Forced air heat exchanging system with variable fan speed control

Liquid refrigerant temperature, measured by a sensor adjacent the flow control device, provides multiple diverse pieces of information which are used to control the indoor fan speed. liquid refrigerant temperature is used as an indication of outdoor air temperature and is also used as an indication of whether the compressor is operational or not.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates generally to heat pump and air-conditioning 
systems (HVAC systems). More particularly, the invention relates to an 
improved forced air system in which the indoor fan speed is variably 
controlled based on measured liquid refrigerant temperature. 
Heat pump and air-conditioning systems have become quite sophisticated in 
recent years, due in part to the desire for high efficiency and improved 
occupant comfort. Many of today's control systems for heat pumps and air 
conditioners use microprocessor-based electronics with a wide assortment 
of temperature, pressure and humidity sensors. On the one hand, these 
microprocessor-based systems, with multiple diverse sensors, are quite 
versatile and are far better able to optimize efficiency and occupant 
comfort than worthy simple systems of the past. On the other hand, 
microprocessor-based systems are becoming increasingly complex. 
By way of example, U.S. Pat No. 5,303,561 to Bahel et al., entitled 
"Control System for Heat Pump Having Humidity Responsive Variable Speed 
Fan," issued Apr. 19, 1994, a microprocessor-based control system is 
described. That patent is assigned to the assignee of the present 
invention. It describes a system which controls indoor fan speed based on 
humidity measurements, to produce a slower airflow when conditions are 
humid, in order to help remove moisture from the air. 
By way of further example, U.S. Pat. No. 5,303,562 to Bahel et al. entitled 
"Control System for Heat Pump/Air-Conditioning System for Improved Cyclic 
Performance," issued Apr. 19, 1994, another microprocessor-based control 
system is described. This patent is also assigned to the assignee of the 
present invention. It describes a system which optimizes efficiency of the 
ON/OFF refrigeration cycle. Indoor fan speed is controlled by a 
proportional electrical signal driving a variable speed motor to optimize 
airflow in relation to the temperature of the heat exchanging elements. 
Both of these systems employ a plurality of temperature sensors and a 
humidity sensor. 
The present invention seeks to retain the advantages of 
microprocessor-based technology, particularly with regard to forced 
airflow control. Departing from the technology of the past, however, the 
present invention seeks to accomplish this purpose by a simplified sensing 
arrangement. As described more fully below, the presently preferred 
embodiment is capable of providing forced airflow control using a single 
sensor measuring condenser liquid refrigerant temperature at the inlet or 
upstream side of the flow control device for example, the thermal 
expansion valve. Compared with existing technology, the present invention 
uses a sensing arrangement which is simpler, easier to manufacture, 
install and maintain and thus lower in cost. 
The present invention provides an improved forced air heat exchanging 
system in which a fan is positioned in the heating/cooling system to 
direct an airflow into heat exchange contact with a heat exchanger of the 
system. The fan has at least two speeds of operation and may provide 
either separate discrete speeds or a continuously variable speed, 
depending on the mode of operation of the system. A temperature sensor is 
coupled to the system so that it will sense the temperature of the liquid 
refrigerant, preferably at the inlet or upstream side of the flow control 
device. The metering device may be, for example, a variable expansion 
device or restricted orifice which delivers refrigerant to the evaporator 
coil. 
A control circuit is coupled to the indoor fan and also to-the temperature 
sensor for controller the speed of the fan based on the temperature of the 
liquid refrigerant. Preferably the control circuit includes a 
microprocessor which processes the condenser liquid refrigerant 
temperature information to select the optimal fan speed. The 
microprocessor selects the optimal speed by using the liquid refrigerant 
temperature to extract information concerning diverse system functions 
ordinarily sent by separate sensors. For example, from the condenser 
liquid refrigerant temperature, the microprocessor is able to infer the 
outdoor ambient temperature and to infer whether the refrigerant 
compressor is running or not. This information is used to select the 
optimal indoor fan speed. 
For a more complete understanding of the invention, its objects and 
advantages, reference may be had to the following specification and to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the improved forced air heat exchanging system is 
shown generally at 10. The system works in conjunction with a heat pump or 
an air-conditioner system of the type having a compressor 12 and an 
outdoor heat exchanger 14 coupled through refrigerant liquid line 16 and 
refrigerant suction line 18 to an indoor heat exchanger 20. The 
refrigerant liquid line is controlled by flow control device 22 which may 
be a thermal expansion valve or other suitable metering device. Outdoor 
heat exchanger 14 is preferably housed in an outdoor unit 24 which 
includes an outdoor fan 26. Similarly, indoor heat exchanger 20 is 
associated with an indoor unit 28 which includes an indoor fan 30. If 
desired, both indoor and outdoor fans may be variable speed fans. The 
presently preferred embodiment uses an, electronically commutated motor 32 
to drive the indoor fan which has five programmable discrete speeds. 
