Compressor Pressure Ratio Boundaries at -60℃: Dividing Criteria Between Scroll and Semi-Hermetic Reciprocating

The conclusion in one sentence

At the -60℃ evaporating temperature level, the governing variable in compressor selection is not capacity itself but whether the pressure ratio {{压比}} at the design condition falls inside the compressor allowable pressure ratio envelope, and whether the discharge temperature can be held below {{排气温度上限}} by the available cooling means. Scroll achieves low vibration and low noise in the small and medium capacity range on the premise that the built-in volume ratio is matched; semi-hermetic reciprocating is the safer choice in the medium and large capacity range and in deep part-load duty, because of its wide pressure ratio tolerance and field serviceability. The dividing line is not a single cut on capacity; it is a joint judgement on four items: capacity range, pressure ratio envelope, cooling method and part-load range.

Scope of applicability

• Machine family: LT-60 IceEdge series -60℃ evaporating temperature class single-stage auto-cascade (SACR) low-temperature refrigeration units.
• Capacity range: the platform spans 1 – 50 kW (planned family range, not measured values of any single model; capacity of a specific model is taken from {{制冷量}} and the accompanying technical documentation).
• Operating envelope: evaporating temperature {{蒸发温度}}, condensing temperature {{冷凝温度}}, ambient temperature {{环境温度}}.
• Compressor routes: two platform routes, scroll and semi-hermetic reciprocating.
• Not covered here: specific values of refrigerant composition and charge quantity, capacity control algorithms, machining and assembly tolerances.

Mechanism

1. Pressure ratio and volumetric efficiency

Pressure ratio is defined as the ratio of discharge absolute pressure to suction absolute pressure:

π = p_d / p_s

At -60℃ the suction pressure {{吸气压力}} is set by the saturation pressure of the low-boiling component at {{蒸发温度}}, an order of magnitude lower than in medium-temperature duty. A rising pressure ratio affects output through two paths:

• Re-expansion of high-pressure gas in the clearance volume (reciprocating machines) and flank leakage (scroll) make volumetric efficiency fall as pressure ratio rises; the general form is η_v = 1 − C·(π^(1/n) − 1) − leakage term. As π ↑, η_v ↓ and mass flow ↓.
• The temperature rise of isentropic compression increases monotonically with pressure ratio; the ideal adiabatic relation is T_d / T_s = π^((k−1)/k), where k is the isentropic exponent. As π ↑, T_d ↑ and the risk of refrigeration oil carbonisation and high motor winding temperature ↑.

One particularity of the auto-cascade cycle is that the suction side carries a mixture whose molar mass and isentropic exponent change with the circulating composition. Composition drift {{工质配比}} therefore changes the pressure ratio and the discharge temperature directly, and selection must be checked at both ends, "design composition" and "composition drift envelope", not at the design point alone.

2. Built-in volume ratio matching

In a scroll compressor the geometry of the fixed and orbiting scrolls determines a fixed built-in volume ratio Vi, and the corresponding optimum pressure ratio is approximately π_opt ≈ Vi^n. When the actual pressure ratio departs from π_opt:

• π > π_opt: over-compression; at the moment of discharge the pressure exceeds condensing pressure, and the extra compression work produces no refrigerating effect.
• π < π_opt: under-compression; discharge gas flows back and volumetric efficiency is lost.

In a semi-hermetic reciprocating compressor the effective built-in volume ratio is set by the relative size of the clearance volume, so the same model decays more gently in efficiency over a wider pressure ratio band, and the effective displacement can be changed by cylinder unloading. This is the fundamental difference between the two routes in pressure ratio tolerance.

3. Motor cooling and cooling means

In semi-hermetic and hermetic compressors the motor sits on the suction side and is cooled by the suction gas flow. At low temperature the suction density is low and the mass flow is small, so motor heat dissipation degrades:

• Suction mass flow ↓ → motor winding temperature ↑ → insulation ageing rate ↑.
• Evaporating temperature ↓ → suction specific volume ↑ → mass flow per unit displacement ↓ → motor cooling capacity ↓.

When natural suction cooling is insufficient, three means are available; their trade-offs are given in the table below. Liquid injection and economized vapour injection (EVI) are not mutually exclusive, but both require the compressor body to provide the corresponding port and internal flow path.

