Howden Compressor Overhaul Manuals:
WRV 255 Compressor Manual, WRV 204 Compressor Manual, WRV 255 Compressor Manual, WRV 321 Compressor Manual, WRV 365 Compressor Manual, WRV 510 Compressor Manual
Howden WRV 255 Compressor Information:
VARIABLE VOLUME RATIO
Two forms of variable volume control are available.
1. ADJUSTABLE VOLUME RATIO (MVI)
2. AUTOMATIC VARIABLE VOLUME RATIO (AVI)
Compressor selections need to take account of the peak operating conditions likely to be encountered. However, actual operating conditions may vary, resulting in lower efficiencies. Control of capacity and volume ratio can maintain high efficiency levels. The Howden variable Vi concept, coupled to slide valve capacity control, offers alternative control methods. Where the pressure ratio across the compressor is consistently high or changes in pressure ratio are infrequent (e.g. the change from winter to summer conditions) then the MVi manually adjustable system will be entirely satisfactory. With lower pressure ratios, or where condensing conditions vary frequently, the Howden automatic control AVi system can be offered.
The Howden superfeed system is a development of the oil injected screw compressor design. All oil injected compressors are equipped with an additional gas port, located along the length of the compression chamber. Feeding refrigerant to this port from a superfeed/economiser vessel within the refrigeration system offers increased evaporator capacity of up to 20 percent, with virtually no increase in absorbed power.
ALL WRV RANGE COMPRESSORS ARE OF DOUBLE WALL CONSTRUCTION AND UTILISE WHITE METAL, SLEEVE TYPE JOURNAL BEARINGS WITH PRESSURISED SHAFT SEAL.
A full range of Vi options from 2.1 to 5.8 is offered for each compressor while slide valve capacity control is a standard feature on all compressors. Many options to standard design are available, some of which are indicated below:
‘Condition controlled’ version with reduced oil flow for dense gases and temperature control.
‘Mirror’ version for reverse rotation with double ended motor drive and two stage design.
‘Higher pressure’ version for high discharge pressure.
‘eXtra high’ discharge design.
‘Tilting pad’ thrust bearing design, e.g. to comply with API 619.
‘Steel casings’ for high pressure or to match specification.
‘Nodular cast iron design” again for specific project specification.
HOWDEN WRV 255 FAILURE ANALYSIS:
The (Mechanical seal) input shaft seal is the fragile component of any screw compressor. It becomes the first point of visible distress. The mechanical seal seen as occurring in two general areas: primary face and secondary seal.
A compressor’s bearings are among vital components of a screw compressor. Bearings locate the position of the rotors with great precision. The main bearing depends on a film of oil to keep the rotor and bearing surfaces separated. This film supports the entire weight of the rotor and its gas load. Bearings normally fail when this oil film becomes contaminated, breaks down, or if the bearing becomes overloaded. The oil film is created and maintained by the compressors lube oil system. The rotor journal is in contact with the bearing surface when at rest. With proper oil pressure that the rotor positions itself. The rotor rides on a lubricant wedge during design loads. The thrust bearings installed in the Howden compressor are paired single-row, angular-contact ball bearings. The steel caged antifriction bearings are fitted face to face at the discharge end of the compressor. The bearings are off loaded by the use of balance pistons. The compressor achieves sufficient off loading by utilizing a single balance piston on the input end of the male rotor. Lubrication failure doesn't normally imply a total lack of lubrication. A thorough investigation may reveal too low a differential oil pressure, improper viscosity due to dilution, or product condensing due to a low discharge temperature.
The vibration and sound levels of a howden compressor are rather low. The maximum vibration level is normally around 0.2165-in./sec in the horizontal and vertical plane and 0.295-in./sec axially. These levels are measured in RMS velocity.While sound levels are subjective, changes in operating sound levels should not be ignored. The cause of abnormal sound/vibration levels can be rooted in the compressor drive, auxiliary equipment, or system gas flow. When investigating abnormal vibration levels, don’t overlook the following: Mechanical mounting and coupling alignment; Piping strain; Throttled suction; Liquid carryover; Excessive oil injection; Discharge line/vessel resonance; and Mechanical interference (rotor to slide valve, rotor to casing).
The conditions and causes listed in the previous failure analysis were kept simple. In reality, several factors often seem to occur, making the root cause of a failed component not always obvious. Only after a complete review of operating conditions and other compressor components can these be determined.