Pressure control override

A pressure control override apparatus for overriding a conventional pressure control starting switch on a refrigeration system is provided. The apparatus includes a power supply that receives power only when the valve in the liquid line of the refrigeration system is open. The apparatus also includes a temperature sensor for sensing the ambient temperature in the area of the compressor and condenser. The apparatus further includes a circuit to start the compressor that is separate from the normal pressure control starting switch when the apparatus is energized and the ambient temperature sensed by the temperature sensor is lower than a preselected temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to refrigeration control devices. More 
particularly, the present invention relates to a device for overriding the 
pressure control starting switch for a compressor on a conventional 
refrigeration system having a pump-down cycle when the compressor is 
subjected to low ambient temperatures. 
Generally, medium temperature refrigeration units have been one of two 
types. A first type, and generally considered outdated now, utilizes a 
compressor and condenser located generally either inside a building, or in 
the basement of a building. Thus, the compressor and condenser unit in 
this first type of system is never subjected to near 0.degree. F. 
temperatures which can cause the refrigerant, normally Freon 12, to change 
to a liquid state when the system is shut off. 
A second type of system, and that normally in use today, utilizes, a 
compressor and condenser unit that is placed outside the building, and 
normally either mounted on the roof of the building, or at the side of the 
building. Therefore, the compressor and condenser unit on a new type of 
system is periodically exposed to ambient temperatures well below 
32.degree. F., and in some portions of the country, below 0.degree. F. 
Because of this exposure to low ambient temperatures, the new type systems 
are configured to include a pump-down cycle to prevent the compressor pump 
from being damaged. 
A pump-down cycle is accomplished by installing a solenoid valve in the 
liquid line of the refrigeration system. When the temperature inside the 
refrigerated area is satisfied, the thermostat in the refrigerated area 
interrupts the power to the solenoid valve, and the solenoid valve closes 
off the liquid line. The compressor continues to run until all of the 
refrigerant is captured inside a receiver tank that is generally located 
with the compressor on the condensing unit. When the liquid has been 
captured in the receiving unit, the compressor is then shut off. The 
shut-off of the compressor is controlled by a pressure control device that 
may be set to shut off the compressor at a preselected pressure. The 
pressure control is normally set to shut off the compressor when the 
pressure in the suction line of the system reaches 0 pounds per square 
inch gauge (psig). When the temperature inside the refrigerated space 
rises to a preselected level, the thermostat in the space returns the 
power to the solenoid valve which opens the liquid line. Normally, the 
refrigerant pressure rises in the system to a pre-set level on the 
pressure control, and the compressor begins to operate and cool the space 
again. 
The pump-down cycle is necessary because in cold ambient conditions, in a 
system without a pump-down cycle, the refrigerant will migrate to the 
coldest point in the system when the compressor is shut off. Where the 
compressor and condenser are located outside, the coldest point will 
normally be these units. If the ambient temperature is low enough, this 
migrated refrigerant will quickly liquify. Upon starting the compressor 
pump again, the pump will be forced to pump this liquid refrigerant. This 
action quickly damages compressor pumps because such pumps are not 
designed to pump any liquids at all, only to pump and compress gases. The 
pump-down cycle prevents damage to the compressor pump by forcing all of 
the refrigerant in the system to the receiver tank, which is separated 
from the compressor pump. Thus, in a system with a pump-down cycle, when 
the compressor pump is turned on again, the pump will not be forced to 
pump the liquid refrigerant. 
One problem with the new type of systems equipped with a pump-down cycle is 
that in very cold ambient conditions, the pressure of the refrigerant may 
not be able to rise to the necessary pressure required to activate the 
pressure control after the solenoid valve is opened. This condition will 
prevent the compressor pump from starting, and therefore will prevent the 
refrigeration system from continued cooling of the refrigerated space, 
because in a pump-down system, the pressure control is the only control 
that starts and stops the compressor. Normally, when this condition 
occurs, a serviceman must be called to start the compressor. 
The serviceman can generally start the compressor by one of two methods. 