System 10 includes a microprocessor-based controller 34 which supplies 
compressor and contactor 24 volt logic signals for turning compressor 12 
on and off and for turning fan 26 on and off at the selected fan speed. 
Microprocessor-based controller 34 also provides a motor drive signal on 
lead 36 coupled to motor 32. Microprocessor 34 also received a plurality 
of signals from room thermostat unit 38. The thermostat unit supplies 
logic signals to the microprocessor, indicating whether the user has 
selected the heating mode, the cooling mode or the Fan-On mode. These 
logic signals are supplied on leads 40. 
Temperature sensor 42 is positioned on the refrigerant liquid line 16 
adjacent the inlet side or upstream side of flow control device 22 
depending upon the mode of operation of heating-cooling system. In this 
regard, liquid refrigerant flows from the outdoor heat exchanger, through 
the flow control device and into the indoor heat exchanger. The 
refrigerant is in the liquid state in refrigerant liquid line 16 adjacent 
the inlet to flow control device 22, because the refrigerant has been 
compressed by compressor 12. Passing through flow control device 22, the 
liquid refrigerant is atomized into tiny liquid droplets. This occurs 
because the liquid is forced under pressure through the restricted orifice 
of the flow control device into the low pressure side of the refrigerant 
loop. These atomized droplets are then passed into the indoor heat 
exchanger where they extract heat from the atmosphere during the cooling 
mode. Extracting atmospheric heat causes the droplets to enter the gaseous 
phase. In the gaseous phase, the refrigerant is then pulled through 
suction line 18 back into the high pressure compressor, which again 
compresses the refrigerant back into the liquid phase. Temperature sensor 
42 may be a thermistor placed in thermal contact with the refrigerant 
liquid line, preferably adjacent to the inlet side of the flow control 
device. The thermistor provides a signal on lead 44 which the 
microprocessor reads to measure the liquid refrigerant temperature. 
The invention can be implemented in a variety of different heat pump and 
air-conditioning systems. To illustrate some of the possibilities, FIG. 2 
depicts a heat pump system of the type using a single bidirectional flow 
control device. In FIG. 2 the flow control device is depicted at 22. The 
temperature sensor for sensing liquid refrigerant temperature is depicted 
at 38. The other refrigeration system components are designated using the 
reference numerals corresponding to like components in FIG. 1. A reversing 
valve 46 is used to control the flow direction. In FIG. 2 the flow arrows 
indicate the direction of flow when the system is in Cooling mode. The 
reversing valve operates so that, switched to Heating mode, refrigerant 
flow through the heat exchangers and flow control device is in the 
opposite direction. Nevertheless, in Heating mode, flow through the 
compressor is in the same direction as flow through the compressor in 
Cooling mode. If desired, a second temperature sensor 48 may be provided 
on the side of flow control device 22 opposite temperature sensor 38. This 
second sensor may also be coupled to the microprocessor circuit and is 
used to sense liquid refrigerant temperature when the system is operating 
in Heating mode (i.e. when refrigerant flow is opposite that depicted in 
FIG. 2). 
By way of further example, FIGS. 3A and 3B illustrate a dual flow control 
device heat pump system. FIG. 3A shows the system in Cooling mode and FIG. 
3B shows the system in Heating mode. As with FIGS. 1 and 2, like 
components have been assigned the same reference numerals. In this case, 
there are two flow control devices, flow control device 22 and flow 
control device 50. The flow control devices are each protected by check 
valves 52 and 54, respectively. The check valves effectively bypass the 
associated flow control device when refrigerant flow is in one of the two 
directions of the heat pump. Thus in FIG. 3A flow control device 50 is 
bypassed and in FIG. 3B flow control device 22 is bypassed. Note that in 
both Figures a single temperature sensor 38 has been provided to sense 
liquid refrigerant temperature. If desired, a second temperature sensor 48 
may also be provided at the location shown so that the two sensors are 
always located at the inlet of the metering device. Depending on the 
system configuration, this second temperature sensor 48 may not be 
required, so long as temperature sensor 38 is positioned at a point where 
it will sense liquid refrigerant temperature in either Cooling mode or 
Heating mode. 
