PULSE WIDTH MODULATION CONTROL FOR HEAT PUMP FAN TO ELIMINATE COLD BLOW

BACKGROUlSfD OF THE INVENTION This application relates to a heat pump, wherein a fan for moving air into a conditioned environment is provided with a pulse width modulation control to address the problem of "cold blow".

Heat pumps are known in the art and utilized to provide cooling to a conditioned environment during time periods of hot weather or excessive internal thermal load generation, and to provide heat to the same indoor environment when the weather is cold. Also, there are known a more simplistic heat pump designs that are able to operate just in a heating mode. Heat pumps have great potential to provide efficient conditioning to the indoor environment, however, there have been impediments to their use. ^' One known problem with existing heat pump designs is so-called "cold blow."

"Cold blow" occurs when the heat pump does not have sufficient heat rejection capability to adequately heat air being driven into the environment to be conditioned.

When this phenomenon occurs, air driven over the indoor heat exchanger and into the environment to be conditioned is not heated to the temperature desired by the occupant of the environment, causing uncomfortable conditions to the occupant, that is of course undesirable. >

It has been known to address "cold blow" by reducing the volume of air delivered into the environment to be conditioned either through the use of a variable speed drive, or through a two-speed fan motor. A two-speed fan motor does not provide sufficient flexibility to adequately tailor the airflow to achieve the desired temperature. A variable speed drive may provide such flexibility, however, it is quite expensive, represent an additional source of potential reliability problems and associated with efficiency losses. Thus, there has not been an adequate cost effective solution offered to resolve this problem. Pulse width modulation controls are known for controlling the amount of refrigerant passing to a compressor in a refrigerant system, such as an air conditioning

system or a heat pump. However, pulse width modulation controls have not been utilized to address the "cold blow" problem mentioned above.

SUMMARY QF THE INVENTION In a disclosed embodiment of this invention, fan moving air over an indoor heat exchanger is operated in a pulse width modulated manner. The use of the pulse width modulation control precisely tailors the amount of air moved over the indoor heat exchanger and into the climate-controlled environment, such that the heat rejected by the indoor heat exchanger, in the heating mode of operation, to the indoor air stream is sufficient to heat the controlled volume of air to the desired temperature. Thus, if less heat is rejected by the indoor heat exchanger and available to heat the air, the amount of air being driven into the environment will be reduced accordingly, such that air is delivered to a climate-controlled environment at the target temperature.

By closely controlling and cycling the fan between "on" and "off positions, in single-speed fan applications, the present invention is able to precisely control the temperature of air delivered to the indoor space. If a two-speed fan is utilized, the pulse width modulation control can cycle the fan between the lower and a higher speed to achieve the desired effect. In the tatter case, the cycling between a lower speed and zero speed as well as a higher speed and zero speed is also permissible, if desired.

The time interval during which the fan is engaged in a full-speed position, for a single-speed fan, or in a higher speed position, for a two-speed fan, is determined by the temperature requirement and comfort level, while the cycle rate is primarily determined by fan assembly reliability requirements and temperature variation tolerance bounds. Further, frequent cycling is not necessary, since refrigerant system thermal inertia compensates for sudden changes in fan speed. Also, the fan does not have to be brought to a full stop state, between activation and deactivation of the pulse width modulation signal, since the mechanical inertia allows for a softer start in a subsequent cycle.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic of a heat pump incorporating the present invention.

Figure 2 shows a cycling sequence for a single-speed fan. Figure 2 shows a cycling sequence for a two-speed fan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in Figure 1 and includes a compressor 22 delivering a refrigerant to a discharge line 23, and through a four-way valve 24 (if a heat pump is dedicated to heating applications only, then a four-way valve is not required) to an indoor heat exchanger 26. Downstream of the indoor heat exchanger

26, the refrigerant passes through an expansion device 28, and then to an outdoor heat exchanger 30. The outdoor heat exchanger 30 is provided with a fan 32 to move air over the external heat transfer surfaces of the outdoor heat exchanger 30.

Downstream of the outdoor heat exchanger 30, the refrigerant passes again through the four- way valve 24, and into a suction line 33 returning the refrigerant to the compressor 22. The refrigerant system 20 is illustrated in Figure 1 in a heating mode of operation. The refrigerant system 20 can be moved to an air conditioning cooling mode of operation by switching the four-valve 24 and routing the refrigerant from the discharge line 23 initially to the outdoor heat exchanger 30, through the expansion device 28, and returning the refrigerant from the indoor heat exchanger 26 to the suction line 33. However, the present invention is directed to an improvement that is particularly applicable when the refrigerant system is in a heating mode of operation.

The Figure 1 schematic for the heat pump 20 is a basic schematic, and as known to a person ordinarily skilled in the art, can be improved by adding a number of enhancement features and various options. All these designs are within the scope and can benefit from the invention.

