Patent Application
20070251251
The present invention relates to control strategies for heat exchangers, and specifically, evaporators, used in air conditioning systems.
An automotive heating, ventilation, and air conditioning (HVAC) system usually comprises heat exchangers such as an evaporator for cooling air and a heater-core for heating air. The evaporator is designed to transfer heat from the incoming air to the passenger compartment in order to cool the passengers and defog the windows of the vehicle. The cold air flows from the evaporator and is discharged through vents into the compartment of the vehicle. Auxiliary evaporators are used to provide cold air to passengers in seating locations other than the front row, or to electronic components such as batteries, motors, cold storage areas or power modules.
Heat exchangers such as evaporators function at varying temperatures due to outside environment conditions. For example, evaporators have a tendency to reach freezing temperatures (below 0° C.) while functioning in an automotive HVAC unit under low load conditions, such as low inlet air temperature, low relative humidity of the inlet air, and/or reduced airflow rates through the evaporator due to blower speed changes.
Control strategies in production exist to prevent auxiliary evaporator freeze in air conditioning systems having primary and auxiliary evaporators as part of the system. In general, auxiliary evaporator freeze-up is abated or reduced by using a refrigerant valve operated with a solenoid, whereby as the temperature decreases the refrigerant valve closes to stop the flow of refrigerant to the auxiliary evaporator thereby eliminating or reducing the heat transfer medium. This solenoid is operated in a manner similar to the compressor clutch in a front HVAC evaporator system utilizing a cycling clutch freeze protection strategy.
In the automotive contexts, an HVAC system is often found having front and auxiliary heat exchangers, such as evaporators. A front evaporator is found in front of the automotive vehicle, and usually serves to provide cold air and therefore, thermal comfort to the occupants in the front row and to provide dehumidified air to defog the windshield and front side windows. An auxiliary evaporator is found in the rear of a vehicle, and usually serves to provide cold air and therefore thermal comfort to occupants in the rear of the vehicle and/or to provide cold air to electronic components such as batteries, motors, or power modules. A front evaporator can be protected from freezing by using either a cycling clutch fixed displacement compressor or variable displacement compressor in a controlled system. The temperature of the evaporator is controlled by a system utilizing either a feedback signal from a temperature sensor mounted in the evaporator fins (or immediately downstream in the air path from the evaporator); or by reaction to a signal received based upon pressure measurements from a low side refrigerant path (e.g. evaporator return line). Current technology has attempted to control auxiliary evaporator freezing through an electromechanical refrigerant valve which is cycled or regulated in a similar manner to a fixed displacement compressor; whereas the compressor/clutch is cycled on and off.
The present invention provides for a method and HVAC system design with a simpler and/or more efficient freeze protection function. Various aspects also reduce the possibility of acoustic phenomena (evaporator hiss for instance) due to refrigerant system cycling (via valve or compressor clutch), from occurring as it functions in a different manner from the prior art and the refrigerant flow is not interrupted or changed.
The present invention, in various aspects, relates to a method and a device to prevent freezing of an auxiliary evaporator in an HVAC system utilizing more than one evaporator.
Though prior art control system may have desired end effects on heat exchange surface temperatures, it has been found that HVAC systems having an auxiliary heat exchanger, and, in particular an auxiliary evaporator, in addition to the primary heat exchanger, such as evaporator, lack an adequate way to assure correct functioning at minimum temperatures that can be reached in mild operating conditions. For example, auxiliary evaporators in such systems can freeze when run at a lower evaporator load condition relative to the primary evaporator. Auxiliary evaporator freeze can result in reduced passenger comfort, evaporator failures including rupture due to freeze/thaw cycles, odor complaints, and performance complaints such as loss of cold air and airflow.
The present invention, in various aspects, provide for an HVAC system having a temperature sensing device, and a method for altering and/or controlling auxiliary evaporator temperature in an HVAC unit. In general, air flows through the heat exchangers of an automotive HVAC unit, having been pulled, pushed or otherwise drawn through the unit, prior to coming out of the evaporator (being ‘discharged’). Air, therefore, is discharged downstream of the heat exchanger. In various aspects of the present invention, air is discharged downstream of an auxiliary heat exchanger of the HVAC unit, and the temperature of the discharged air or surface temperature of the heat exchanger is measured via a temperature sensing device. The temperature sensing device monitors the temperature and sends a signal to a control device. The control device causes a response to be sent out as a command to the blower device based on the relative value of the signal. The blower device then causes the speed of the blower, and, therefore, the flow volume of the discharged air to be increased, linearly or by increments, for example, percent of full speed until the temperature of the sensing device, is above a predetermined threshold value.
Aspects of the present invention include, therefore, a method for regulating the temperature of an heat exchanger of an automotive HVAC system, having a blower and a temperature sensing device capable of detecting temperatures that drop below a desired minimum operating temperature, by: measuring the surface temperature or the discharge air temperature of the heat exchanger using the temperature sensing device; detecting temperature conditions where the temperature in or around the heat exchanger drop below that minimum; sending a signal from the temperature sensing device to a control logic device; calculating a response to send as a command from the control logic device to the blower motor or blower motor controller based on the signal; increasing incrementally or linearly the airflow volume from the blower device in response to the response signal, thereby altering the operating temperature of the heat exchanger by the increased flow of air through the heat exchanger; and, preventing the temperature of the heat exchanger from reaching or being sustained at defined threshold level in normal operation. In various aspects of the present invention, where the blower device has a motor, the parameter or parameters to be regulated or controlled are selected from the group consisting of speed, current or voltage of the blower motor.