Referring next to FIGS. 4A through 4C, the presently preferred 
microprocessor control routines are illustrated commencing at step 100. 
The routine first initializes variables at step 102 and then enters a 
series of tests to determine what mode the system is in. In step 104, a 
test is performed to determine whether the system is in the cooling mode 
or not. If not, control proceeds through step 106 to step 108 (FIG. 4B) 
where a similar test is performed to determine if the system is in Heating 
mode. If not in Heating mode, then the system proceeds to step 110 where a 
determination is made whether the system is in Fan-On mode. These modes 
are user-selectable via the room thermostat. Thus, the microprocessor 
software simply reads the state of the user-settable thermostat switches. 
If the Fan-On mode has been set, then the routine sets the indoor fan to 
the minimum circulation flow rate at step 111 and then returns to location 
2 on FIG. 4A. If, at step 110, the Fan-On mode has not been selected, then 
control simply returns to location 2 on FIG. 4A, without changing the fan 
speed. The process continues to loop or cycle from cooling mode test 104, 
to heating mode test 108, to Fan-On test 110 until a selection has been 
made. 
Assuming the Cooling mode has been selected, control proceeds to step 112 
where a determination is made whether the system demand is met. This 
determination is made based on a signal from the thermostat. If the 
thermostat is calling for cooling, then control will branch to step 122. 
On the other hand, if the thermostat is not calling for cooling, then 
control will branch to step 113 where control branches depending on 
whether the system is on or not. If the system is on, control branches to 
step 114 where a determination is made whether the system has been on for 
less than six minutes. If the system has not been on for six minutes, the 
system is turned off at step 115 and, control simply branches back where 
it enters the mode test loop. 
If the system has been on for less than six minutes, step 114, control 
branches to step 116 where a one minute timer is set and tested at step 
118. After the one minute time has elapsed, the system on-time variable 
"Time" is incremented by one minute at step 120 and control branches back 
to point 2. 
If the thermostat is calling for cooling at step 112, control proceeds to 
step 122, where a determination is made whether the system is in start-up 
mode or not. This may be done by setting and reading a flag. If the system 
is in start-up mode, the indoor fan is set at the low cool airflow rate at 
step 124, selected by lookup table which stores a value used for low flow 
rate in the Cooling mode. Next, control proceeds to step 126 where a 
sensor check routine is performed as depicted in FIG. 5 and discussed 
later. After the sensor integrity has been verified, control proceeds to 
step 128 to measure the condenser liquid temperature. Thereafter, a three 
minute timer is set at step 130 and the program then cycles at step 132 
until the three minute timer has elapsed. Then, the liquid temperature is 
measured again and stored as value Tnew, at step 134. A delta T is then 
calculated by subtracting Tnew--Tinitial, where the initial value is a 
predetermined stored value representing a low temperature condition. This 
occurs at step 136. If the calculated delta T is greater than 5.degree. F. 
as determined at step 138, a determination is made that the compressor is 
not running (step 140). In this instance, a malfunction light is turned on 
in step 141, indicating a compressor start malfunction has occurred, and 
the indoor airflow is set at the minimum circulation rate at step 142. 
Otherwise, if the delta T is greater than 5.degree. F., a determination is 
made that the compressor is running (step 148), whereupon the malfunction 
light is turned off at step 149. 
If the system is not in start-up mode at step 122, then a simpler procedure 
is followed. At step 144, the sensor check routine is performed and at 
step 146 the condenser liquid temperature is measured. Then, since the 
compressor may be assumed to be running, the system simply proceeds to 
step 150 where the look-up table is used to set the appropriate fan speed. 
Thereafter, in step 152, the indoor airflow is set to the high cool or low 
cool setting based on the airflow relationship determined in step 150. 
The operation is essentially the same for the heating mode as it is with 
the cooling mode, proceeding beginning at step 154 as illustrated. One 
difference in heating mode is that the system must also control the 
auxiliary heat, which may be electric resistance heaters, for example. See 
steps 165, 183 and 189 for example. After step 165, if the auxiliary heat 
is on, the indoor fan is set at the high heat air flow rate at step 166 
and if the auxiliary heat is off, the indoor fan is set at the low heat 
air flow rate at step 167. After step 183, if the auxiliary heat is on, 
the indoor fan is set at the high air flow rate at step 185 and if the 
auxiliary heat is off, the indoor fan is set at the minimum air 
circulation flow rate at step 184. After step 189, if the auxiliary heat 
is on, the indoor fan is set at the high heat air flow rate at step 190 
and step 150' is skipped. If, at step 189, the auxiliary heat is off 
control moves to step 150'. Also, the system provides an Emergency Heat 
mode, tested for at step 162, to insure that the fan is operating at high 
heat airflow rate (step 163) in emergency conditions, to prevent pipes 
from freezing, for example. The remaining steps illustrated in FIG. 4B and 
4C are essentially the same as those for the cooling mode and are 
identified using the same reference numerals with the reference numerals 
being primed. A with the reference numerals being primed. A description of 
these steps will therefore not be repeated. 
