Patent Application
20110095010
The present application relates generally to immersion type resistance heaters used for heating water and, more particularly, to water heating systems used in dishwashers or ovens.
Resistive heating elements are commonly used in connection with commercial food equipment for generating heat. For example, such elements are used in steam ovens and dishwashers for heating water, where the resistive heating elements are immersed in the water that is to be heated. Lime scale or other insulating material tends to build up on the elements degrading the performance of the element by impeding the transfer of heat to the surrounding water.
In the past, temperature sensing devices have been used in attempt to identify abnormal or degraded heating elements. While this approach provides an accurate temperature measurement, it does so only at a single location on the element. If the buildup occurs at another location on the element, the degradation may go undetected. Using multiple temperature sensors along the element would be expensive and impractical.
Accordingly, it would be desirable and advantageous to provide a water heating system that more effectively identifies degradation of immersion type resistive heating elements.
In one aspect, a water heating system includes a tank for holding water and a resistive heating element associated with the tank for heating water in the tank. A control is connected for monitoring at least one resistance parameter of the resistive heating element as power is applied to the resistive heating element, the control configured to output a fault signal if the monitored resistance parameter exceeds a set threshold.
The monitored parameter may be one or more of (i) a rate of resistance change during heater start-up, (ii) a rate of resistance change during steady state heater operation or (iii) a heater resistance corresponding to a set point in time following heater start-up.
Referring to
A delime agent source 40 is connected to the heating chamber 18 via inlet 42 and associated valve 44. In some embodiments, the delime agent source 40 may be connected to the water inlet 20. In other arrangements, an opening (e.g., with removable cap) could be provided to facilitate manual introduction of the delime agent into the heating chamber 18. During a cleaning and draining operation, delime agent is introduced to the water in the heating chamber 18 to help remove lime and scale from the heating element.
A controller 50 is connected for controlling energization of the heating elements 32, 34 (e.g., in accordance with a cooking program of the steam oven) and may also be connected with the various valves and sensors (e.g., temperature sensors and/or water level sensors) of the system for proper oven control and may be connected with a communication device or channel 52 for sending and receiving messages to other computer devices (e.g., personal computers, cell phones, PDAs etc.). The controller 50 is also programmed or otherwise configured to monitor the resistive heating elements as will be described in further detail below.
Referring to
In a alternative dishwasher implementation, the dishwasher may be formed as a box-type machine (also known as a batch-type or door-type machine) in which dishes are manually placed in a chamber for washing and rinsing sprays, and the dishes are then removed after cleaning. Such machines, which could be hood-type machines or undercounter type machines, may include one or more resistive heating elements in the sump tank of the machine (e.g., from which water is pumped and delivered to nozzles for spraying in a recirculated manner) and/or in a booster heater tank used for heating rinse water that is delivered to spray nozzles of the machine.
The electrical resistance of a conductor is dependent on the subatomic collisions within the material. The energy of subatomic collisions is related to the temperature of the conductor and as such the electrical resistance of a conductor is proportional to its temperature as defined by the following equation ΔR/R0=αΔT, where:
ΔR=change in resistance;
R0=Initial Resistance of heating element;
α=Temperature Coefficient of Resistance; and
ΔT=Change in Temperature.
By measuring voltage across and current through the resistive element of a heater it is possible to calculate the resistance of the heater. Any localized increase in resistance will reduce the current flow in the overall element.
Referring to
During normal operation of a resistance heating element in a machine, the controller 50, 92 monitors the element to determine if one or more resistance parameters or metrics of the element changes in a manner that is indicative of significant degradation of the element. This monitoring operation would, preferably, take place during start-up and/or before the element reaches thermal equilibrium with the water (or other fluid) being heated. Specifically, the controller 50, 92 would monitor voltage and current, calculate resistance values and, based upon the calculated resistance values calculate or otherwise define the rate change metrics and/or actual resistance at a point in time metric, or indicators thereof, and compare them to the corresponding stored characteristic metrics for the element. As a heater becomes insulated through extended use (e.g., due to significant lime build up) the resistance verses time graph translates as shown in
The controller 50, 92 compares the newly measured and calculated metric values to those in memory. If the change in the metric from the stored characteristic metric is within an acceptable or set window, range or tolerance of the stored characteristic metric, the monitored metric is determined to be within or less than a set threshold that has been established for the metric. On the other hand, if the monitored metric is outside the window or range, the metric is determined to exceed a set threshold for the metric, and the controller outputs a fault signal.
In one implementation, the fault signal is used to alert a machine operator to potential element failure by effecting energization of an annuciator element 100 (see
The subject monitoring operation could also be utilized to identify a condition when the resistive heating element is not immersed. In such an embodiment the temperature of the resistive heating element will rise rapidly and thus the resistance of the element will also rise rapidly. A rate of change in excess of a certain threshold rate of change, or a total resistance in excess of a certain threshold resistance, would indicate a lack of immersion of the element, prompting shut down of power to the element and/or triggering of an annunciator or service person communication that identifies the problem. The threshold levels might typically be higher than the set threshold levels that merely indicated a degraded non-failure state of the element.
Although the invention has been described and illustrated in detail it is to be clearly understood that the same is intended by way of illustration and example only and is not intended to be taken by way of limitation. For example, in some implementations, such as where the voltage applied to the resistive heating element is constant and does not vary over time, the current may only need to be monitored to determine resistance, and the determination could be from a calculation or other means, such as a look-up table. Moreover, while the system has been described and discussed in the context of a steam oven and/or dishwasher, the element monitoring technique described herein could be utilized in other type of machines. It is recognized that numerous other variations exist, including both narrowing and broadening variations of the appended claims.