Key variables and direction of influence

• Evaporating temperature ↓ → suction pressure ↓ → pressure ratio ↑ → volumetric efficiency ↓, mass flow ↓ → capacity ↓
• Condensing temperature ↑ → discharge pressure ↑ → pressure ratio ↑ → discharge temperature ↑, power input ↑
• Pressure ratio ↑ → departure from the optimum pressure ratio for the built-in volume ratio ↑ → over- or under-compression loss ↑ → COP ↓
• Suction superheat ↑ → discharge temperature ↑ → risk of refrigeration oil carbonisation ↑, motor winding temperature ↑
• Suction mass flow ↓ → motor cooling capacity ↓ → motor winding temperature ↑ → insulation life ↓
• Liquid injection rate ↑ → discharge temperature ↓, while the risk of wet compression ↑, oil dilution ↑ and useful capacity ↓
• EVI engagement ↑ → intermediate injection flow ↑ → discharge temperature ↓, mass flow ↑, capacity ↑, while component count ↑ and controlled objects ↑
• Running frequency ↓ → share of scroll flank leakage ↑ → volumetric efficiency ↓; at the same time return gas velocity ↓ → oil return capability ↓
• Running frequency ↓ → motor cooling mass flow ↓ → motor winding temperature ↑ (an effect opposite to the falling load)
• Discharge oil content ↑ → oil entering the heat exchangers ↑ → oil film thickness on heat transfer surfaces ↑ → heat transfer coefficient ↓ → evaporating temperature ↑ (worse)
• Low-boiling mole fraction in the circulating refrigerant ↑ → suction pressure ↑ → pressure ratio ↓, while discharge temperature ↑ (the isentropic exponent varies with composition)

Engineering practice

Selection and verification proceed in the following order; each step states its acceptance criterion.

1. Fix the design condition: define {{蒸发温度}}, {{冷凝温度}}, {{环境温度}} and the secondary refrigerant / heat transfer fluid (HTF) inlet and outlet temperatures. Criterion: the condition set covers both the highest {{冷凝温度}} of the year and the lowest load, not the rated point alone.
2. Calculate the pressure ratio envelope: calculate the pressure ratio at the design composition and at the upper and lower bounds of composition drift. Criterion: the whole pressure ratio band falls inside the allowable envelope of the candidate compressor, with {{压比设计余量}} of margin left.
3. Check the built-in volume ratio: for scroll models, compare the design pressure ratio with π_opt. Criterion: |π − π_opt| ≤ {{压比偏差允许值}}; if exceeded, move to a high pressure ratio model or to a semi-hermetic reciprocating machine.
4. Estimate the discharge temperature: estimate from the isentropic relation and superimpose the cooling means. Criterion: discharge temperature ≤ {{排气温度上限}}, motor winding temperature ≤ {{电机绕组温度上限}}.
5. Decide the cooling scheme: first try to solve it by lowering condensing temperature and suction superheat; if insufficient, choose liquid injection or EVI according to the ports the model provides. Criterion: liquid injection rate ≤ {{喷液量上限}}, injection pressure {{喷气压力}} within the economiser design window.
6. Check the part-load range: determine the lowest load the unit must run at continuously and the corresponding minimum frequency {{最低运行频率}}. Criterion: return gas velocity ≥ {{最小回气流速}}, oil level within {{油位范围}}, motor winding temperature within {{电机绕组温度上限}}.
7. Check the oil circuit: confirm the refrigeration oil grade, the oil separator efficiency {{油分效率}} and the oil return path. Criterion: discharge oil content ≤ {{排气含油量上限}}, oil sump temperature within {{油温窗口}}.
8. Fix the route against the dividing criteria: compare the table below item by item and record the reason for each choice. Criterion: the four criteria agree; where they conflict, the pressure ratio envelope and the discharge temperature take priority.
9. Archive and re-check: keep the selection calculation sheet and the manufacturer's confirmation letter. Criterion: the calculation inputs match the operating assumptions of the accompanying technical documentation.

Note: the upper limits for discharge temperature and motor winding temperature are taken from the compressor manufacturer's stated values, which differ between models and between cooling methods. This page carries {{排气温度上限}} and {{电机绕组温度上限}} as placeholders; actual values follow the accompanying technical documentation.

Danger: the compressor and the discharge-side piping are at high pressure and high temperature during operation; before disassembly, leak detection or live measurement, the supply must be disconnected, pressure relieved and the equipment tagged out. Contact with -60℃ low-temperature parts causes adhesive frostbite; dry cryogenic protective gloves must be worn.

