Refrigerating apparatus with single compressor and multiple evaporators

Refrigerating apparatus including a compressor, condensor and a plurality of evaporators which are connected in parallel and controlled by individual electromagnetic valves in response to temperature changes of individual evaporators. The refrigerating apparatus also includes an inverter circuit and a bypass circuit which causes refrigerant to flow from output side of the condenser to the input side of the compressor. When the temperature of the bypass circuit drops because of operation of the electromagnetic valves, the inverter circuit adjusts the operating frequency of the compressor.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to refrigerating apparatus, and in 
particular, to an improvement in a multiple-type refrigerating apparatus 
for an air-conditioner, freezer or display case. 
2. Description of the prior art 
A well known prior art refrigerating apparatus, a so-called 
multiple-compressor refrigerator, is shown in FIG. 1. The exhaust-sides of 
each of a plurality of compressors 1, 3 and 5 are communicated to parallel 
connected condensers 7, 9 and 11 through a refrigerant discharge pipe 13. 
The output sides of condensers 7, 9 and 11 are collectively connected to 
parallel connected electromagnetic valves 15, 17 and 19, through a 
refrigerant pipe 21 which includes a capillary element (not shown). 
Electromagnetic valves 15, 17 and 19 are connected to an accumulator 23 
through individual evaporators 25, 27 and 29. Electromagnetic valves 15, 
17 and 19 include individual thermostats (not shown) which are provided on 
evaporators 25, 27 and 29 respectively. When the temperatures of 
evaporators 25, 27, or 29 reach a predetermined low temperature, a 
corresponding electromagnetic valve 15, 17 or 19 is closed automatically. 
Accumulator 23 is connected to the suction sides of compressors 1, 3 and 5 
through a refrigerant intake pipe 31. 
A pressure sensor 33, communicated to refrigerant intake pipe 31, is 
electrically connected to an electronic control circuit 35. Pressure 
sensor 33 detects refrigerant suction pressure of the suction sides, i.e. 
low pressure side, of the compressors. Electronic control circuit 35 is 
connected to magnetic switches 37, 39 and 41 which are individually 
provided on compressors 1, 3 and 5. Magnetic switches 37, 39 and 41 turn 
corresponding compressors 1, 3 and 5 on and off respectively in response 
to the output signal of electronic control circuit 35. 
The operation of the above-described refrigerating cycle will be described 
hereinafter. When compressors 1, 3, and 5 are actuated, refrigerant 
circulates through evaporators 25, 27 and 29. At this moment, compressors 
1, 3 and 5 are actuated at the same operating frequency and at the same 
rotary speed. Respective evaporators 25, 27 and 29 therefore operate 
identically. If the temperature of one of evaporators, e.g. evaporator 25, 
falls below a prescribed temperature due to its refrigeration load or 
other environmental condition, the corresponding electromagnetic valve 15 
is closed and the refrigerant flowing into evaporator 25 is stopped. The 
low pressure side of the refrigerant (the suction sides of the 
compressors) therefore falls below a prescribed value and pressure sensor 
33 detects this pressure fall and sends a signal to electronic control 
circuit 35. The electronic control circuit compares the signal with a set 
value to control respective compressors 1, 3 and 5. That is, one of 
compressors 1, 3 and 5 is stopped and the others are operated. The amount 
of refrigerant circulated is reduced, the pressure of the suction side of 
the compressors gradually rises and the refrigerating cycle suited to the 
number of evaporators which have been opened by the electromagnetic valves 
is effective. 
In this arrangement, a variable refrigerating cycle is provided by a 
plurality of compressors and condensors corresponding to the number of the 
compressors. Thus, installation space and operating costs are very large. 
There is also the disadvantage that it is not always possible to achieve 
optimum conditions because it is only possible to carry out stepwise 
control in correspondence to the number of compressors. 
It has been proposed to apply an analogue control system, a so-called 
inverter control system, in which the operating frequency of a single 
compressor is controlled by an inverter circuit and thus achieve optimum 
operating conditions. 
According to this control system, a single compressor, and a common 
condensor can be used to reduce installation space and manufacturing 
costs. It further has another advantage that the optimum control is 
possible because of stepless frequency control. However, the control 
section of the inverter circuit receives signals from a pressure sensor 
which detects the pressure side (suction side) of the compressor as in a 
conventional system. There are many errors in pressure measurement, and 
sensors in which errors are reduced are extremely expensive. These 
improved sensors have an adverse effect on costs. The detection of 
pressure of the low pressure side means detecting the pressure at the 
return side of a refrigerating cycle, i.e. the pressure of the refrigerant 
just before return to the compressor. Response is slow and it is difficult 
to detect slight changes of pressure. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved refrigerator 
with accurate compressor drive control and good response. 