Turning now to FIG. 5, the sensor check routine begins at step 206. If the 
sensor temperature reading is greater than -77.degree. F. (step 208 ), 
then the thermistor is deemed to be operational at step 210 and the sensor 
malfunction light is switched off at step 112. In this case the indoor fan 
speed may be regulated based on refrigerant temperature measurements as 
indicated at step 214. 
On the other hand, if the sensor temperature reading is not greater than 
-77.degree. F., then the thermistor is assumed to be bad at step 216. In 
this case, the indoor fan speed is set to the High/Cool airflow speed or 
the High/Heat airflow speed, depending on whether the system is in Cooling 
mode or not. This is illustrated in steps 218, 220 and 222. Also, in the 
event of a bad thermistor condition, the sensor malfunction light is 
turned on at step 224. 
Reference will next be made to FIGS. 6, 7, 8A-8B and 9A-9B. These Figures 
give information on how the presently preferred look-up table values are 
arrived at. Of course, different values may be chosen from those 
illustrated here, depending on the operation of the system and design 
preferences. The values illustrated here represent the presently preferred 
embodiment. 
In FIG. 6 the relationship between condenser liquid temperature and indoor 
airflow is shown. Note that the condenser liquid temperature is a linearly 
increasing function which may be treated as a function of outdoor air 
temperature. Indoor airflow is switched from the low speed to the high 
speed at a liquid temperature corresponding to an outdoor air temperature 
of approximately 88.degree.. The illustrated airflow speed is 70% of 
maximum for outdoor air temperatures below 88.degree., and 100% (maximum) 
at outdoor air temperatures above 88.degree.. The switch-over point 
occurs, as illustrated, at a liquid refrigerant temperature of 94.degree.. 
The values illustrated in FIG. 6 correspond to a system operating in 
Cooling mode. If desired, different speeds may be programmed in Heating 
mode. Also, if desired, a different liquid temperature may be programmed 
to signify the switchover point. 
FIG. 7 illustrates two families of curves, one family for the Heating mode 
and one family for the Cooling mode. In each family reference numeral 300 
designates a normally charged system, reference numeral 302 designates a 
significantly undercharged (-30%) system and reference numeral 304 
designates an (+30%) overcharged system. This Figure illustrates the 
effect of refrigerant charge on condenser liquid temperature. In some 
systems where a certain variance in the fan speed switchover point can be 
tolerated, it is not necessary to modify the measured liquid refrigerant 
temperature to compensate for undercharged and overcharged conditions. 
However, for more precise control of the indoor airflow, the system can be 
programmed to compensate for undercharged and overcharged conditions by 
adding or subtracting a suitable compensation value to or from the 
measured liquid refrigerant temperature. 
FIGS. 8A and 8B show the indoor airflow rate as a function of liquid 
refrigerant temperature for Cooling mode and Heating mode, respectively. 
As illustrated, in both cases a 4.degree. F. dead band 306 is provided. In 
other words, taking the Cooling mode (FIG. 8A) as an example, airflow is 
switched from the low speed to the high speed when the liquid temperature 
is 96.degree. F. Fan speed is switched from high speed to low speed when 
the liquid refrigerant temperature falls to 92.degree. F. Comparing FIGS. 
8A and 8B, note that the switchover point for the Cooling mode is 
preferably at 94.degree. F. whereas in the Heating mode the switchover 
point is preferably 77.degree. F. Of course, these switchover points 
represent the presently preferred embodiment. Other embodiments are 
possible. 
FIGS. 9A and 9B show the condenser liquid temperature response during the 
heating-cooling modes along with the indoor fan strategy on system 
startup. It may be seen that the indoor fan operates at the (user 
selected) default low airflow rates during the first three minutes from 
system start-up. These Figures also show the condenser liquid temperature 
takes about two minutes to reach the steady-state condenser liquid 
temperature. This built-in period of three minutes assures the condenser 
liquid temperature has reached its steady-state value thus preventing 
nuisance fan speed switch-overs caused by any transient conditions. An 
additional benefit for the low-airflow during heating cycle is: it reduces 
the cold draft on system startup. After the elapse of this initial period 
the indoor air-flow is based on condenser liquid line temperature 
relationship shown in FIGS. 8A and 8B. 