As shown in Figure 1, an air-moving device, such as fan 34, moves air over the indoor heat exchanger 26 and into an environment to be conditioned 36. In the heating mode of operation, the heat exchanger 26 performs a condenser (or a gas cooler, for transcritical applications) function. At times (for instance, at lower ambient temperatures or at high heating load demands in the conditioned space), there may be insufficient heat rejection capacity provided by the heat exchanger 26 to adequately heat a nominal volume of air driven by the fan 34 over external heat transfer surfaces of the heat exchanger 26 and into the indoor environment 36. In such situations, the indoor air stream does not reach the desired temperature creating uncomfortable so- called "cold blow" conditions for an occupant of the indoor environment 36. A control 38 (that could be a stand-alone control or a refrigerant system control) is provided with a feedback communication loop from a temperature sensor 49 and is capable to operate the indoor fan 34 in a pulse width modulation mode. Therefore, the control 38 can detect a lower than desired temperature of the air being delivered into the indoor environment 36. In such cases, the control 38 is operable to provide a pulse width modulation control to a motor for the indoor fan 34 such that the average volume of air supplied by the indoor fan 34 to the conditioned environment 36 is reduced. As the average volume of air is sufficiently reduced the heat rejected by the indoor heat exchanger 26 is sufficient enough to heat that reduced volume of air to a temperature desired by an occupant of the conditioned environment 36. As known, the desired temperature may be set by a thermostat 50. Alternatively, the thermostat 50 can be utilized as a feed back device for the control 38.

By utilizing the pulse width modulation control, a single-speed motor for the indoor fan 34 is rapidly cycled between "on" and "off (or fully engaged and fully disengaged) positions. In the case of a two-speed fan, the indoor fan motor may be rapidly cycled between its higher and lower speed positions, as well as between the lower speed position and an "off position and between the higher speed position and an "off position". In either case, the volume of air delivered into the environment 36 is precisely adjusted, such that the heat rejected by the indoor heat exchanger 26 is adequate to heat this adjusted air volume to the desired temperature. As mentioned

above, when a multi-speed fan motor is used, the pulse width modulation cycling can be executed between any of the speeds, including a speed of zero.

It is proposed to control the indoor fan 34, in the heating mode of operation, by pulse width modulation method to precisely adjust the temperature of the conditioned (heated) air delivered to the indoor environment 36. This control is straightforward and does not require additional components. The cycling frequency is determined by the indoor fan assembly reliability and temperature variation tolerance requirements. The time interval at each speed position is defined by the required air temperature values and comfort level to be achieved and efficiency considerations. Frequent cycling will not be necessary and is avoided, since refrigerant system thermal inertia smoothes sharp variation of operational parameters and compensates for sudden abrupt peeks and valleys. Cycling can be executed between a zero and full speed, for a single-speed fan, and between the lower and the higher speed positions, as well as between the lower speed position and an "off position and between the higher speed position and an "off" position", for a dual-fan speed fan. Multi-speed fans provide even a higher degree of flexibility and precision control. Lastly, indoor fan mechanical inertia may assist in continuous rotation of the indoor fan (although at a constantly reducing speed), while the pulse width modulation signal is activated and deactivated allowing for a softer start in a subsequent cycle. Figure 2 is an exemplary chart of a temperature of air supplied to the conditioned space versus time, for a refrigerant system wherein the indoor fan motor is operable at a single, full speed, in an "on" or fully engaged position, and at a zero speed, in an "off or fully disengaged position. As can be seen, initially, the indoor heat exchanger is not capable to provide sufficient amount of heat to the nominal volume of air supplied to the conditioned environment, and the actual temperature is below the desired temperature, promoting "cold blow" conditions. Also, as can be seen, when the pulse width modulation control for the indoor fan is engaged, the indoor fan motor begins to cycle between a full speed (or an "on" position) and a zero speed (or an "off position), reducing the time-average amount of air supplied to the conditioned environment. As a result, the actual supply air temperature, increases, as

well as approaches and reaches the desired temperature, while the pulse width modulation cycle parameters are adjusted to the correct values.

Similarly, Figure 3 shows another embodiment, wherein the fan motor is operable at two speeds and can be cycled by the pulse width modulation control between the higher and lower speed positions, as well as between the lower speed position and an "off position and between the higher speed position and an "off position". As an example, here, the fan is cycled between the lower speed position and the higher speed position. Again, the actual supply air temperature increases and soon approaches the desired temperature, with this arrangement. It has to be noted that although a square waveform is used in Figure 2 and 3 embodiments for the pulse width modulation control, other waveforms are also feasible and within the scope of the invention. For instance, a trapezoidal, a triangular, a rounded square or any other waveform can be utilized instead.

Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.