A of  FIG. 6  according to the present invention. 
         FIG. 7  shows a metal heater housing and an insulated electrode accord according to the present invention. 
         FIG. 8  shows a method according to the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims. 
     A spa  10  is shown in  FIG. 1 . The spa  10  includes drains  12   a  and  12   b . The drains  12   a ,  12   b  are in fluid communication with a pump  14  through first lines  16   a  and  16   b  carrying flows  17   a  and  17   b  respectively, through a filter  13  and to the pump  14 . A spa heater unit  18  is in fluid communication with the pump  14  through a second line  20  carrying second flow  21 . A spa-side control/display  11  is electrically connected to the spa heater  18  by control wires  11   a  for controlling the spa  10 , or may be wirelessly connected to the spa heater unit  18 . The control/display  11  generally includes user controls for controlling the mode of operation of the spa  10 . The heater unit  18  is in fluid communication with at least one jet  22  through line  24  carrying a third flow  25 . Water  26  is thereby circulated, filtered, and heated. 
     The pump  14  may be powered by a single speed motor at maximum 1725 rpm or a variable speed motor. The variable speed may be as low as 1000 rpm and as high as 5000 rpm. The low speed, 1000 rpm, is usually used to circulate the water from the drains  12   a  and  12   b  through the spa heater unit  18  to cool down the spa heater. Also, providing a flow of water through the spa heater using low speed saves electricity compared to providing a flow of water through the heater at high speed. Because time is required to increase the temperature of a typically 400 gallon hot tub regardless of pump speed, low speed may be used in order to save electricity. 
     A side view of a first spa heater unit  18   a  is shown in  FIG. 2  and a cut-away view of a first spa heater  40   a  of the spa heater unit  18   a . The spa heater unit includes a connection  30  receiving power through wires  31 . The power may be 110 volt AC power or 220 volt AC power, but is preferably 220 volt AC power. Wires  31  carry power to a main power relay  32  which controls switched power through wires  51   a  and  51   b  to the spa heater  40   a  to energize the spa heater  40   a . A neutral (e.g., ground) wire  45  runs from the connection  30  to current collectors  44   a  and  44   b  in the spa heater  40   a.    
     A processor  38  is connected temperature sensors  48   a  and  48   b  by wires  49   a  and  49   b  to monitor the spa heater  40   a  for over temperature and electrodes  46   a  and  46   b  are connected to the processor  38  by wires  47   a  and  47   b  to monitor for entrapped air in the spa heater  40   a . Continuity in a circuit including the electrodes  46   a  and  46   b  is monitored and if either electrode  46   a  or  46   b  is surrounded by entrapped air, high resistance between the electrode  46   a  or  46   b  will result. When the processor  38  detects the high resistance, a second relay  34  is actuated which opens the main power relay  32 . 
     The spa pump  14  will come on and tends to clear the entrapped air from the spa heater  40   a . The processor  38  continues to monitor the electrodes  46   a  and  46   b  and if sufficient continuity is measured between the electrodes  46   a  and  46   b , power will be provided to the spa heater  40   a . The processor  38  may also control the spa pump  14  speed when the spa pump is a variable speed pump, and turn the speed to high to help clear the entrapped air from the spa heater  40   a.    
     The processor  38  is further electrically connected to the spa-side control/display  11  and the spa pump  14 . When the processor  38  detected entrapped air in the heater housing  50 , and message may be displayed on the spa-side control/display  11  informing a user that the spa heater  40   a  should be purged of entrapped air. The processor  38  may also be connected to the spa pump  14  to turn the spa pump to high speed to clear the heater housing  50  of entrapped air when the spa pump is a variable speed pump, and to turn the spa pump  14  back to low speed after the entrapped air has been cleared from the housing  50 . The housing  30  is preferably made from a non-electrically conductive material, and more preferably made from a plastic material. 
     A side view of a second spa heater unit  18   b  according to the present invention is shown in  FIG. 4  and a cut-away view of a second spa heater  40   b  of the spa heater unit  18   b  is shown in  FIG. 5 . The second spa heater unit  18   b  and second spa heater  40   b  are similar to the first spa heater unit  18   a  and first spa heater  40   a , with the exception that the second spa heater  40   b  includes temperature sensors  48 ′ a  and  48 ′ b  which are moved to opposite ends of the heater housing  42  and include electrodes  46 ′ a  and  46 ′ b.    
     The heater element  50  is shown in  FIG. 6  and a cross-sectional view of the heater element  50  taken along line  6 A- 6 A of  FIG. 6  is shown in  FIG. 6A . The heater element  50  comprises an inner wire  52   a  separated from an outer wall by insulating material  54 , for example, Magnesium Oxide (MgO) insulating material. When even a small portion of the heater element  50  is exposed to entrapped air when receiving power, the temperature of the exposed portion rises very quickly and damage to the heater element  50  results. 
       FIG. 7  shows a portion of a metal (or any electrically conductive) heater housing  42   a . Because the heater housing  42   a  itself is electrically conductive, at least one of the electrode  47   a  and  47   b , and preferably both electrode  47   a  and  47   b , are mounted using electrical insulators  43 . 
       FIG. 8  shows a method according to the present invention. The method includes monitoring a circuit including two electrodes residing at opposite ends of a heater interior at step  100 , measuring the electrical resistance between the electrodes at step  102 , and delaying providing power to a spa heater element if the measured resistance is above a threshold at step  104 . The method may further include turning a spa pump to high speed if the measured resistance is above the threshold at step  106 , and if the measured resistance is below the threshold for a period of time: turning the spa pump to low speed; and providing power to the spa heater element at step  108 . If after turning the spa pump to low speed, the measured resistance increases to above the threshold, power to the spa heater element will continue to be delayed and the spa pump will be returned to high speed. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.