YLG Two-Stage Screw Compressor: Inside a Single-Body Machine

Most explanations of two-stage compression describe two machines: a booster, a discharge line, an intercooler, and a high-stage compressor. That arrangement exists and it works. It is also not what this series is.

The YLG is a single-body machine. Two rotor sets sit on one axis with the drive motor between them, and the space between the stages is an intermediate-pressure chamber machined into the body — not a vessel in the pipework. That one fact changes the piping, the oil circuit, the capacity control and the failure modes, so it deserves more of a buyer's attention than the phrase "two-stage" on its own.

This article takes the machine apart at the level a plant engineer needs: what each stage does, where the 2.18 to 1 stage ratio comes from, what the intermediate pressure chamber is for, how the slide valve unloads, where the oil goes, and where the envelope stops.

One machine, two stages. The two rotor housings share an axis; the motor sits between them rather than on a separate skid with an intercooler in between.

Key takeaways

· Two stages in one casting. The motor sits between the rotor sets and the intermediate pressure chamber is inside the body, so there is no interstage pipework and no intercooler vessel to design, insulate and leak-test.

· 2.18 to 1 is a displacement ratio, not a pressure ratio. What each stage actually sees is set by the intermediate pressure, and holding that pressure is a control problem rather than a hardware one.

· The envelope splits into two regions, and that is not a headline temperature. In Region I the machine needs liquid injection and oil cooling together; in Region II either one will do.

· The compressor arrives bare. Oil separator, oil cooler, level controller, heater, interstage controls, piping and insulation are still yours to build.

Two stages in one casting, not two machines in series

A section through the machine makes the architecture obvious in a way that a photograph cannot.

Section through the single body: low-stage rotor set on one side, high-stage rotor set on the other, motor between them, and oil routed to bearings, rotor, seal and the intermediate pressure chamber.

Both rotor sets are coupled to the same motor, in the same pressure boundary. Three consequences follow immediately.

There is no interstage pipework. On a two-machine arrangement the gas leaving the booster has to travel to the high stage through a line that is usually insulated, often large, and always a place where a joint can weep. Here that path is a machined passage a few hundred millimetres long, inside the casting.

There is no external intercooler to size. The interstage volume is fixed by the design. In practice this is the biggest difference in the plant room: the layout a two-machine system needs, with its vessel, its supports and its insulation, simply does not exist.

The motor is inside the refrigerant boundary. That is what makes the machine semi-hermetic, and it removes the shaft seal — the classic leak path on an open-drive machine. It also means the motor is cooled by suction gas, which shapes the electrical design and is the reason a motor swap means opening the circuit rather than unbolting a coupling.

What you give up is flexibility. The stage ratio is fixed when the machine is built, so a duty point that moves a long way after the order does not move with it. On a plant where the product mix is still being decided, that is worth knowing before the purchase order rather than after.

Why the stage ratio is 2.18 to 1, and why that is not a pressure ratio

The series uses a coupled two-stage design with a low-stage to high-stage displacement ratio of 2.18 to 1. Buyers routinely read that as "the pressure is split 2.18 to 1", which is not what it means.

A screw rotor pair has a fixed built-in volume ratio: the shape of the flute and the position of the discharge port decide how much the gas is compressed before it leaves. Two rotor sets of different displacement, running at the same speed, move different volumes. The displacement ratio is a statement about how much gas each stage can carry, chosen so that for the design duty the interstage pressure lands somewhere sensible for both rotors.

The pressure each stage sees is then set by that interstage pressure, and the interstage pressure depends on the whole circuit: suction, condensing, the ratio the rotors are built for, and how much gas is being injected. That is why the number on the drawing and the number in the commissioning report are allowed to differ, and why a machine that looks "correct on paper" can still run with a high stage that is doing too much work.

The symptom of a drifting interstage pressure is easy to recognise. One stage absorbs more of the lift, its discharge temperature climbs, and the motor current on that stage climbs with it. Nothing fails immediately. The oil degrades faster, the protection module starts to earn its keep, and the machine's service life quietly shortens.

The control end in section: oil pressure drives the piston, the spring returns it, CR1 to CR3 switch the piston chambers, CR4 feeds the circuit, and the piston chamber is tied to the intermediate pressure chamber through the balance line.

What the intermediate pressure chamber is actually for

The chamber between the stages is not just a gap. In this design it does three jobs, and all three are visible in the diagrams.

It gives the capacity control something to push against. The piston chamber behind the slide valve is connected to the intermediate pressure chamber through a balance pipe. That connection is what stops the valve from fighting the gas force: the piston sees intermediate pressure on one side instead of suction or discharge, so a small solenoid can move a large valve.

It is where excess oil goes. Oil that has done its work in the high stage and been separated is discharged back to the intermediate pressure chamber rather than to suction. Returning it to suction would mean dumping oil into gas the low stage then has to pump; returning it to the intermediate chamber puts it back at a pressure the circuit can accept.

