Ejemplos acad´ emicos

Allow 5 minutes to elapse before starting calibration.

To start calibration, press the calibration button once. The green LED by the calibration button will go off and the red Led by the calibration button will come on steady for approximately 15 seconds and then start flashing.

Now move the slide valve to the maximum load position. During this movement the cylinder capacity array lights will start to light. When the slide valve is in the max. load position, push the calibration button once. The red LED by the calibration button will stay on for approximately 15 seconds and then go off. The green LED light will come on, possibly flashing.

The calibration is now complete. Refit the calibration button cover.

Note: If the operating Vi is changed, then the LPI will have to be re-calibrated as from 6 above.

February 2003 7-7

February 2003 7-8 LPI Wiring Diagram

Red LED Green LED

Calibration Button

be run until alignment readings taken before and after all pipes have been fitted are identical.

A spacer coupling may be used where access to the compressor shaft seal is required without moving either the compressor or the main drive motor.

In the event that a torsional analysis of a complete compressor and driver system suggests that a change in natural frequency would be advantageous, the torsional stiffness of the coupling would be the area to be changed.

February 2003 8-1

The gas strainer should have gaps no greater than 0.25 mm (0.010"). A suitable wire mesh is 37 S.W.G. (0.173 mm dia., 0.0068" dia) and 24 wires/cm (60 wires/inch) giving a gap size of 0.24 mm (0.0096"). This fine mesh is too light in construction on its own and requires support from heavier material. One method of support is to sandwich the fine mesh between two sheets of heavier mesh of 20 S.W.G. (0.91 mm dia., 0.036" dia) and 8 wires/cm (20 wires/inch).

This "sandwich" can be formed into a plain or corrugated cylinder as shown in the sketches below. It is important that the free area of the fine mesh should be not less than 2.5 times the area of the incoming suction pipe. The perforated plate support should have a free area not less than 1.5 times the pipe area.

A typical construction of suction strainer is shown below. This allows the strainer basket to be removed for cleaning without disturbing either the pipework or compressor.

February 2003 8-2

found that some disc types where the disc is mounted on a spindle, can suffer from chatter at part load unless some damping is provided. An 'O' ring fitted to the spindle can solve this problem.

The suction non-return valve should always be positioned before the suction strainer.

This is to ensure that should any part of the valve come loose it is caught in the strainer and does not pass into the compressor.

The size of the non-return valve should be the nominal size of the suction ductwork.

The maximum recommended gas velocity through the suction system is 15 metres/sec.

(50 ft/sec). When handling very light gases such as helium (He) pressure losses are low and higher velocities are acceptable.

February 2003 8-3

temperature are normally connected into this pipe section.

February 2003 8-4

Separator Vessel Diameter - Oil Reservoir Capacity

This vessel has two main functions. It contains the oil reservoir and separates the oil from the gas before it passes to the rest of the system. The diameter of the vessel has to take both of these functions into account and is normally set by the area of the demister and the height, as required for the necessary oil volume and other physical considerations.

The first stage in the oil separation process is to arrange for oil which is in large droplet form to return by gravity to the main oil reservoir in the bottom of the separator vessel.

At least 97% of the oil is separated out in this way. The method of feeding the gas/oil mixture from the compressor into the separator is carefully designed to reduce velocity, reverse flow direction etc. to achieve maximum effect from this first separation stage.

Details of how to do this are provided later in these notes.

The quantity of oil in the reservoir should be a minimum of 114 litres (25 Imperial gallons) or half the oil pump capacity per minute whichever is the greater.

Separator Element Selection

There are two main alternative types of separator concepts in use, and sometimes both are used in combination when extra high efficiency of separation is required. The two types are wire mesh pads and coalescing cartridges (sometimes called cylinders or candles).

February 2003 8-5

The following design comments apply to vertical separator design.

The knitted wire pad operates in the following way. As the gas/oil mixture flows through the pad, the small oil droplets impinge or catch on the strands of wire and build up to large droplets as more small drops collect. Once large enough they fall off the bottom of the pack into the oil reservoir. The gas with the oil removed carries on through the top of the pack. It is important, therefore, to have a thick enough pad to ensure that there is time for all the small droplets to come in contact with the wire mesh and also that there are as many wires as possible in the pad to achieve maximum collection of droplets.

It is also necessary to ensure that the ideal velocity of the gas through the pad does not exceed the separating effect of gravity on the oil in the gas. If this does occur the pad does not drain and very high oil carry over results.

The knitted wire pad should be made from 2 ply stainless steel wire of 38 S.W.G. (0.152 mm dia., 0.006" dia) the pack having a density of 0.192 gm/cm³ (12 lb/ft³). The pack thickness should be not less than 300 mm (12"). The pack should be produced in the form of a continuous knitted sleeve which is half crimped and then rolled compactly into a circular pack. This would be fitted with grids above and below which provide basic support for the pack in operation. The grids would be smaller in diameter than the vessel but the pack itself should be a tight fit in the vessel to ensure that all gas/oil mixture has to pass through the pack and cannot by-pass it. There should be no openings through the pad, either because the pad is not rolled tightly enough or because the attachment of the grids to the pack (usually these are tied to the pack by wire) distorts it to produce a through opening.

