Refrigeration – Automatic control – Trapping and discharging refrigerant batches
Reexamination Certificate
2000-03-29
2002-08-06
Tanner, Harry B. (Department: 3744)
Refrigeration
Automatic control
Trapping and discharging refrigerant batches
C062S196400, C062S509000, C062S510000, C062S175000
Reexamination Certificate
active
06427460
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a refrigeration system with a refrigeration cycle which provides optimized consumption.
More particularly, the invention relates to a refrigeration system which allows to save energy with respect to conventional refrigeration systems, which are adapted to refrigerate enclosed spaces or masses in general and to keep them at a lower temperature than the outside environment by using an external power source.
Among refrigeration systems or refrigeration machines, the most widely used type is the so-called vapor-compression type, which operates by converting mechanical energy into heat energy by means of the single or multiple reverse thermodynamic cycle in which a refrigerant fluid evolves.
In some of the conversions of the cycle, the fluid is in the superheated saturated vapor phase. Of the heat energy produced by the conversion, a certain fraction, determined on the basis of the first and second laws of thermodynamics, is used at a temperature below the temperature outside the system in order to refrigerate enclosed spaces or masses.
FIG. 1
is an enthalpy chart of the conventional type, in which enthalpy H per unit mass is plotted on the X-axis and the Y-axis plots the logarithm of pressure.
FIG. 1
plots two isothermal lines H
1
and H
2
on which state transitions occur at a different pressure and with different temperatures.
The region between the upper and lower limiting curves is known as wet vapor region, since liquid and vapor are simultaneously present. It can also be noted that the enthalpies required to change state vary according to the pressure and tend to decrease (H
2
is smaller than H
1
) as one approaches the critical pressure C, at which the state transition occurs without requiring heat.
FIG. 2
is an enthalpy chart which plots a conventional refrigeration cycle.
The various steps of a refrigeration cycle, which can be provided for example by means of a refrigeration system as shown in
FIG. 3
, are now described.
The refrigeration system comprises a refrigeration cell inside which an evaporator
2
and an expansion valve
3
are arranged.
The evaporator is connected to a compressor
4
, which is in turn connected to a condenser
5
and to a reservoir
6
which is connected to the expansion valve
3
.
The connection between the evaporator
2
and the compressor
4
is a connection in which gas at low pressure flows, while the connection between the compressor
4
and the condenser
5
is a connection in which high-pressure gas is present.
A liquid line is provided between the reservoir
6
and the expansion valve
3
.
The refrigeration cycle provided by means of the system shown in
FIG. 3
is illustrated schematically in the enthalpy chart of FIG.
2
.
The point
1
of the chart corresponds to a step during which the refrigerant gas is at a pressure P
1
in the dry saturated steam region; in this condition, it is aspirated by the compressor and compressed to the point
2
, which corresponds to a pressure P
2
>P
1
.
The compression work applied to the gas by the compressor
4
is given by the difference in enthalpy H
2
−H
1
and approximately corresponds to the energy expenditure of the refrigeration system.
The temperature of the gas at the point
2
increases with respect to the temperature at the point
1
: compression normally occurs in a very short time and it is therefore possible to consider this as an adiabatic transformation (i.e., involving no heat exchange).
The high-pressure gas is then sent to the condenser
5
, in which heat is removed.
Desuperheating occurs in the first part of the condenser
5
, and the rigerant gas passes from the point
2
on the chart of
FIG. 3
to the point
3
the same chart, and actual condensation begins, until all the gas is in the liquid state and is therefore subcooled until it reaches the point
4
.
The expansion valve
3
, arranged upstream of the evaporator
2
, is designed to create a pressure drop at constant enthalpy; i.e., the energy content of the refrigerant gas does not vary in this step.
With reference to the chart of
FIG. 2
, the cycle passes from point
4
to point
5
, at which it is in the wet vapor region.
Then the partially evaporated gas at low pressure and at low temperature is sent to the evaporator
2
, where the actual refrigeration effect occurs.
Inside the evaporator
2
, the gas, by absorbing heat from outside, evaporates completely and therefore superheats to the point
1
of the chart of FIG.
2
.
The refrigeration effect produced by this cycle is given by H
1
−H
5
. At this point the cycle is repeated.
The theoretical efficiency of the described cycle is H
1
−H
5
/H
2
−H
1
.
The main variables to be defined, in addition of course to the refrigerating capacity of the system, are the evaporation pressure and the condensation pressure.
The evaporation pressure is chosen according to the temperature of the refrigeration cell
1
to be cooled, in which the evaporator
2
is immersed, and according to the characteristics of the evaporator.
The condensation pressure is instead usually determined by the maximum temperature that can be reached by the medium used to cool the condenser
3
(usually air), plus the temperature difference required by the condenser to operate.
Therefore, for example in the case of an external air temperature (on the condenser
5
) of at most 35° C., and of a condenser characterized by a temperature difference of 5° C., the condensation pressure corresponds to the temperature of 40° C.
However, the temperature of the condensation air can drop considerably, accordingly entailing a decrease in the condensation pressure.
In this case, the values of the condensation pressure may become so low as to prevent the correct operation of the expansion valve
3
and therefore the entire system stalls.
It should be noted that the purpose of the expansion valve is to provide the isenthalpic expansion of the refrigerant fluid and requires a minimum pressure difference between the inlet and the outlet for its correct operation.
This problem is normally solved by appropriately varying the ventilation on the condenser
5
so as to maintain a condensation pressure that corresponds to approximately 35° C.
Ventilation is normally reduced by sequentially shutting down the fans
7
, in the case of a multiple-fan condenser, or by reducing the rotation rate of said fans.
The actual refrigeration cycle, therefore, does not occur at the lowest possible condensation pressure (determined by the temperature of the condensation air), but is limited downwards so as to ensure circulation of the refrigerant gas.
Optimized energy consumption is not achieved in this way.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a refrigeration system in which energy consumption is optimized with respect to conventional systems.
Within the scope of this aim, an object of the present invention is to provide a refrigeration system which allows gas refrigeration to occur at the lowest possible pressure.
Another object of the present invention is to provide a refrigeration system which allows the compressor of the system to operate at lower temperatures and pressures than normally provided in conventional systems, thus reducing the wear of said compressor.
Another object of the present invention is to provide a refrigeration system whose overall refrigeration capacity is higher than that of conventional systems.
Another object of the present invention is to provide a refrigeration system in which the main section of the system is subjected to lower operating temperatures than those of conventional systems.
Another object of the present invention is to provide a refrigeration system in which the main compressors are quieter than the compressors of conventional systems.
Another object of the present invention is to provide a refrigeration system which can be used as a high-efficiency conditioning system.
Another object of the present invention is to provide a refrigeration system which can allow to provide high
Josif Albert
Modiano Guido
O'Byrne Daniel J.
Tanner Harry B.
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