By way of further illustrating the principles of the invention, FIGS. 
10A-10B and FIGS. 11A-11B show another presently preferred look-up table 
may be arrived at. As illustrated in these Figures, the indoor airflow 
rate can have a portion that is continuously variable, in this case 
linearly variable, over a predetermined range of condenser liquid 
refrigerant temperature (FIGS. 10A-10B) and outdoor air temperature (FIGS. 
11A-11B). Thus, referring to FIG. 10A, the indoor airflow rate holds 
steady at a 70% of rated capacity for temperatures below about 92.degree.. 
The indoor airflow rate then ramps up linearly until the condenser liquid 
refrigerant temperature reaches about 98.degree.. For condenser liquid 
refrigerant temperatures above 98.degree., the indoor airflow rate is set 
at 100% of its rated speed. A similar airflow rate curve may be 
implemented for Heating mode. This is shown in Figure 10B. The indoor 
airflow rate is set at 85% of rated capacity for condenser liquid 
refrigerant temperatures below about 74.degree.. The indoor airflow rate 
then ramps up, in this case linearly, until the condenser liquid 
refrigerant temperature reaches about 83.degree.. Above 83.degree. the 
indoor airflow rate is set is at 100% of rated speed. 
FIGS. 11A and 11B illustrate further the embodiment shown in FIGS. 10A and 
10B. These Figures illustrate the outdoor air temperature at which the 
indoor airflow speed is switched from constant speed to variable speed. 
Specifically, in the Cooling mode (FIG. 11A) the indoor airflow is 
operated in variable speed mode in the range between about 85.degree. to 
92.degree.. In the Heating mode (FIG. 11B) the indoor airflow is operated 
in a variable speed mode between the outdoor air temperatures of 
21.degree. to 44.degree.. Also superimposed on FIGS. 11A and 11B are a 
family of straight line graphs depicting the condenser liquid temperature 
as a function of outdoor air temperature, for the normally charged system, 
for the overcharged system and for the undercharged system. 
In the embodiment illustrated in FIGS. 10A-10B and FIGS. 11A-11B the 
inflection points at which indoor airflow speed is switched from constant 
mode to variable mode are determined in relation to the system rating 
points. For example, the inflection point in FIG. 10A is set at a 
predetermined 98.degree. F. condenser liquid temperature corresponding to 
92.degree. F. outdoor temperature. This inflection point is selected 
because of the 3.degree. F. temperature measurement tolerance to account 
for errors arising from: sensor accuracy, analog-to-digital conversion 
location and method of sensor mounting on system tubing. The built-in 
measurement tolerance ensures the indoor fan operates at the rated airflow 
(100%) at the system capacity rating point of 95.degree. F. outdoor 
temperature. Similarly, the predetermined inflection point of 91.degree. 
F. condenser liquid temperature (corresponding to 85.degree. F. outdoor 
temperature) ensures that the indoor fan speed change occurs ahead of the 
82.degree. F. system efficiency rating point. 
In the embodiment illustrated in FIGS. 10A-10B and FIGS. 11A-11B the 
variable speed range is a proportional range, in this case a linear 
function (straight line). If desired, other variable speed relationships 
can be implemented, including nonlinear relationships. Also, while the 
illustrated embodiment breaks the indoor airflow rate control into two 
fixed speed ranges and a variable speed range, other combinations are also 
possible. Thus the look-up table may be alternatively programmed to 
achieve a fully variable range (no fixed speed portions). Alternatively, 
other combinations, such as multiple variable speed ranges of different 
slope, or multiple discrete steps may be used. In this regard, a multiple 
discrete step "stairstep" function can be used to approximate the 
proportional variable speed range. 
From the foregoing, it will be seen that the present invention provides an 
indoor fan speed control means which is capable of optimally setting the 
forced airflow based on a single temperature sensor measurement. The 
sensor measurement provides an indication of diverse system conditions 
(outdoor air temperature and compressor operation) which are then used to 
optimally control the indoor fan speed. While the invention has been 
described in its presently preferred embodiment, it will be understood 
that the invention is capable of certain modification without departing 
from the spirit of the invention as set forth in the appended claims.