First, the serviceman can set the pressure control to turn on at a very 
low pressure, for example at 0 psig. This method has the disadvantage of 
causing the compressor to run much longer than necessary, and particularly 
causes the system to operate in a vacuum much of the time. Operating the 
system in a vacuum is highly undesirable because contaminants can be drawn 
into the system which can cause system breakdown and system replacement. 
The second method that may be used by the serviceman is to install a 
jumper wire across the pressure control. This method has a disadvantage of 
causing the compressor to run continuously. This method also has the 
disadvantage of causing the compressor to run much longer than normal, and 
causing the system to operate in the vacuum much of the time. 
Because of these problems with the new type of systems in low ambient 
temperature conditions, it would be advantageous to have a device that 
could bypass the pressure control and start the compressor motor under 
these conditions. One type of bypass device for bypassing the pressure 
control of a refrigeration system is disclosed in U.S. Pat. No. 2,191,965 
to McGrath. However, the device disclosed in McGrath is usable only on the 
first type, or older type, of refrigeration systems without a pump-down 
cycle. McGrath discloses a system in which either the thermostat 20 or the 
low pressure control 21 can regulate the temperature of the space to be 
cooled. The respective settings on these two controls determine which of 
the controls will regulate the temperature inside the refrigerated space. 
McGrath discloses that the use of two controls to start the compressor is 
to insure that the evaporator is adequately defrosted during each off 
cycle of the system. 
In the McGrath device, when the refrigerated space temperature rises to a 
preselected level, the compressor does not immediately turn on. Instead, 
the low pressure control 21 causes the starting of the compressor to be 
delayed until the refrigerant pressure reaches 30 psig. Forcing the 
compressor to delay starting until the refrigerant pressure reaches 30 
psig insures that the evaporator is adequately defrosted before the 
compressor starts. Because the old type of refrigeration systems sometimes 
located the compressor and condenser units in the basement of the 
buildings, it was possible for the basement temperature to sometimes fall 
below about 30.degree. F. If the temperature fell below 30.degree. F., the 
refrigerant pressure sometimes would not reach the required 30 psig to 
cause the low pressure control 21 to start the compressor. To insure that 
the compressor would start under these conditions, McGrath discloses a low 
ambient temperature control 22 that bridges across the low pressure 
control 21 whenever the ambient temperature is below 30.degree. F. In 
McGrath's device, the low pressure control 21 is continuously bridged as 
long as the temperature remains below 30.degree. F. Thus, the thermostat 
20 becomes the sole control for starting and stopping the compressor, and 
therefore the defrost period for the system is bypassed. 
It is apparent from the above discussion that the device disclosed in 
McGrath could not possibly be used on a new type refrigeration system 
having a pump-down cycle. If the McGrath device were so installed, 
whenever the ambient temperature was below 30.degree. F., the compressor 
would be forced to run continuously because on a pump-down system, the low 
pressure control is the only control that starts or stops the compressor. 
Because it is undesirable to run the compressor continuously, the McGrath 
device would not solve the problems related to low ambient conditions on a 
new type refrigeration system. 
It is therefore one object of the present invention to provide a pressure 
control override apparatus that is usable on a refrigeration system having 
a pump-down cycle, and in which the pressure control is the only control 
that starts and stops the compressor. 
It is another object of the present invention to provide a pressure control 
override apparatus that is activated only when both the ambient 
temperature around the compressor and condenser units is below a 
preselected level, and when the refrigerated space temperature rises to a 
preselected level indicating that cooling within the space is required. 
SUMMARY OF THE INVENTION 
According to the present invention, a pressure control override apparatus 
is provided for overriding a conventional pressure control starting switch 
on a refrigeration system that includes a compressor having a pump-down 
cycle and a refrigerated space to be cooled. The apparatus includes means 
for selectively energizing the apparatus sensitive to the temperature in 
the refrigerated space when the temperature rises to a selected level. The 
apparatus further includes means for sensing the ambient temperature in 
the area of the compressor, and the means for starting the compressor 
separate from the normal pressure control starting switch when the 
apparatus is energized and the ambient temperature is less than a 
predetermined temperature. 
