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.
CAUTIONARY NOTE:
One of the disadvantages of this system is that it is very easy for the flexible pipe to come out of the oil or for the oil container to empty and air is then drawn into the system.
If not noticed, quite large amounts can be drawn in and will have to be purged out of the condenser in a refrigeration system. Care has to be taken that the main suction isolating valve is fully opened when the addition of oil is complete.
February 2003 8-16
Where height is restricted and it is necessary to mount the vessel as low as possible, an internal drain pipe may be used, although it must be remembered that with this arrangement full draining will only be achieved using a pump.
Where an internal pipe drain is used the end of the pipe should be close to the lowest point, and the recommended level is 25 mm (1") above the tank base.
Simple drain plug Drain piped to Internal isolating valve drain pipe
When the internal drain pipe is used it is recommended that a drain plug is still fitted to allow proper flushing of the vessel prior to its installation on the baseplate.
February 2003 8-17
Information concerning the number and size of the coalescing cartridges (sometimes called elements or candles) is provided by their manufacturer.
In arrangement (1) the space 's' between the knitted wire pad and the plate holding the candles should be not less than 300 mm (12").
In arrangement (3) the space 'x' between the deflector and the plate holding the candles should be 0.5D or 350 mm (14") whichever is the greater.
February 2003 8-18
continuous but restricted flow back to the compressor, and the second uses a small vessel with level control.
February 2003 8-19
return oil from this vessel to the compressor.
Discharge of Gas/Refrigerant from Coalescing Separators
The discharge pipe from the coalescing section has to be carefully positioned to avoid preferential flow causing high local velocities. If the pipe is simply positioned out of the side of the vessel adjacent to the candles or elements there will be an automatic pattern of flow towards that point. This results in gas flowing preferentially to the elements closest to the discharge pipe. The gas velocity through these is higher than elsewhere and in fact the elements furthest away can have very little flow passing through them.
The result of the high velocity through some elements is to reduce their separation efficiency and can also cause some of the oil separated from the gas to be re-entrained.
A baffle should be positioned over the outlet pipe to prevent this problem arising.
February 2003 8-20
An alternative and simpler approach is shown in view (2). This gives an ideal gas flow pattern but does involve a slightly taller vessel and it must be designed to permit the coalescer elements to be replaced. These coalescer elements are not self cleaning and over a period of time the pressure drop across the elements increases and they require to be replaced. The separator vessel design, therefore, must be flanged to permit access to the elements for replacement.
Separator Discharge Non-Return (Check Valve)
A non-return (check) valve is always fitted in the gas line after the separator. This valve ensures that the compressor pressure is at low level for starting.
February 2003 8-21
The heater should be located below minimum oil level. It should be fitted with its own temperature sensing and control. In order to avoid overheating the oil at the heater surface, the maximum heat rating should be 1.25 watt/cm² (8 watt/in²). The minimum temperature for the oil tank should be 35°C.
The heater rating has to be calculated on the basis of the time required for heating the contents of the oil tank and the capacity of the tank. If the time is not specified for any given installation it is suggested that a time of approximately 4 hours is used.
Sometimes more than one heater has to be fitted to comply with the combination of maximum heater surface rating and time to raise oil tank to temperature.
February 2003 8-22
This relief valve can pass gas to the compressor suction (upstream of the suction strainer) or to some other part of the system. This is not the main system relief valve in a refrigeration plant which must be located elsewhere in the system.
February 2003 8-23
flow to the system to zero. This valve need only be sized to handle 10% of the mass flow and is therefore quite small in size.
The other reason for fitting a by-pass valve is for applications which require the compressor capacity to be reduced almost instantaneously but where it is desirable for the compressor to be kept running. A by-pass line sized for the full mass flow will achieve this and the normal off loading by means of the slide valve will occur in the normal way over a slightly longer period. Although the by-pass valve has to pass the full mass flow, a high pressure drop is acceptable and therefore the by-pass line can be much smaller than the compressor discharge line.
It is worth noting that a by-pass line can ensure the minimum possible starting torque.
The compressor should always have the slide valve in the fully off load position for starting and starting torque is therefore low. With a by-pass line there can be no build up of pressure during the starting phase. However this is rarely needed.
February 2003 8-24
particularly with refrigerants that are highly soluble in oil, such as Freons. If it is necessary to be able to clean the oil strainer without depressurising the whole system, isolating valves have to be fitted. One would be located before the strainer and the second after the oil pump. A pump relief should be fitted at the pump discharge, upstream of the isolating valve. This arrangement permits the oil pump to be maintained without depressurising the total system.
February 2003 8-25
February 2003 8-26
thermosyphon arrangement for circulation. With this system the lowest connection on the cooler is connected to the bottom of the refrigerant liquid receiver and the upper connection to a point high up in the liquid receiver. It is necessary for the liquid receiver to be located at a higher level than the oil cooler and the sizing of the refrigerant pipes is important. Also, the cooler and receiver must not be too far apart. If a small refrigerant pump is fitted in the circuit, flow is guaranteed and the system design is much simpler.
The third system uses a refrigerant cooled oil cooler but with the refrigerant flow being controlled by an expansion valve varying flow to keep the oil temperature leaving the cooler at a controlled value. The refrigerant leaving the cooler is at a low pressure and may usually be passed to the compressor superfeed connection if superfeed is not being used.
In the vast majority of applications a straightforward water cooled oil cooler is used.