It is the injection point. When the duty point falls in the region that requires liquid injection, the injection lands in the interstage volume, where it cools gas before the high stage rather than after it.

Three functions, one machined cavity, no external vessel. This is the part of the design that a two-machine layout has to reproduce with pipework, and it is the part that is hardest to explain to someone who has only seen a cascade.

Capacity control: a slide valve moved by four solenoids

The slide valve changes how much of the rotor length the gas actually passes through, and therefore how much gas the machine moves. It is driven by a hydraulic piston against a spring, and the piston is switched by solenoid valves labelled CR1 to CR4 on the schematic.

It is worth being precise about what this is and is not. It is a stepped control, not a stepless dimmer: each solenoid combination gives a defined position, and the machine unloads in steps rather than sliding continuously across the whole range. Any description of this class of machine as stepless is describing the valve's motion, not the delivered capacity.

In a freezing plant that is usually fine, and the reason is thermal rather than mechanical. A blast freezer or a cold store carries far more inertia than a compressor step, so the evaporator absorbs a step change without the suction pressure collapsing. What matters is not the smoothness of the curve but the size of the smallest step and the behaviour at low load, because that is where a machine either holds the room or starts cycling.

Three questions worth asking before the order: how many steps the control provides, what the smallest step is as a fraction of full capacity, and what the machine does below the lowest step — hold, cycle, or hand over to a second machine.

Oil: pressure-differential supply, and the half of the circuit you own

The oil circuit is where a deep-freeze screw compressor is won or lost, and it is also where most buyer specifications stop at "forced lubrication".

The feed here is by pressure differential: the oil is pushed through the circuit by the difference between discharge and the lower pressures it serves, rather than by a separate oil pump on the shaft. The section drawing shows the branches — bearing sets at both ends, rotor cooling and sealing, the coupling, the shaft seal area — each fed from the same gallery.

What a complete oil circuit looks like around the compressor. Only the machine in the middle is in the crate; everything with a number on it is a decision somebody still has to make.

The compressor carries a 12 litre charge, an oil flow switch and the internal galleries. Everything else in that diagram is plant: separator, filter, flow switch, solenoid valve, sight glass, level controller, temperature controller, heater, and the oil cooler — which is optional, and which becomes mandatory the moment the duty point runs hot.

At an evaporating temperature below minus forty, the oil is the component that decides whether the machine survives the year. Two things matter more than the grade written in the oil specification: whether the oil stays viscous enough at the coldest point it reaches, and whether it comes back from the evaporator at all. The second is a piping and defrost question as much as a compressor question, and it is the reason a plant can pass commissioning in July and starve in January.

Specify the oil grade, the heater rating and the level control philosophy in writing. If the answer to "who supplies the level controller" is "somebody will", the answer to "why is the machine tripping on oil flow" is already written.

The envelope differs by refrigerant, and splits into two regions

The series carries separate envelopes for R22 and for R404A/R507A. Both read the same way: evaporating temperature across the bottom from minus 70 to plus 5 degrees Celsius, condensing temperature up the side from 10 to 60 degrees. Inside each envelope the working area is split into two regions by a diagonal boundary.

The practical reading is a two-step check, and most enquiries only do the first step. First, does the duty point fall inside the boundary at all. Second, which region is it in — because the two regions do not carry the same equipment list. A point that sits in Region II needs one auxiliary circuit; the same point pushed up by a warmer condensing temperature can cross into Region I and need two.

That is why the condenser belongs in the same conversation as the compressor. On a plant with a seawater condenser running cool, a deep evaporating temperature may sit comfortably in Region II. Retrofit the same compressor behind an air-cooled condenser that idles at 50 degrees in summer, and the same duty point can move into Region I — same machine, different auxiliary equipment, different price, and a different commissioning conversation.

So "how low can this machine go" does not have a single answer. It has an answer at a stated condensing temperature, in a stated region, on a stated refrigerant, and the envelope is the authority rather than a headline figure.

What the rotor profile buys, and what it does not

The rotor pair uses the YZ57 profile, built from arcs and higher-order curves, with area utilisation values of 0.41 and 0.45 and a deliberately small leakage triangle.

In plain terms: more of the space between the flutes carries gas, and less of it leaks back from the high-pressure side to the low-pressure side while the rotors turn. Both effects matter most at high pressure ratio, which is exactly the deep-freeze condition — a single-stage machine at the same lift would be losing a much larger share of its swept volume to leak-back and re-expansion.

What the profile does not do is rescue a bad installation. It will not fix oil return, it will not fix a condensing temperature above the envelope, and it will not compensate for a machine selected at the wrong stage ratio. Rotor geometry sets the ceiling; the plant decides how close to it you run.

Semi-hermetic: what it removes, what it costs

Because the motor sits inside the pressure boundary, there is no shaft seal between motor and compressor. On an open-drive machine that seal is a wear item and a leak path, and on a deep-freeze plant a slow refrigerant loss is both an expense and a slow degradation of capacity that nobody notices until the room is warm.

