Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-06-13
2003-02-11
Walberg, Teresa (Department: 3742)
Refrigeration
Automatic control
Refrigeration producer
Reexamination Certificate
active
06516622
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to compressor controls and more particularly to compressor controls in compressed gas systems having refrigerated dryers.
BACKGROUND OF THE INVENTION
Refrigerant compressors are used in a variety of systems. One type of system that uses refrigerant compressors is a compressed gas system. Compressed gas systems typically provide high volumes of dry, pressurized air or other gases to operate various items or tools (while a multitude of gases can be used, this application typically refers to air as a matter of convenience). Conventional systems dry the air using heat exchangers first to cool the air and lower the dew point of the air, which causes water vapor to condense out of the air, and second to reheat the air and raise the outlet temperature of the air. This system provides a relatively dry air source.
FIG. 1
shows a conventional refrigerated dryer
100
for a compressed gas system. Refrigerated dryer
100
includes both an air heat exchanger circuit
110
and a refrigerant heat exchanger circuit
120
. Air heat exchanger circuit
110
includes an inlet
112
, an air-to-air heat exchanger
114
, a air-to-refrigerant heat exchanger or evaporator
116
, a water separator
120
a
and an air outlet
118
. Refrigerant heat exchanger circuit
120
includes evaporator
116
, a compressor
122
, a condenser
124
, a throttling device
126
, and a hot gas by-pass valve
128
.
Notice that temperatures used below to describe the operation of dryer
100
are exemplary only. Many different air temperatures and saturation levels are possible. The temperatures and saturation levels of the final operating system depend on a large variety of factors including for example system design specifications and local environmental factors. The factors that determine actual temperatures are beyond the scope of this patent application and, in any event, are well known in the art.
In operation, dryer
100
receives a high temperature, saturated, pressurized air or gas stream at inlet
112
. For example, the air or gas may be at 100 degrees (all degrees represented are degrees Fahrenheit) with a dew point of 100 degrees (i.e., 100% humidity), although any inlet temperature and dew point is possible. The air or gas stream passes through an inlet side of air-to-air heat exchanger
114
. The air or gas stream cools down to, in this example, 70 degrees with a dew point of 70 degrees (i.e., still 100% humidity). However, because 100 degree air or gas can carry a larger volume of water vapor than 70 degree air or gas, some water vapor condenses. The condensed moisture precipitates out and collects in the separator
120
a
. The 70 degree air or gas then travels through the air side of evaporator
116
where the air or gas is further cooled to approximately 35 degrees with a dew point of 35 degrees (i.e., still at 100% humidity). Again, moisture condenses out of the air or gas stream and collects in the separator
120
a
. The 35 degree air or gas then travels through the outlet side of air-to-air heat exchanger
114
. This reheats the air or gas stream to approximately 85 degrees with a pressure dew point of 35 degrees. The air or gas stream then exits the dryer
100
at air outlet
118
. Because 85 degree air can hold significantly more moisture vapor than 35 degree air, dryer
100
provides a source of dry, unsaturated, pressurized air or gas at air outlet
118
.
In refrigerant heat exchanger circuit
120
, refrigerant enters the refrigerant side of evaporator
116
as a cool liquid. While passing through evaporator
116
, the refrigerant heats up and is converted to a gas by the exchange of heat from the relatively hot air side to the relatively cool refrigerant side of evaporator
116
. The low pressure gas travels to compressor
122
where the refrigerant is compressed into a high pressure gas. The refrigerant than passes through air or water cooled condenser
124
where the refrigerant is condensed to a cool, high pressure liquid. The cool, high pressure refrigerant passes through throttling device
126
(typically capillary tubes or the like) to reduce the pressure and boiling point of the refrigerant. The cool, low pressure, liquid refrigerant than enters the evaporator and evaporates as described above.
When air heat exchanger circuit
110
and refrigerant heat exchanger circuit
120
operate at or near full capacity, hot gas by-pass valve
128
has no particular function. However, as the demand on air heat exchanger circuit
110
decreases, refrigerant heat exchanger circuit
120
has excessive capacity that could cause the liquid condensate in dryer
100
to freeze. Thus, when used in this situation, hot gas by-pass valve
128
functions to prevent the liquid condensate in dryer
100
from freezing. In particular, the hot gas by-pass valve opens feeding hot, high pressure gas around the evaporator (i.e., by-passes) maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing. The particulars regarding the operation of hot gas by-pass valve
128
are well known in the art. Typically, a temperature sensor associated with the hot gas by-pass valve (not specifically shown in
FIG. 1
) monitors the refrigerant temperature at the outlet of evaporator
116
. When the temperature at the outlet decreases below a predetermined threshold, the hot gas by-pass valve
128
opens feeding hot, high pressure gas around the evaporator maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing.
The capacity of compressor
122
depends, in large part, on the maximum required capacity or expected air flow (measured in standard cubic feet per minute) of air heat exchanger circuit
110
. At full capacity (or air flow), compressor
122
operates at 100% capacity and the air temperature and dew point of the air stream is, for example, approximately as described above. The demand on the air system, however, is not always 100% of the designed capacity. Frequently, the demand on air heat exchanger circuit
110
is somewhat below full capacity. With less than 100% demand on air heat exchanger circuit
110
, the refrigerant heat exchanger circuit
120
described above still operates at 100% capacity, thus wasting energy or electric power because compressor
122
does not need to operate at full capacity. Some systems, as described above, compensate using hot gas by-pass valve
128
. Hot gas by-pass solves the problem of providing to much cooling through refrigerant heat exchanger circuit
120
, but does not solve the problem that the compressor is operating at a higher than necessary capacity and consuming a larger amount of electrical power than necessary. Other systems cycle the compressor on and off when the system operates at less than 100% capacity. These systems reduce power consumption somewhat, but cause excessive on and off cycling of compressor
122
, wide fluctuations in the dew point at air outlet
118
, and introduce inefficiencies associated with the heat exchange of mass media. Thus, it would be beneficial to control operation of compressor
122
based on the demand of air heat exchanger circuit
110
to reduce the power consumed by compressor
122
and increase the overall power efficiency of dryer
100
.
SUMMARY OF THE INVENTION
To attain the advantages of and in accordance with the purpose of the present invention, as embodied and broadly described herein, apparatus for controlling the operating speed of a variable speed compressor in a refrigerated air drying system having changing demands on an air supply, include a demand sensor capable of sensing changes in the demand on the air supply and generating a change in demand signal. A motor speed controller receives the generated change in demand signal and generates a motor speed signal. The motor speed controller sends the motor speed signal to a motor of the variable speed compressor to change the speed of the variable speed compressor.
Other embodiments of the present invention provide methods for controlling the operati
Neve Donald A.
Thomas William B.
Wilson James J.
BelAir Technologies, LLC
Dorsey & Whitney LLP
Robinson Daniel
Walberg Teresa
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