Every throttling valve in a refrigeration plant throws pressure energy away as heat. The fluid enters at high pressure, leaves at low, and the difference is dissipated into the liquid instead of being captured. A screw expander is the one piece of equipment that captures it: the refrigerant expands inside a rotating twin-screw machine, and the machine turns the pressure drop into shaft power that can be generated as electricity. On MOON-TECH’s oil injection screw expander line the recovery envelope runs from 50 to 1,250 kW at inlet pressures of 0.4 to 5.0 MPa — the band that covers most medium and large ammonia and hydrocarbon plants.
The size of the prize is easy to overstate, so the thermodynamics come before the hardware.
How much pressure energy is actually in the line
Throttling is isenthalpic: the fluid’s enthalpy does not change across the valve, but the work potential of the pressure difference is lost. That work potential is bounded by an isentropic expansion, and in most refrigeration circuits it is on the order of 1–2% of the total compressor power. A real expander converts a portion of that limit, set by its isentropic efficiency at your operating point — which is why the vendor’s curve at your pressures matters more than the nameplate.
The practical reading: pressure energy recovery is not a step-change saving, it is a permanent offset against the largest power bill in the plant. It scales with the pressure differential and the mass flow — high condensing pressure, deep evaporation and large flow all raise the recoverable amount — and it is earned every hour the plant runs, with no consumables.
Why a screw machine, specifically
Only positive-displacement expansion turns a pressure drop into shaft work; every other arrangement converts it into fluid friction. The twin-screw type is the workhorse for industrial duty: two intermeshing rotors move the refrigerant continuously, there are no suction or discharge valves to wear, and the machine rides partial load without the pulsation a reciprocating design would produce.
Oil injection does three jobs inside the machine — sealing the rotor mesh, cooling the expansion and lubricating the bearings — and the same oil leaves with the discharge gas and returns to the compressor’s oil system. That oil loop is the part of an expander installation that most first-time specifications get wrong, and it is why the checks below all touch it.
When recovery pays
Run the arithmetic on your own operating data before asking for a quote. The recoverable power follows the pressure differential across the expansion point, the mass flow, and the expander’s isentropic efficiency at that point — not the nameplate maximum. Three situations where the case is weakest: a small differential between condensing and evaporation pressure; a plant that runs at partial load for most of the year, because efficiency dips away from the rated point; and a site where the generated power has nowhere to go, so the capital is paid for an offset that only shows up in theory.
The opposite picture is where the case is strongest: a high-differential ammonia or hydrocarbon plant on continuous duty, with grid power expensive enough that a few percent of compressor power paid back inside a few years is an easy approval. The 50–1,250 kW output range of the oil injection screw expander exists for exactly that band — small enough for a standard machine room, large enough to matter on the electricity bill.
Four checks to run on site
- Inlet state. The expander wants a defined two-phase inlet. A flash fraction that is too high — for example after a liquid surge upstream — hits the rotors with liquid, and efficiency and bearing life both pay for it. Verify the liquid level and pressure at the expander inlet under real load swings, not just the design point.
- Oil return. The oil film on the rotors leaves the machine with the discharge gas. Confirm the separator and the return line get it back to the compressor loop at the rate it leaves; slow oil return is the most common reason an expander’s efficiency drifts down over its first year.
- Load range. Ask for the efficiency curve across 40–100% of flow, not a single rated point, and weight it against the plant’s real load profile.
- Mounting and access. These are rotating machines with bearings that will be serviced. The foundation, the acoustic environment of the machine room and the physical access for rotor work should all be checked before the position is fixed, because moving the unit later means re-doing the inlet and outlet lines.
Where expanders are already running
The oil injection screw expander is built for the oil, gas and chemical sector, where both the pressure differentials and the duty hours are at their highest. It belongs to the same pressure-energy-recovery family as MOON-TECH’s heat exchange and waste-steam lines — the common thread is that an industrial plant already pays for its energy, and the question is how much of it comes back.
Questions plant managers ask
How much power can we actually expect to recover?
Start from the thermodynamic limit — in most refrigeration circuits the pressure energy is on the order of 1–2% of the total compressor power — then apply the expander’s isentropic efficiency at your operating point. Ask the vendor for the expected kilowatts at your actual pressure differential and flow, not the 1,250 kW nameplate ceiling.
Does the expander replace the expansion valve?
In a full-recovery configuration, yes — the expander takes the whole pressure drop and the downstream heat exchanger is re-sized for the new operating point. In a partial configuration a small throttling device stays downstream to take the residual drop. The choice changes the system’s operating point, so it is made at system level, not at machine level.
Will it work with our refrigerant?
Screw expanders run on ammonia and on hydrocarbons. The deciding factors are the pressure differential, the mass flow and oil compatibility at your operating conditions — not the refrigerant name by itself. The first engineering step is a compatibility check between your cycle data and the machine’s oil system.
Sizing a recovery unit?
Send the condensing and evaporation pressures plus the compressor power. The engineering team responds with the expected recovery at your operating point.