The cost is on the service side. Motor windings see refrigerant and oil, the motor is cooled by suction gas, and a motor failure means opening the circuit rather than unbolting a coupling. For a plant that plans to run twenty years with a yearly service, that is a fair trade. For a plant that expects to swap motors on a weekend, it is not.

What arrives, and what the site still builds

The compressor arrives as a compressor. It does not arrive as a package, and the expectation that it might is the single most common misunderstanding on this class of machine — usually imported from descriptions of two-machine systems that ship with their intercooler and their skid.

In the crate: the machine, its service valves, the terminal box, the nameplate, the test record and the oil specification. On site, still to be built: the oil separator, the oil cooler if the duty point needs one, the suction and discharge piping with its supports and flexibility, the liquid injection circuit if the point is in Region I, the electrical panel and motor protection, the controls that hold the interstage pressure, the foundation, and the commissioning charge and leak test.

That list is not a complaint about the product. It is the difference between buying a compressor and buying a plant, and it is worth putting in the enquiry so that two quotes can be compared on the same scope.

Where a two-stage screw is the wrong answer

This is a machine for a specific corner of the map, and it is worth saying where the corner ends.

Above roughly minus twenty degrees evaporating, a single-stage screw with an economiser is simpler, cheaper and easier to service, and we will say so rather than sell the bigger machine. The single-stage screw range exists for exactly that band.

At small capacities, a two-stage reciprocating machine or a single-stage screw is usually the better commercial answer. Our two-stage reciprocating series covers the smaller end of deep-freeze duty, and the trade between the two architectures is set out in the screw versus piston comparison.

Where the site cannot support the oil circuit — no space for a separator, no water for a cooler, no-one to own level control — the machine will underperform and the compressor will be blamed for it.

Where the load swings across the whole range every hour, the stepped slide valve may be the wrong control. That is a conversation about the load profile, not about the compressor, and it belongs in the enquiry rather than after commissioning.

What to put in the enquiry

Seven items turn a quote into a selection:

1. Refrigerant, and whether the plant is new or existing.

2. Design evaporating and condensing temperatures, both of them, at the worst case rather than the average.

3. Required capacity at those two points.

4. Load profile — how far and how often the duty moves.

5. Cooling medium and its temperature in summer.

6. Electrical supply, voltage, frequency and starting method.

7. Who supplies the oil circuit and the controls: separator, cooler, level controller, heater, panel and interstage pressure control.

The envelope is the two-stage screw compressor range, part of the wider two-stage screw category. If you already own the machine and it is tripping rather than underperforming, the step-by-step check starts at the terminal box; and if you want the reciprocating equivalent, what is inside a semi-hermetic compressor walks through the same anatomy part by part. The YLG series was shown at CRH 2026 in Shanghai.

Conclusion

Two-stage compression in a single body is a different proposition from two machines in series, and the differences are all in the details: an intermediate pressure chamber doing three jobs, a stage ratio that is a displacement figure rather than a pressure figure, a slide valve that steps, and an oil circuit that is half machine and half plant.

None of it is exotic. All of it has to be specified, because none of it is delivered by the word "two-stage" on a quotation.

Give us the refrigerant, both design temperatures in the worst case, the capacity at those points and the cooling medium, and we will tell you which region the duty point falls in and what that adds to the scope — including when the honest answer is that a single-stage machine belongs there instead.

FAQ

Does the YLG use two compressors or one body?

One body. The two rotor sets are coupled to a single motor inside one semi-hermetic casting, with the intermediate pressure chamber machined between them. There is no separate booster, no interstage line and no external intercooler vessel.

Is 2.18 to 1 the pressure ratio between the stages?

No. It is the displacement ratio between the low-stage and high-stage rotor sets. The pressure each stage sees is set by the intermediate pressure, which depends on suction, condensing and the built-in volume ratio of the rotors — which is why holding that pressure is a control task.

Can this compressor run on ammonia?

No. The series is offered for R22 and for R404A/R507A, and the envelope is drawn for those refrigerants. Ammonia needs a different materials set, a different oil and a different safety regime; nothing in an R22 or R404A approval transfers to it.

How low an evaporating temperature can it reach?

It depends on the refrigerant and on your condensing temperature, so the answer is a point on the envelope rather than a single number. Read the evaporating temperature across the bottom, the condensing temperature up the side, and check which region the point lands in — that determines whether you need liquid injection, oil cooling, or both.

Is the capacity control stepless?

The valve motion is continuous but the delivered capacity steps, because the piston is switched by solenoid valves rather than modulated. On a freezing plant the evaporator absorbs those steps, so the practical question is the size of the smallest step, not its smoothness.

Does it ship as a skid or a packaged unit?

It ships as a compressor. The oil separator, oil cooler, level controller, heater, panel and interstage controls are plant items, and the piping between them is yours to design. If you need a package, that is a separate scope and should be quoted as one.