Normas de origen rígidas

2.8.1 Vi (Internal Volume Ratio)

For a screw compressor, the volume of refrigerant gas trapped between the M and F rotor lobes decreases while the pressure increases as the rotors turn. When the trapped volume decreases to the designed Vi (internal volume ratio), the volume is exposed to the discharge side and the refrigerant gas is discharged.

As described above, for a screw compressor, the Vi (internal volume ratio) indicates the ratio of the volume of sucked refrigerant gas to the compressed minimum volume when the interlobes connect to the discharge outlet port (the volume before discharged).

Vi is shown as follow.

(Volume of refrigerant gas (after suction) when compression begins) Vi =

(Volume of refrigerant gas when discharge port opens (end of compression))

Vi is the ratio of the volume between rotor lobes and casing after completion of suction and the volume when discharge port starts to open. The J-series basically adopts three Vi values of 2.50, 3.50, and 5.0 which are called ‘L port’, ‘M port’, and ‘H port’ respectively.

Vi is expressed as follows with the compression ratio.

As shown above, Vi is related to the specific heat ratio (κ) of the refrigerant gas. Therefore, Vi values vary not only with the compression ratio but also the type of refrigerant gas. The J-series compressors are designed to automatically adjust Vi according to operating conditions using the specified controller (MYPRO-CP IV or succession machine).

Figure 2-23 Explanation of Vi (Internal Volume Ratio)

Figure 2-24 Relationship between Vi Settings and Operation Conditions

Screw Compressor J-series 2.8 Variable Vi Control

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2.8.2 Reasons for Adjusting Vi According to Operating Conditions

Operating conditions for the compressor differs with applications. The same compressor may be operated under a variety of pressure conditions such as air conditioning, cold storage and freezing. Also, the discharge pressure may vary depending on the type of condenser or climate conditions. Even in the same application, some conditions may vary. For example, a compressor for air conditioning has different operating conditions for the cooling mode and the heating mode. And a compressor for refrigeration may have different operating conditions depending on the storage room temperature. Compressors are always required to operate with maximum efficiency in these varying operating conditions.

For a compressor without a variable Vi mechanism, power is wasted when the fixed Vi does not match the operating conditions (refer to Figure 2-23 and Figure 2-24). For example, if a compressor with a high Vi discharge port (suitable for a high compression ratio) is operated at low compression ratio conditions, the gas between the rotor lobes increases internal pressure to a value higher than the discharge pressure before it is released to the discharge port, this results in extra compression power. On the other hand, if a compressor with a low Vi discharge port is operated at high compression ratio conditions, the gas between the rotor lobes is released to the discharge port before the internal pressure reaches discharge pressure. This results in backflow of the high pressure gas to the discharge side which wastes extra power to push back the backflow gas. These characteristics are inevitable for a fixed Vi screw compressor and become obvious when the discharge port Vi does not match the actual operation conditions.

The J-series has a structure that can vary Vi of the discharge port according to operating conditions, therefore, it provides the advantage of operating at a wide range of conditions with high efficiency.

2.8.3 Details of Variable Vi Mechanism

The J-series varies the Vi by moving the variable Vi slide valve with a hydraulic cylinder thus changing the radial discharge port located in the unloader slide valve.

The Vi is increased when oil is supplied to the Vi increase side of the hydraulic cylinder. The variable Vi slide valve moves towards the discharge side, and the unloader slide valve moves so that the radial discharge port becomes smaller and Vi is increased.

On the other hand, the Vi is decreased when oil is supplied to the Vi decrease side of the hydraulic cylinder and the variable Vi slide valve moves towards the suction side. The radial discharge port becomes large and Vi is decreased.

The position of the variable Vi slide valve can be detected by the Vi position sensor. The J-series can always control Vi to an optimum value with a specified controller according to measured operating conditions (such as suction and discharge pressure).

2.8.3.1 Automatic Variable Vi Mode

In the automatic variable Vi mode, the J-series calculates the optimum Vi from the measured operating pressures using a specified controller and adjusts the hydraulic pressure on the Vi increasing side or the Vi decreasing side of the hydraulic cylinder to move the variable Vi slide valve to a suitable position for the calculated Vi.

Note 1: Basically, the automatic variable Vi control is achieved in three steps, L, M, and H.（Not continuously）

Note 2: Do not frequently vary the Vi automatically. This may lead to deterioration of reliability such as early wear to the related components.

Screw Compressor J-series 2.8 Variable Vi Control

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2.8.3.2 Fixed Vi Mode

The J-series also allows for fixed Vi if the operating conditions are stable and the automatic variable Vi control is not necessary.

Note, however, a specified controller is required to adjust the variable Vi slide valve to the desired Vi value. It is able to set Vi to the following three ports: L port (Vi=2.5), M port (Vi=3.5), H port (Vi=5.0).

Note 1: For the J-series, a specified controller is always required.

2.8.4 Vi Position Sensor

Figure 2-25 Vi Sensor Wiring Diagram

Table 2-10 Relationship between Vi and Vi Sensor Output

P o r t Vi R a t i o t o f u l l s t r o k e ( %) R e s i s ta n c e (Ω) O u t p u t vo l ta g e r a t i o ( %)

L 2.5 0 830 83

M 3.5 56 550 55

H 5.0 100 330 33

Table 2-11 Specifications of the Potentiometer I t e m Sp e c i f i c a t i o n s Total resistance 1kΩ ±20%

Rated power 0.3W/50°C

Insulation resistance 100MΩ or more 500V DC Withstanding voltage 500V AC for 1 minute Working temperature range -40—100°C

Screw Compressor J-series 2.9 Solenoid Valve

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In document MÓDULO DE CAPACITACIÓN EN EL COMERCIO DE LOS TEXTILES Y EL VESTIDO (página 64-120)