COPYRIGHT (c) ALL RIGHTS RESERVED. REPRODUCTION OR TRANSMISSION BY PRINTED, , ELECTRONIC OR OTHER MEANS STRICTY PROHIBITED WITHOUT THE WRITTEN PERMISSION OF UNICLA/SCA Australia PTY LTD.
The information contained herein is presented to assist service personnel in the selection of the correct compressor for specific applications and to ensure correct fitting procedures are adopted to maximise the life of both the compressor and associated componentry.
All Unicla compressors are manufactured to exacting standards using quality materials with test programs to ensure both reliability and durability are optimised. It is important to recognise poor fitment or servicing procedures and / or compressors mismatched to systems can seriously jeopardise both reliability and performance. This may result in premature compressor failure or unacceptable performance losses.
The following guidelines of fitment must be strictly adhered to. Unicla warranty against faulty product (Materials and Workmanship) - Subject to system compatibility and refrigerant and lubricant compatibility.
Principle Causes of Premature Compressor Failure
The broad categories of causes of premature compressor failure are:
- Excessive Pressure Loading
- Excessive Thermal Loading
- Liquid Slugging of Compressor
- Lack of Oil / Oil Return
- Contamination
- Overspeed
- Incorrect direction of rotation
Failure Cause 1
Pressure Loading
Excessive pressure loadings are directly attributable to one or more of the following system design or servicing faults.
- Overcharge
- Inadequate condenser capability (condenser too small)
- Inadequate condenser efficiency due to either
- Poor airflow dynamics (fans and / or ram air)
- Poor condenser placement (air damming and / or excessive
- external heat loading eg oil coolers etc)
- Oil flooding (reduced thermal efficiency)
- Flow restriction (condenser internal)
- Contaminated refrigerants — either non condensables (air / nitrogen), flushing agent contamination or refrigerant mixtures (cocktails)
The following chart should be used as a guide for analysing normally acceptable High Side (head Pressures) for given ambient. Allow 20% Tolerance for Humidities above 60% RH
|
° C
|
° F
|
kPa
|
PSI
|
|
15° C
|
(60)
|
600 — 800
|
90 — 115
|
|
18° C
|
(65)
|
750 — 950
|
110 — 135
|
|
21° C
|
(70)
|
900 — 1100
|
130 — 160
|
|
24° C
|
(75)
|
1050 — 1300
|
155 — 190
|
|
27° C
|
(80)
|
1200 — 1550
|
185 — 220
|
|
30° C
|
(85)
|
1400 — 1750
|
200 — 250
|
|
33° C
|
(90)
|
1500 — 1900
|
215 — 275
|
|
35° C
|
(95)
|
1700 — 2100
|
245 — 300
|
|
38° C
|
(100)
|
1850 — 2250
|
265 — 325
|
|
41° C
|
(105)
|
2000 — 2400
|
290 — 350
|
|
44° C
|
(110)
|
2250 — 2650
|
325 — 385
|
|
47° C
|
(115)
|
2500 — 2900
|
370 — 420
|
Alternatively
Condensing temperatures can be used for high side evaluation. Refer to additional technical information regarding condensing to air differentials for specifications.
Cautionary Notes
Liquid line access valves will not identify condenser internal restrictions, nor will liquid line line high pressure cut switches offer protection. Internal flow restrictions or blockages in the condenser rely on compressor mounted P.R.V’s (pressure relief valves) or compressor / discharge line high pressure cut switches for protection.
FAILURE CAUSE 2
Thermal Loadings
The enemy of the modern system. Thermal loadings are often misinterpreted as being directly attributable to excess pressures. This is not the case. Excess discharge temperatures may be pressure driven or maybe superheated vapours generated due to inadequate cooling vapour return to the compressor or excessive external thermal loads on the suction line and / or the compressor. NOTE: Modern compressors generate superheat as a normal operating condition — this is referred to as superheat of compression.