Parameter tables

Table 1: Dividing criteria between scroll and semi-hermetic reciprocating (thresholds pending)

Criterion	Threshold	Points to scroll	Points to semi-hermetic reciprocating
Capacity range	dividing capacity {{机型分界冷量}}	required capacity ≤ dividing value	required capacity > dividing value
Pressure ratio envelope	allowable pressure ratio {{涡旋允许压比}}	design pressure ratio ≤ allowable and close to π_opt	design pressure ratio outside the scroll allowable envelope
Built-in volume ratio matching	deviation {{压比偏差允许值}}	deviation within the allowable value	deviation exceeded, or a wide envelope needed for a broad condition span
Discharge temperature	upper limit {{排气温度上限}}	natural cooling plus liquid injection is enough	intermediate cooling / EVI / a wide-envelope model is required
Deep part load	minimum frequency {{最低运行频率}}	capacity control mainly by variable speed above 30Hz	cylinder unloading or prolonged low-frequency running required
Field serviceability	—	replacement as a unit is the norm	valve plates, piston rings and shaft seals replaceable on site
Vibration and noise constraint	{{噪声声功率级}}	reciprocating inertia forces are small, source strength lower	additional vibration isolation and acoustic measures required

Table 2: Comparison of motor cooling methods

Cooling method	Applicable condition	Discharge temperature	Capacity	COP	Main risk
Natural suction cooling	suction mass flow sufficient to carry away motor losses	no additional reduction	no loss	baseline	insufficient cooling at low temperature and low load
Liquid injection	discharge temperature above the limit and the model provides an injection port	↓	↓ (part of the liquid does not take part in evaporation)	↓	wet compression, oil dilution, liquid slugging
Economized vapour injection EVI	model with an intermediate injection port, fitted with an economiser	↓	↑ (injection raises mass flow)	↑ (within the applicable range)	more components and controlled objects, risk of liquid carry-over in the injected vapour

Table 3: Selection verification parameters pending

Parameter	Placeholder	Source of value
Design condition pressure ratio	{{压比}}	condition calculation
Suction pressure / discharge pressure	{{吸气压力}} / {{排气压力}}	measurement or property calculation
Discharge temperature and its limit	{{排气温度}} / {{排气温度上限}}	manufacturer's specification
Motor winding temperature limit	{{电机绕组温度上限}}	manufacturer's specification
Scroll built-in volume ratio	{{内容积比}}	manufacturer's specification
Minimum continuous running frequency	{{最低运行频率}}	manufacturer's specification plus measured oil return verification
Minimum return gas velocity	{{最小回气流速}}	measurement
Oil separator efficiency	{{油分效率}}	component specification plus measurement
Compressor model	{{压缩机型号}}	accompanying technical documentation

Common mistakes

1. Symptom: the compressor is selected on the nominal capacity in the catalogue and the installed unit delivers insufficient capacity. Misjudgement: applying the nominal capacity of a medium-temperature condition directly to -60℃ duty. Correct approach: at low temperature suction specific volume rises and volumetric efficiency falls, so values must be taken from the performance table at {{蒸发温度}} / {{冷凝温度}}. Criterion: the selection calculation inputs match the design condition, and the pressure ratio envelope is checked.
2. Symptom: discharge temperature runs high, so the liquid injection rate is simply increased. Misjudgement: treating liquid injection as a pure gain. Correct approach: liquid injection simultaneously brings oil dilution, wet compression and capacity loss; check first whether pressure ratio, condensing temperature, suction superheat and refrigerant composition have drifted. Criterion: liquid injection rate ≤ {{喷液量上限}}, with oil dilution and suction superheat still within their windows.
3. Symptom: the variable speed frequency is driven down to obtain a smaller capacity. Misjudgement: assuming capacity can be scaled down linearly with frequency all the way. Correct approach: at low frequency the leakage share rises, return gas velocity falls and motor cooling worsens, so a lower limit exists. Criterion: do not go below {{最低运行频率}}; meet lower capacity demand with cylinder unloading or hot gas bypass, and provide buffer volume on the secondary refrigerant side.

Further reading

• Single-Stage Auto-Cascade (SACR) Cycle Overview — cycle state points and the effect of the mixture on pressure ratio
• Refrigerant Pairing Constraints — how composition drift changes suction pressure and discharge temperature
• Troubleshooting — where the compressor branch sits in the fault tree and its criteria

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The technical material on this page is for selection reference; the accompanying technical documentation and the contractual technical agreement govern.