One feature of the foregoing structure is that the pressure control 
override apparatus is not energized until the temperature within the 
refrigerated space reaches a selected level. One advantage of this feature 
is that the apparatus is energized only during the period of time that the 
compressor should be running. This selective energizing of the apparatus 
permits the compressor to function normally during the pump-down cycle, 
and to shut off normally when the pressure in the system reaches the 
normal shut off pressure. 
In preferred embodiments of the present invention, the starting means 
includes a relay circuit that is closed only when both the apparatus is 
energized, and the sensing means senses an ambient temperature that is 
lower than a predetermined level. One advantage of this feature is that 
the apparatus does not interact with the compressor to start the 
compressor until both conditions are met. This permits the refrigeration 
system to function normally at all other times. 
Applicant's invention provides an apparatus that solves the problems with 
conventional refrigeration systems having pump-down cycles with the 
compressors and condensers located in areas where they are exposed to low 
ambient temperature conditions. Applicant's invention is only activated 
during the period of time that the compressor would normally be running, 
and is deactivated both during the pump-down cycle, and at all times while 
the compressor is shut off. Applicant's invention solves the problems 
associated with such systems, without resorting to the measures now 
normally taken to overcome this problem.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIG. 1, FIG. 1 shows a prior art, conventional medium 
temperature refrigeration system equipped with a pump-down cycle. 
Normally, this type of system 10 is used to cool a large refrigerated 
space, typically a walk-in type cooler. Such walk-in type coolers are 
typically installed in supermarkets, convenience markets, and the like. 
The system 10 includes a compressor 12 that compresses a refrigerant gas, 
illustratively Freon 12. An electric motor 14 is provided to drive the 
compressor 12. Illustratively, the electric motor 14 is a 3-phase, 240 
volt motor that is connected to a 240 volt, 3-phase power supply (not 
shown) by wires 16, 18, and 20. 
The output of the compressor 12 is connected by a connecting tube 22 to a 
condenser 26. The condenser 26 operates in a customary manner to remove 
heat from the compressed refrigerant at a constant pressure until the 
refrigerant becomes a saturated liquid. The liquid refrigerant passes from 
the condenser 26 to a receiver 30 through a connecting tube 32. The liquid 
refrigerant then passes from the receiver 30 through a liquid line 34 to a 
thermal expansion valve 36. As the liquid refrigerant passes through the 
thermal expansion valve 36, it is expanded adiabatically, which reduces 
the pressure of the refrigerant. Thus, the refrigerant exits the expansion 
valve 36 as a highly cooled gas. The cooled gas refrigerant then passes 
through an evaporator 38 where air-cooling occurs. The evaporator 38 is 
shown installed in a cooler 40 (shown only in dotted line). 
Illustratively, the cooler 40 is a walk-in type cooler used to store 
perishable items at a refrigerated temperature, normally slightly above 
freezing. 
After the refrigerant passes through the evaporator 38, its pressure is 
lowered considerably. This low pressure refrigerant is then routed by a 
suction line 42 to the intake side of the compressor 12. The low pressure 
refrigerant is then compressed by the compressor 12 to begin the 
above-described cycle. Thus, the system 10 is a conventional, closed 
system that operates to cool the cooler 40. 
As stated previously, the compressor motor 14 is powered by a 240 volt 
power supply (not shown) through wires 16, 18, and 20. Contacts 44, 46, 
and 48 are provided in the wires 16, 18, and 20, respectively, to control 
the power to the motor 14. It will be understood that the contacts 44, 46, 
and 48 collectively function to start and stop the motor 14. The contacts 
44, 46, and 48 are controlled by a contactor coil 50 that operates in a 
conventional manner to open and close the contacts 44, 46, and 48 
simultaneously. One terminal of the contactor coil 50 is connected 
directly to the power wire 20 of the 240 volt power supply. The other 
terminal of the contactor coil 50 is connected to the power wire 16 of the 
240 volt power supply through a dual-pressure control 52. 
