BACKGROUND OF THE INVENTION
[0001] This application relates to an algorithm for ensuring adequate oil return in a refrigerant
system including a capacity modulated compressor.
[0002] Compressors are utilized as an integral part of a refrigerant system. In general,
a compressor compresses a refrigerant and passes it downstream to a condenser. Refrigerant
from the condenser passes through an expansion device, and then through an evaporator.
From the evaporator, the refrigerant returns to the compressor.
[0003] Lubricant is included in the refrigerant system, typically an Air Conditioner, Heat
Pump or refrigeration application, and is particularly important to lubricate moving
parts in the compressor. However, the lubricant becomes entrained in the refrigerant,
and can flow with the refrigerant throughout the refrigerant system. As such, there
may sometimes be an inadequate supply of lubricant returned to the compressor. Lubricant
can sit in other areas of the refrigerant system, and in particular in the condenser
and evaporator.
[0004] Recently, system energy efficiency improvements have resulted in the compressor and
refrigerant systems being of a variable capacity. Typical applications of the compressor's
ability to vary capacity include variable speed, multi-stepped modulation, PWM of
compression element engagement, or other means of affecting capacity and the mass
flow rates of the compressor. Thus, should the cooling demand on the refrigerant system
be low, the compressor may be operated at a lower capacity to improve energy efficiency.
While operating at a higher capacity, the refrigerant may well drive sufficient lubricant
back from the condenser and evaporator to the compressor such that there is an adequate
lubricant supply. However, at lower capacities, it may sometimes be difficult to adequately
drive the lubricant back.
[0005] One known system periodically provides an increase in speed when the compressor is
operating at the lower speed. The increased speed is maintained for a short period
of time to drive lubricant back to the compressor. However, the increase in speed
is not tied to any existing conditions in the refrigerant system, and thus may be
operated too frequently, or not frequently enough. Of course, operating at the increased
speed too frequency somewhat defeats the purpose of operating the compressor at the
lower speed. On the other hand, running at the higher speed to return lubricant too
infrequently would be even more undesirable.
SUMMARY OF THE INVENTION
[0006] In a disclosed embodiment of this invention, system conditions are monitored to predict
an amount of oil migration from the compressor during low capacity operation. When
a particular amount of lubricant is deemed to have migrated from the compressor, an
increased compressor capacity is run for a period of time. As an example of a particular
application of the invention, a variable speed compressor is used.
[0007] 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
[0008]
Figure 1 schematically shows a refrigerant system.
Figure 2 is a chart showing oil loss per hour at various compressor speeds.
Figure 3 is a flowchart.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0009] A refrigerant system 20 is illustrated in Figure 1. A compressor shell 23 includes
a lubricant sump 22, which maintains a quantity of lubricant. A compressor pump unit
24 compresses refrigerant and delivers the refrigerant to a discharge tube 26. From
the discharge tube 26, the lubricant passes through a condenser 28, an expansion device
30, an evaporator 32, and back through a suction tube 33 into the compressor shell
23.
[0010] A control 36 for a compressor motor 37 may operate the compressor motor 37 at various
speeds. Thus, the compressor may be operating at a relatively low speed to increase
energy efficiency when a cooling demand is also low.
[0011] Sensors 34 and 35 monitor a condition, such as pressure, at the evaporator 32 and
condenser 28, respectively. Alternatively, sensors that monitor temperature or an
estimated saturated refrigerant temperature can be used to detect the conditions the
compressor is operating at, These conditions may be sent to the control 36 to assist
in quantifying the amount of oil which has likely migrated outwardly of the compressor
shell 23 into other system components at low speed operation.
[0012] As shown in Figure 2, a curve may be developed which equates oil loss for a period
of time for various low compressor speeds. Thus, as shown, at some point, here 2400
rpm, the oil loss no longer occurs. However, at other speeds, there is pronounced
oil loss.
[0013] The present invention integrates the amount of oil loss over time based upon a relationship
such as shown in Figure 2, and actuates the motor control 36 to increase the speed
of the motor for a short period of oil return time when that integrated amount passes
a particular limit.
[0014] Thus, as shown in the flowchart of Figure 3, the amount of oil loss is calculated
and the speed is increased when the amount passes a limit.
[0015] In one embodiment, the compressor speed may be ramped up from the low energy efficiency
speed to a predetermined amount, e.g., 2400 rpm in a compressor having the characteristics
as shown below, for a short period of time (e.g. 3 seconds).
[0016] The point at which the oil return speed-up will occur can be defined as a function
of the pressure at the suction and discharge of the compressor, as a function of compressor
capacity or based upon other variables. A ten coefficient map can be utilized to set
a curve similar to that shown in Figure 2.