February 2003 8-6

ft³/sec = ft² ft/sec

The volume flow passing through the separator is the compressor mass flow multiplied by the specific volume of the gas at discharge pressure and temperature.

The design velocity for the wire mesh pad is calculated from the following equations.

Metric Units

Design vapour velocity, metres/sec = 0.7 x 0.11 P P P

i v

- v

Where P_L is density of entrained liquid Kg/m³ PV is density of vapour Kg/m³

Imperial Units

Design vapour velocity, ft/sec = 0.7 x 0.35 P P P

i v

- v

Where PL is density of entrained liquid lb/ft³ PV is density of vapour lb/ft³

The design velocity calculated by the above equation incorporates a safety margin to cope with variation in velocity which can occur during normal operation.

February 2003 8-7

The entry of the gas/oil mixture into the vessel is important in order to achieve effective preliminary separation and also in relation to the wire pack position and shape. A simple and effective method is shown below.

This design causes the gas/oil mixture firstly to flow down towards the main oil reservoir and then to reverse direction upwards at low velocity through the wire pack and out of the separator vessel. The large oil droplets, which comprise nearly all the oil, do not reverse direction and tend to continue to flow down in to the oil reservoir, assisted by gravity.

February 2003 8-8

In this case the Gas Flow Area B and the Gas Flow Area C should be not less than 1.5 times the pipe area A.

It is important that the pipe cover is not positioned too close to the wire pack or it will mask part of it and cause excessive velocity through the remainder.

The following rules should be applied.

The distance L should be 0.5D or 350 mm (14") whichever is the greater.

February 2003 8-9

In all of these arrangements, the gas/oil mixture first flows towards the oil surface and then reverses direction. If the velocity against the oil surface is too high, there is the danger that re-entrainment of oil can occur causing more oil to pass through the separator instead of less.

The height 'h' should be not less than twice the diameter of the incoming pipe.

February 2003 8-10

The length 'h' from the top of the separator pad to the top of the vessel should be 0.5D or 450 mm (18") whichever is the greater. The space 'S' should not be less than the diameter of the exit pipe i.e. not less than 'd'.

February 2003 8-11

Some form of oil level indicator should be fitted to the oil tank separator which can be a sight glass or "level eyes" i.e. small transparent windows. When the refrigerant is Ammonia, which is less dense than oil, it can condense in the sight glass and sit on top of the oil in the glass tube. If the top of the ammonia level is used as a guide, it will be reasonably accurate but will show a level slightly higher than the oil level internally.

February 2003 8-12

February 2003 8-13

The oil pump suction pipe should not be located in the centre of the vessel. The centre is the lowest point in the tank and is the area in which any dirt etc. will tend to gather. It is better to have the pipe offset a little to avoid drawing in any dirt. Piping systems for oil supplies to the compressor should be designed to limit pressure losses to < 1 bar and reduce foaming.

February 2003 8-14

It is necessary to be able to add oil to the system while it is pressurised with gas or refrigerant. This can be done in three ways:

1. Topping up Tank

A small tank of capacity of not less than 5 litres (1 Imperial gallon), or 10 to 15 litres (2 to 3 gallons) in larger sets is mounted on the side of the oil tank/separator. This small vessel, which is designed to be suitable for the system maximum pressure, has connections top and bottom, through isolating valves, into the oil tank/separator. It also has a connection in the top, flanged or screwed, of, say 50 mm (2") diameter. For convenience of operation this top connection can be fitted with a valve rather than a blank flange.

In order to add oil to the system, the valves on the two pipes into the oil tank/separator are closed and the topping up tank is filled through the large connection on the top. The top connection is then closed, the two side valves opened and the oil flows into the main tank by gravity.

This process is repeated until the required quantity of oil has been added.

February 2003 8-15

A hand or motor driven pump capable of developing the required pressure may be used.

Only clean oil should be added in this way as it is passing directly into the compressor.

3. Drawing in Oil By Low Suction

An even simpler method of adding oil is to draw in the oil directly. If the compressor suction pressure is below atmospheric pressure the same valved connection described above is used. A flexible pipe is connected and the other end placed in a container of CLEAN oil. When the valve is opened oil will be drawn into the system.

When the compressor suction pressure is above atmospheric pressure, the compressor suction isolating valve has to be closed sufficiently to reduce it to below atmospheric pressure to approximately 0.5 atmospheres absolute. It must not be closed completely or cavitation in the compressor will occur. Once this sub-atmospheric pressure is reached, the small valve can be opened to draw in oil as described above.