The dual-pressure 52 includes a low pressure contact 54 and a high pressure 
contact 56 that are capable of individually interrupting the power to the 
contactor coil 50. The high pressure contact 56 is coupled by a capillary 
tube 58 to a high pressure sensor 60 that is installed in the liquid line 
34. The high pressure contact 56 is normally closed, and opens only when 
the high pressure sensor 60 is subjected to a pressure that exceeds a 
predetermined, excessive level. It will be understood that in normal 
operation, the high pressure contact 56 will be closed. The low pressure 
contact 54 is coupled through a capillary tube 62 to a low pressure sensor 
64 that is installed in the suction line 42. The low pressure contact 54 
is configured to open when the pressure in the suction line 42 drops to a 
preselected, low pressure. Illustratively, the low pressure contact 54 is 
configured to open when the pressure in the suction line 42 reaches 0 
psig. The low pressure contact 54 is also configured to close when the 
pressure in the suction line 42 increases to a preselected level, 
illustratively 6-7 psig. 
Thus, in normal operation, the high pressure contact 56 and the low 
pressure contact 54 will be closed to provide power to the contactor coil 
50 when the pressure in the suction line 42 is above 6-7 psig, and the 
pressure in the liquid line 34 does not exceed the predetermined, 
excessive pressure level. When power is supplied to the contactor coil 50, 
the contacts 44, 46, and 48 will be closed, providing power to the motor 
14 to drive the compressor 12. When the pressure in the suction line 42 
drops to approximately 0 psig, the low pressure contact 54 will open to 
interrupt the power to the contactor coil 50. When power is interrupted to 
the contactor coil 50, the contacts 44, 46, and 48 open, interrupting 
power to the motor 14. When the pressure in the suction line 42 increases 
to above 6-7 psig, the low pressure contact 54 closes, providing power to 
the contactor coil 50 to again close the contacts 44, 46, and 48. 
The system 10 is configured to include a conventional pump-down cycle. In a 
pump-down cycle, the flow of liquid refrigerant from the condenser 26 to 
the evaporator 38 is interrupted, while the compressor 12 continues to 
operate. The operating compressor 26 will draw all of the refrigerant from 
the evaporator 38 through the suction line 42 into the receiver 30. The 
refrigerant is then stored in the receiver 30, away from the compressor 
12. When the refrigerant liquifies due to the cold temperatures, it will 
then be separate from the compressor 12. 
To provide the pump-down cycle, a valve 68 is installed in the liquid line 
34 between the condenser 26 and the expansion valve 36. The valve 68 is 
controlled by a conventional solenoid coil 70 that opens and closes the 
valve 68 selectively. One terminal of the solenoid coil 70 is connected 
directly to one terminal of a power supply, illustratively a 120 volt 
power supply (not shown). The other terminal of the solenoid coil 70 is 
connected to the other terminal of the power supply (not shown) through a 
thermostat 72 which includes a switch 74. 
The thermostat 72 is installed within the cooler 40, and operates in a 
conventional manner to interrupt the power to the solenoid coil 70 
whenever the temperature within the cooler 40 drops below a preselected 
temperature. It will be understood that this preselected temperature is 
the desired temperature to be maintained within the cooler 40. Whenever 
the temperature within the cooler 40 drops below the preselected 
temperature, power to the solenoid coil 70 is interrupted, closing the 
valve 68. Whenever the temperature within the cooler 40 increases above 
this preselected temperature, the switch 74 closes to provide power to the 
solenoid coil 70, which then opens the valve 68. It will be understood 
that when the valve 68 is open, the liquid refrigerant is allowed to pass 
through the evaporator 38 to cool the cooler 40. When the valve 68 is 
closed, liquid refrigerant is prevented from passing through the 
evaporator 38, and no cooling in the cooler 40 occurs. 