[0017] Thus, the oil loss rate may be calculated as:
[0018] The oil loss rate shown above is based upon the pressure at the evaporator and the
pressure at the condenser. The same form of equation could be applied to evap and
condensing temperatures. It could also be multiplied by a multiplier which brings
in a capacity level factor. One such multiplier factor may be determined by the following
equation: Multiplier factor = b1 (Rc) ^3 + b2 (Rc) ^2 + b3 (Rc) + b4.
Where
[0019] As an example of this, for variable speed compressors this would be the current rpm/max
RPM.
[0020] With the present invention, and once an adequate curve is developed, the quantity
of oil which is "lost" or which has migrated from the compressor is calculated, or
integrated, over time while the compressor is operating at a low capacity. Once that
quantity exceeds a predetermined limit, then the compressor capacity is ramped up
to the oil return speed for a short period of time.
[0021] In one application, the amount of oil lost is calculated in a plurality of discrete
time units when the compressor is operating at a lower speed. As an example, this
can occur every five seconds.
[0022] Although an 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.
1. A compressor comprising:
a compressor pump unit, and said compressor pump unit and said motor being housed
within a shell, said shell having a sump for maintaining a quantity of lubricant;
a control for said compressor, said control being programmed to operate at at least
one low capacity and at least one higher capacity, and said control also being operable
to estimate an amount of oil which will have migrated from said compressor shell at
least when said compressor is operating at said at least one lower capacity, and said
motor control being operable to move said compressor motor from the at least one lower
capacity to the at least one higher capacity once the estimated quantity of oil exceeds
a predetermined limit.
2. The compressor as set forth in claim 1, wherein said at least one lower capacity and
said at least one higher capacity are achieved by varying the speed of the motor.
3. The compressor as set forth in claim 1 or 2, wherein said estimate of the amount of
oil is based upon pressures in the system, or
is based upon the pressures at both a condenser and an evaporator that are connected
to said compressor.
4. The compressor as set forth in claim 3, wherein the amount of oil estimate is based
upon the estimated saturated temperature at both a condenser and an evaporator that
are connected to said compressor
5. The compressor as set forth in any preceding claim, wherein said estimate of the amount
of oil is based upon temperature measurements.
6. The compressor as set forth in any preceding claim, wherein a multiplier factor which
includes a Capacity ratio quantity multiplied by an oil loss rate calculated from
the pressures or estimate of saturated temperatures at said condenser and said evaporator.
7. A refrigerant system comprising:
a condenser, an evaporator, an expansion device and a compressor;
the compressor having a motor for driving a compressor pump unit, and said compressor
pump unit and motor being housed within a shell, said shell having a sump for maintaining
a quantity of lubricant, a control for said motor, said control being programmed to
operate at at least one low capacity and at least one higher capacity, and said control
also being operable to estimate an amount of oil which will have migrated from said
compressor shell at least when said compressor is operating at said at least one lower
capacity, and said motor control being operable to move said compressor motor from
the at least one lower capacity to the at least one higher capacity once the estimated
quantity of oil exceeds a predetermined limit.
8. The refrigerant system as set forth in claim 7, wherein said estimate of the amount
of oil lost is based upon pressures in the system, or
is based upon the pressures at both the condenser and the evaporator.
9. The refrigerant system as set forth in claim 7 or 8, wherein a multiplier factor which
includes an RPM quantity is multiplied by an oil loss rate calculated from the pressures
at said condenser and said evaporator.
10. The refrigerant system as set forth in claim 7, 8, or 9, wherein said at least one
lower capacity and said at least one higher capacity are achieved by varying the speed
of the motor.
11. A method of operating a compressor comprising the steps of:
(a) operating a motor for a compressor at at least one low capacity and at least one
higher capacity;
(b) estimating an amount of oil which will have migrated from a compressor shell at
least when said compressor is operating at said at least one lower capacity rate;
and
(c) moving said motor from the at least one lower capacity to the at least one higher
capacity once the estimated quantity of oil exceeds a predetermined limit.
12. The method as set forth in claim 11, wherein said estimate of the amount of oil lost
is based upon pressures in the system, or
is based upon the pressures at both a condenser and an evaporator that are connected
to said compressor.
13. The method as set forth in claim 11 or 12, wherein a multiplier factor which includes
an RPM quantity is multiplied by an oil loss rate calculated from the pressures at
said condenser and said evaporator.
14. The method as set forth in claim 11, 12 or 13, wherein said at least one lower capacity
and said at least one higher capacity are achieved by varying the speed of the motor
15. The method as set forth in claim 11, 12, 13 or 14, wherein said estimate of the amount
of oil is based upon temperature measurements.