In some cases cooling water is either in short supply or simply not available. In a refrigeration plant refrigerant oil cooling can be used as mentioned above. Another alternative is air cooled oil cooling. This involves a heat exchanger with air being blown across it to remove the heat. This is usually quite practical but air cooled heat exchangers are usually more expensive than water cooled designs. If an air cooled system is sometimes required to operate with very low ambient temperatures, it is recommended that a thermostatic by-pass is fitted. This is set to control the oil temperature to the desired level and will by pass the cooler until the oil heats up.
February 2003 8-27
It is recommended that a differential oil pressure gauge is fitted across the oil filter(s) to give an indication of when the elements need replaced. If a differential gauge is not available a pressure gauge before and after the filter will serve the same function.
February 2003 8-28
pressure and temperature connections.
It should be noted that the piping from the manifold to the compressor should have a bore no smaller than the nominal bore of the compressor connection, e.g. if a compressor connection is 1/2" BSP then the pipe bore must be at least 1/2" diameter.
On the small diameter pipes from the manifold to the compressor, screwed connections are used and because some gases and refrigerants are very difficult to seal, 'O' ring connections are recommended.
In some specifications all oil piping downstream of the oil filter must be manufactured from stainless steel. The comments on cleanliness etc. still apply to this material.
February 2003 8-29
Compressor Suction Pressure Oil Manifold Temperature
Compressor Discharge Pressure
Discharge Pressure (downstream of separator outlet) Oil/gas differential pressure gauge
Oil pressure before filter
The high pressure side of the differential pressure gauge should be connected to the oil manifold and the low pressure side to the compressor discharge.
If it is not possible to obtain a suitable differential pressure gauge this can be replaced by a normal pressure gauge in the oil manifold. The differential pressure of the oil relative to the compressor discharge pressure can then be obtained by the difference between the two gauge readings.
It is also possible to replace the oil pressure gauge before the oil filter with a differential pressure gauge across the oil filter giving a direct reading of oil filter pressure drop.
Care should be taken to ensure that instrument sensing points are not placed in dead ends of piping.
February 2003 8-30
As mentioned elsewhere in these notes, an oil pressure relief valve and a system gas pressure relief valve are required.
February 2003 8-31
February 2003 8-32
February 2003 8-33 described in Section 8.9.
4) The compressor external arrangement drawing shows the appropriate connection sizes for lubricating piping connecting to the compressor. It is essential that pipe sizes relate to the connection sizes and that no reduced section pipe fittings are utilised in piping between the manifold and the compressor. Where the connection size is 1" BSP, 1" N.B. piping must be used.
It is also essential that oil supply connections on both the male and female rotor sides are piped to the oil manifold as indicated on the external arrangement drawing.
5) Gas piping must not be allowed to strain the compressor and sufficient pipe supports must be utilised to prevent allowable flange loadings from being exceeded.
6) Gas suction strainer elements should be rated at 250 microns maximum and consideration should be given to fitting a felt or polypropylene protective sock during initial commissioning runs to prevent contamination from the system pipework. All suction strainer elements must have a free surface area of 2.5 times the suction pipe area and should of the sandwich construction described in Section 8.2.
7) Oil filter elements should have a minimum rating of 15 microns nominal and should have sufficient surface area to ensure a satisfactory operating life. A duplex filter arrangement is preferred, to allow changeover without stopping the compressor, when low oil/gas differential pressure is experienced.
8) The oil side of the oil differential above gas discharge pressure must be measured at the oil manifold. Certain operating conditions dictate that an oil/gas differential pressure of 3.0 to 5.0 bar is required and it is essential that alarm and trip settings reflect this higher than normal pressure difference.
All WRVT510 compressor selections should be referred to Howden Compressors for confirmation that the oil/gas differential pressure specified is adequate for the operating conditions.
February 2003 9-1
compressor start up.
11) It is recommended that oil is circulated prior to compressor start up in order to achieve a minimum oil temperature of 30 deg.C. at the compressor bearings. A temperature bypass valve around the oil cooler may be required in order to achieve this.
12) Cleanliness of the lube oil system is of the utmost importance and must be established prior to any operation of the compressor. During pre-commissioning the pipes between the compressor and the manifold should be disconnected and the system flushed and proved clean before being reconnected to the compressor.
13) Consideration is required to be given to the relative heights of the compressor and the reservoir oil level to ensure drainage of the compressor on shut down.
14) To avoid high starting torque caused by the compressor filling with oil prior to start up, consideration should be given to fitting a solenoid valve and a pipe returning to the oil reservoir to the underside connection on the compressor suction casing. As in 10. control is required to ensure that this valve closes prior to compressor start up.
15) Oil/gas differential pressure must be controlled by a pressure actuated control valve sensing the difference between manifold and discharge pressure. A relief type valve mounted immediately downstream of the oil pump must not be used to control oil/gas pressure difference.
16) Consideration should be given to fitting pre-alarms and alarms in conjunction with any protective trip switches fitted. This would give adequate warning of any fault condition, thus preventing unnecessary shutdowns and subsequent high transient loads associated with the large masses and inertias in a 510 compressor system.
February 2003 9-2
February 2003 9-3
Tel: 0044 (0)141 882 3346 Fax: 0044 (0)141 882 8648
E-mail: [email protected] Web site: www.howdencompressors.co.uk
or
Howden Compressors LLC 1850B North Gravers Road Plymouth Meeting
PA 19462 USA
Tel: 001 610 313 9800 Fax: 001 610 313 9215
E-Mail: [email protected] Web site: www.howdencompressors.com
February 2003 10-1