In aspects of the present invention, developed control strategies utilize a thermistor or other temperature sensing device capable of providing a signal corresponding to the propensity of the evaporator to reach freezing temperatures. The temperature sensing device, is generally located in, on or just downstream of the auxiliary evaporator. A temperature sensing device can be a detector, such as an infra-red or other such a device, or sensor, or any device capable of directly or indirectly determining the parameters necessary to indicate a danger of “freeze condition” or freezing of an operator unit.
An evaporator temperature sensing device, such as a thermistor, detects conditions where the auxiliary evaporator could freeze via measuring temperature, and providing such information, as signal to a control device; the control device processes the signal via a control algorithm operation and provides incremental commands that cause specific parameters to be incrementally increased, such as to incrementally increase the auxiliary blower speed to prevent auxiliary evaporator freezing. For example, parameters such as blower motor voltage, operating speed or current may be regulated, depending on the motor technology used, to prevent the freeze condition. For example, brushless blower motors may regulate blower speed based upon a blower speed control set point signal provided by the HVAC control logic. In various aspects of the present invention, a defined threshold level is correlated to the freezing point of the auxiliary heat exchanger, particularly where the auxiliary heat exchanger is an evaporator.
In various aspects of the invention, an evaporator thermistor is an existing part of an automotive or air conditioning system, which measures various other parameters for control purposes in the system. In aspects of the present invention, control logic involves implementation and validation of software algorithms used for the blower control in response to the thermistor measurements.
A temperature sensing device is useful to monitor auxiliary evaporator discharge air temperature, for example, by means of a thermistor (of same design as used for main evaporator freeze protection). When thermistor temperature corresponds to the evaporator freeze threshold, (or a certain minimum threshold) the auxiliary blower motor speed is increased until the auxiliary evaporator discharge air temperature is above the freeze threshold.
Blower device parameters can be regulated in their responses by varying a supply voltage or a command signal. In various aspects of the present invention, wherein the blower device has a motor, the parameter to be regulated or controlled can be speed or voltage of the blower motor. Also in various aspects wherein control logic device is present, the control logic device can use control logic that involves implementation and validation of software algorithms used for blower control.
The HVAC unit has, in various aspects, other devices that are regulated by one or more control parameters other than blower device parameters. In aspects wherein the output of the temperature sensing device affects one or more control parameters other than blower device parameters, it also can affect at least one blower device parameter.
In automotive HVAC systems having auxiliary evaporators, the use of the methods of regulating of the heat exchangers, in accordance with the present invention, reduce or prevent auxiliary system thermal transients or variation in the temperature of the air discharged from the auxiliary evaporator due to cycling which occurs in prior art solutions to the problem of auxiliary evaporator freeze control wherein a refrigerant valve is used.
In automotive HVAC system having auxiliary heat exchangers, and, in particular, evaporators, aspects of the present invention provide auxiliary evaporator freeze protection without impacting the main evaporator performance. In such a system, embodiments of the present invention prevent thermal transients of the front AC system associated with rear system cycling using a valve as the refrigerant mass flow rate through the primary evaporator does not change.
In an embodiment of the invention, a second set of actual operating parameters comprises a speed of a blower motor or volume of the air discharged from a blower through an evaporator. The advantage of this embodiment of the present invention is to provide a method to prevent auxiliary evaporator freeze-up which requires the use of a single temperature sensing device. Therefore, the present invention can be implemented at low cost.
Preferably, a control method is implemented within an HVAC control device, which comprises memory means storing a software program adapted to implement the method and processing means adapted to execute the program, to process received signals, and to generate a command or commands to the blower device, particularly such that the blower device airflow output is affected in an incremental or linear fashion. A control logic device is often located in an auxiliary or main control head of the HVAC module.
While the refrigerant system of the HVAC system (1) is operating, the evaporator will operate with a surface temperature dependant upon refrigerant compressor speed, airflow through the evaporator, temperature of the air entering the evaporator, humidity and several other parameters.
This temperature (2) can be measured with various types of sensors, and compared with a desired minimum operating temperature (3).
A temperature below the selected minimum is referred to as “Tevaporator Deficiency” (4) as a positive or negative value
An additional boundary (5) can be applied allowing only controller actions to increase the evaporator temperature.
Output signal “Bounded Tevaporator Deficiency” is monitored by a controller (6) (in this instance an integrating controller) that computes a bias value associated with the signal properties.
Tuning parameter (7) in the form of “Blower Bias Gain” is applied to allow for varying the aggressiveness of the algorithm.
A “Bias Limiter” (8) is applied to limit the maximum bias that may be added by the algorithm to a specified value (9).
“Bias” value is added (10) to the baseline blower command resulting in an incremental or linear increase of the blower speed command depending on the application.
The higher operating speed of the blower will cause an increase of the airflow through the evaporator resulting in an increased air discharge and increased heat transfer from the air to the evaporator. This raises the operating temperature of the evaporator core.
As can be envisioned from the above, when a blower device parameter is regulated in its response by varying a supply voltage or a command signal, and the temperature sensing device, for example, is a thermistor, thermocouple, or infra-red sensor, the auxiliary evaporator core average temperature is, under appropriate circumstances, operated at a time averaged temperature above 0° centigrade.
Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. Plural structural components can be provided by a single integrated structure. Alternatively, a single integrated structure might be divided into separate plural components. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention.
The preferred embodiment of the present invention has been disclosed. A person of ordinary skills in the art would realize, however, that certain modifications will come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.