This is achieved by detecting the temperature of refrigerant flowing 
through a bypass at the outlet of the compressor. When one evaporator is 
closed the bypass temperature falls and the operating frequency of the 
compressor is adjusted.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENT 
A preferred embodiment of the present invention will be now described in 
more detail with reference to FIG. 2. The discharge port of a single 
compressor 51 is communicated to a liquid tank 53 through a condensor 55 
by means of a forward side refrigerant pipe 57, and the intake port 
thereof is communicated to the refrigerant discharge side of an 
accumulator 59 by a return side refrigerant pipe 61. One of the output 
sides of liquid tank 53, constituting a liquid line, is connected to an 
intermediate portion 62 of refrigerant pipe 61 located at the refrigerant 
intake side of accumulator 59 through a bypass pipe 63 which has a 
throttle 65 such as e.g. a capillary tube. A bypass circuit 67 is formed 
of the bypass pipe 63 and throttle 65. Compressor 51 is electrically 
connected to electronic control circuit 69 through inverter circuit 71 
which controlls its operating frequency. A temperature sensor 73, e.g. a 
thermister, which is electrically connected to electronic control circuit 
69, is coupled to the intermediate portion of bypass pipe 63 between 
throttle 65 and refrigerant pipe 61, so that it detects the temperature of 
the refrigerant in the intermediate portion of bypass pipe 63. 
Refrigerator unit A includes all of the above-described elements. 
Another output side of liquid tank 53 is connected through refrigerant pipe 
57 to parallel connected electromagnetic valves 75a, 75b and 75c, each 
connected to the input sides of respective evaporators 76a, 76b and 76c, 
e.g. display cases, through expansion valves 77a, 77b and 77c 
respectively. The output sides of evaporators 76a, 76b and 76c are 
collectively connected to the input side of accumulator 59 through 
refrigerant pipe 61. Temperature detectors 79a, 79b and 79c of individual 
electromagnetic valves 76a, 76b and 76c detect the temperature of 
respective evaporators 76a, 76b and 76c. As there are three systems 
including an electromagnetic valve, expansion valve and evaporators in 
this Figure, for convenience of description these systems will be referred 
to as system a, system b and system c. 
The operation of the above-described refrigerating cycle is now described. 
When compressor 51 is actuated at a suitable operating frequency 
corresponding to the number of evaporators to be operated, refrigerant 
circulates. If the temperatures of individual evaporators 76a, 76b and 76c 
are above a predetermined value refrigeration is performed by each of 
evaporators 76a, 76b and 76c. 
If the temperature of evaporator 76a (system a), for example, drops because 
of the load conditions or for other reason, temperature detector 79a 
detects this temperature drop and electromagnetic valve 76a of system a is 
closed in response to this detection. Hereupon, the pressure of low 
pressure side 62, i.e. the intake side of compressor 51, falls because 
refrigerant flows through only evaporators 76b and 76c of system b and c. 
The amount of the refrigerant flowing into bypass circuit 67 from liquid 
tank 53, at this time, is increased with the fall of the pressure of low 
pressure side 62. Therefore, the temperature of the refrigerant flowing 
through throttle 65 falls and the temperature of bypass pipe 63, where 
temperature sensor 73 is provided, also drops. Temperature sensor 73 
detects this temperature-drop, and sends a signal corresponding to the 
detected temperature to electronic control circuit 69. Electronic control 
circuit 69 compares the signal, i.e. detected temperature fed from sensor 
73 with a prescribed temperature. If the detected temperature is lower 
than the prescribed temperature, electronic control circuit 69 changes a 
control signal in correspondence to the difference of the temperatures and 
sends it to inverter circuit 71. 
Based on the control signal, therefore, inverter circuit 71 lowers the 
operating frequency of compressor 51. When the operating frequency of 
compressor 51 is lowered, the refrigerant circulating in the entire 
refrigeration circuit is reduced and the pressure of low pressure side 62 
rises again. Consequently, the amount of refrigerant flowing into bypass 
circuit 67 from liquid tank 53 is reduced, and the temperature of bypass 
pipe 62 rises and finally returns to the prescribed temperature. 
As described in detail above, the operating frequency of the compressor is 
changed in accordance with number of operating evaporators, so that it 
enables a saturated temperature (vapor temperature of evaporator) in 
correspondence to the pressure of the low pressure side to be kept at a 
constant value. Further, the temperature sensor provided on the bypass 
pipe can respond quickly to and detect accurately the slight temperature 
changes of refrigerant at the low pressure side, thereby achieving optimum 
control of the operating frequency of the compressor. 
Many changes and modifications in the above-described embodiment can be 
carried out without departing from the scope of the present invention. 
That scope is intended to be limited only by the scope of the appended 
claims.