Thus, it will be understood that the thermostat 72 acts as the sole control 
to directly govern and regulate the flow of refrigerant through the 
evaporator 38. The thermostat 72 also acts as the sole control to 
indirectly start and stop the compressor 12. Whenever the valve 68 is 
closed by the thermostat 72, the compressor 12 runs until the pressure in 
the suction line 42 drops to 0 psig. At that time, the low pressure 
contact 54 opens to shut off the compressor 12. The compressor 12 will 
remain off until the valve 68 is opened by the thermostat 72. When the 
valve 68 is opened, the pressure of the refrigerant normally increases 
within the suction line 42 above the 6-7 psig, required and the low 
pressure contact 54 closes to again start the compressor 12. 
One major problem that occurs in refrigeration systems similar to system 10 
is that, when the compressor 12 and condenser 26 are exposed to very cold 
ambient temperatures, the refrigerant pressure may not increase to the 
required 6-7 psig after the valve 68 is opened by the thermostat 72. When 
this occurs, the compressor 12 does not start, and no refrigerant is 
circulated through the evaporator 38 to cool the cooler 40. Thus, the 
temperature within the cooler 40 continues to increase which, if left 
unchecked, can spoil the goods in the cooler 40. Normally, when this 
condition occurs, a serviceman must be called to start the compressor 12. 
Typically, the serviceman will perform one of two steps to start the 
compressor 12. First, the serviceman may simply jump around the 
dual-pressure control 52 so that power is continuously supplied to the 
contactor coil 50. This causes the compressor 12 to run continuously. 
Because the thermostat 72 and valve 68 are unaffected, the valve 68 will 
open and close normally under the direction of the thermostat 72. Whenever 
the valve 68 is closed, and the compressor 12 is forced to run 
continuously, the system 10 will operate in a vacuum much of the time. 
When operating in a vacuum, the system 10 is susceptible to drawing 
contaminants into the refrigerant, with possible damage occurring to the 
compressor 12 and other components of the system 10. 
A second alternative for the serviceman to start the compressor 12 is to 
readjust the low pressure sensor 64 so that the low pressure contact 54 
will close when the pressure within the suction line increases to only 
around 0 psig. This alternative also forces the system 10 to operate in a 
vacuum much of the time because the low pressure contact 54 will not open 
to shut off the compressor until the pressure within the suction line 42 
falls to well below 0 psig. Thus, both alternatives conventionally 
available to a serviceman have highly undesirable affects on the system 
10. 
Referring now to FIG. 2, FIG. 2 shows a pressure control override apparatus 
80 of the present invention adapted to the system 10 of FIG. 1. The 
override apparatus 80 includes a step-down power transformer 82 that 
provides power to the device 80 selectively. One terminal of the input 
side of the transformer 82 is connected directly to one terminal of the 
120 volt power source (not shown) that provides power to the solenoid coil 
70. The other terminal of the input side of the transformer 82 is 
connected to the other terminal of the 120 volt power supply between the 
thermostat 72 and the solenoid coil 70. Thus, whenever the switch 74 of 
the thermostat 72 is closed, and the solenoid coil 70 is receiving power, 
the input side of the transformer 82 will also receive power. 
Illustratively, the transformer 82 reduces the 120 volt input voltage to 
24 volts to increase the safety of the apparatus 80. It will be understood 
that a different input voltage could be used, as well as a different 
output voltage of the transformer, without affecting the function of the 
apparatus 80. 
One terminal of the output side of the transformer 82 is connected to one 
terminal of a relay coil 84. The other terminal of the output side of the 
transformer 82 is connected through a thermostat 86 to the other terminal 
of the relay coil 84. The thermostat 86 includes a switch 88 that opens 
and closes in response to a temperature sensor 98 that is mounted adjacent 
the compressor 12. The switch 88 is configured to close whenever the 
temperature sensor 98 is subjected to a temperature lower than a 
preselected temperature, illustratively below 30.degree. F. The switch 88 
is configured to open whenever the temperature sensor 98 is exposed to a 
temperature above this preselected temperature. 