Please refer to the chart below for normal superheat / discharge line temperatures (factory test data)
|
Compressor
|
R/min
|
Discharge Temp ° C
|
Condensing Pressure Kgf/cm2
|
Condensing Temp ° C
|
Evaporator Pressure Kgf/cm2
|
Compressor Inlet
Temp °C
|
Superheat (discharge line)
|
|
UX310 (310cc)
|
1800
|
73
|
15.5
|
58
|
1.86
|
8.9
|
15
|
|
2200
|
81
|
15.5
|
58
|
1.86
|
8.9
|
23
|
|
3500
|
98
|
15.5
|
58
|
1.86
|
8.9
|
40
|
|
UX/UP200 (200cc)
|
1800
|
77
|
15.5
|
58
|
1.86
|
8.9
|
19
|
|
2000
|
78
|
15.5
|
58
|
1.86
|
8.9
|
20
|
|
3500
|
93
|
15.5
|
58
|
1.86
|
8.9
|
35
|
|
UP/UX170 (170cc)
|
1800
|
75
|
15.5
|
58
|
1.86
|
8.9
|
17
|
|
2500
|
83
|
15.5
|
58
|
1.86
|
8.9
|
20
|
|
3500
|
98
|
15.5
|
58
|
1.86
|
8.9
|
25
|
|
UP/UX150 (150cc)
|
1800
|
77
|
15.5
|
58
|
1.86
|
8.9
|
19
|
|
2400
|
81
|
15.5
|
58
|
1.86
|
8.9
|
23
|
|
3600
|
88
|
15.5
|
58
|
1.86
|
8.9
|
30
|
From this chart we can readily identify normal superheat levels as opposed to excessive superheat (superheating that may lead to premature compressor failure due to the thermal overload.
Causes of Excessive Superheat (Discharge)
It is important to note that superheat levels increase as a normal condition
- at higher compressor speeds
- at higher ambient temperature
Abnormal Superheat Generation may be caused by one or more of the following conditions — exaggerated under high heat loads.
- Low Charge Rates — insufficient flow of refrigerant to the compressor to provide adequate cooling for 2 reasons.
- The flow (volume) is reduced giving less cooling medium
- Low flow relates to excessive low side superheating — the suction vapours are no longer cold.
- With the compressor placed in environments of low external airflow (ie behind transverse mounted engines) adequate charge rates must be maintained to ensure adequate compressor cooling by the refrigerant.Restricted TX Valves / Orifice Tubes — will give the same conditions as above. It is strongly recommended the TX Valve or Orifice (Expansion) Tube be replaced at the time of compressor fitment. Partially blocked valves / tubes may provide adequate flow under moderate heat load conditions, but starve the compressor under high heat load conditions when pump cooling is most critical.
- Poor Condensing — In addition to excessive pressure generation poor condensing will result in vapour feed to the TX / Orifice Tube causing excessive evaporator superheating under high heat loads.
- Contaminated Refrigerants — May result in loss of compressor cooling particularly if Air or other Non condensables (ie Nitrogen) are present in the refrigerant stream.
Cautionary Notes — Superheat Testing
Excessive discharge line temperatures may be either excessive vapour superheating (as above) or due to excessive discharge line pressures. (as previously mentioned) In the event of condenser internal restrictions or oil slugging a liquid line access valve will not identify excessive discharge line pressures. Dual high side test points may be required in cases where condenser restrictions need to be identified.
Compressor relief valve activation (if fitted) may be an indicator of condenser internal restrictions.
The photographs above clearly shows a high degree of discharge line superheat relative to the pressure — however if the pressure is being sampled from the liquid line to excessive discharge line temperature may be pressure generated.
Calculation
121° C on the Discharge Line
68° C on the Gauge
58° C Superheat
Failure Cause 3
Liquid Slugging of Compressor
To prevent the risk of liquid slugging the following precautions must be observed when fitting any compressor from the UNICLA range.
- The TX Valve must be correctly sized and superheat set (if adjustable) to manufacturers specifications (TX Valves jammed open must be replaced).
- Refrigerants with positive glide characteristics are not to be used unless a TX superheat adjustment is made as glide compensation.
- Systems must not be overcharged
- Suction piping must be in accordance with accepted refrigeration standards particularly in applications where liquid migration is deemed a possibility.
- Suction line accumulators are the recommended alternative to piping design that may act as an oil trap, or where migration levels are high.
- In applications where suction line system design indicates a risk of either liquid migration or liquid slugging in operation a lock detection sensor (optional kit P / N 55106 — 000070) must be fitted.
This sensor is a protective device in case of adverse operating conditions allowing liquid to reach the pump. It must not be used as a "Front Line" defence against liquid slugging of the pump. If it is deemed there is a significant risk of liquid reaching the pump the suction line must be redesigned, the TX adjusted and / or a suction line accumulator used.
The lock detection sensor will disengage the clutch if the control box does not receive a signal from the sensor for 2 seconds.
Lock detection sensors can be considered on essential component where the compressor drive belt is a common drive for associated components (ie water pump). Liquid entry into the pump may cause belt breakage (on compressor engagement) which obviously must be avoided at all cost in common belt drive systems.
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COPYRIGHT (c) ALL RIGHTS RESERVED. REPRODUCTION OR TRANSMISSION BY PRINTED, , ELECTRONIC OR OTHER MEANS STRICTY PROHIBITED WITHOUT THE WRITTEN PERMISSION OF UNICLA/SCA Australia PTY LTD. |