When the switch 88 is closed, the relay coil 84 will receive power from the 
transformer 82. When powered, the relay coil 84 will close a switch 90 in 
a circuit that bridges across the dual-pressure control 52. One terminal 
of the switch 90 is connected to one terminal of the contactor coil 50, 
between the dual-pressure 52 and the contactor coil 50. The other terminal 
of the switch 90 is connected through a high pressure relay 92 to the 
power wire 16 of the 240 volt power supply (not shown). The high pressure 
relay 92 includes a switch 94 that is normally closed, and only opens when 
the pressure sensed by the high pressure sensor 60 exceeds a predetermined 
excessive level. The high pressure relay 92 is coupled to the high 
pressure sensor 60 by a capillary tube 96. Typically, the switch 94 will 
open at the same predetermined excessive pressure level as the high 
pressure contact 56 in the dual-pressure control 52. Both of the switches 
92, 56 are designed to interrupt the power to the compressor 12 when the 
refrigerant pressure within the system 10 reaches an excessive, dangerous 
level. Because the override apparatus 80 bypasses the contact 56 under 
certain conditions, the high pressure relay 92 and switch 94 are included 
within the override apparatus 80 to perform the function of the high 
pressure contact 56 during the periods of time that the high pressure 
contact 56 is overridden. 
In operation, the transformer 82 receives power only when the switch 74 and 
a thermostat 72 are closed. It will be understood that this corresponds to 
the period of time that the valve 68 is open, and when the compressor 12 
should be running. Assuming first that the compressor 12, and consequently 
the temperature sensor 98, are exposed to ambient temperatures below 
30.degree. F., the switch 88 will be closed. When the switch 74 in the 
thermostat 72 closes to provide power to the solenoid coil 70, the 
transformer 82 will receive power from the 120 volt power supply and 
convert this power to 24 volts. With the switch 88 closed, the relay coil 
84 will receive power to close the switch 90. With the switch 90 closed, 
and assuming that the switch 94 is closed, the contactor coil 50 will 
receive power from a portion of a 240 volt power supply and the contacts 
44, 46, and 48 will close to start the compressor 12. Because the ambient 
temperature around the compressor 12 is below 30.degree. F., the 
compressor 12 would normally not start when the valve 68 was opened 
because the refrigerant pressure in the suction line 42 would not rise to 
the required 6-7 psig. Thus, the override apparatus 80 functions to start 
the compressor 12 immediately upon opening of the valve 68 under these 
conditions. 
With the compressor 12 now running, the pressure within the suction line 42 
will normally rise above the 6-7 psig required to close the low pressure 
contact 54. Thus, after a short period of running time of the compressor 
12, the low pressure contact 54 will close. When the temperature within 
the cooler 40 decreases to the desired level, the thermostat 72 will 
interrupt the power to the solenoid coil 70 to close the valve 68. At this 
time, power to the transformer 82 will also be interrupted, and 
consequently the power to the relay coil 84 will be interrupted. This will 
cause the switch 90 to open to interrupt the override circuit. The 
compressor 12 will continue running, however, because the low pressure 
contact 54 is now closed. 
Thus, the override apparatus 80 only operates to start the compressor 12, 
and as soon as the refrigerant pressure reaches the required 6-7 psig to 
close the low pressure contact 54, has no further function in the system 
10. After the valve 68 is closed, the compressor 12 will continue to run 
during the pump-down cycle until refrigerant pressure reaches 0 psig, as 
described previously. Assuming that the ambient temperature around the 
compressor 12, and consequently around the temperature sensor 98, 
increases to above 30.degree. F., the switch 88 in the thermostat 86 will 
open. In this configuration, when power is supplied to the solenoid coil 
70 through the thermostat 72, the relay coil 84 will not receive power, 
and the override device 80 will be inactive. However, when the ambient 
temperature is above about 30.degree. F., the compressor 12 is capable of 
starting normally without the aid of the override device 80. 
Thus, the override apparatus 80 only functions to start the compressor 12 
immediately upon opening of the valve 68 when the compressor 12 is 
incapable of starting on its own. Also, the override apparatus 80 
functions only to start the compressor 12 under these conditions, and does 
not interfere with the operation of the compressor 12, or the system 10 in 
general, in any other manner. 
Although the invention has been described in detail with reference to a 
preferred embodiment and specific examples, variations and modifications 
exist within the scope and spirit of the invention as described and 
defined in the following claims: