Screw Compressor Economizers and Part-Load Efficiency: A Used-Buyer’s Guide

Two used screw compressors can carry the same nameplate capacity and the same asking price and still deliver very different operating economics once installed. The difference usually comes down to how the machine handles part load and whether its economizer is actually doing useful work across the range the plant runs at. Most industrial refrigeration systems spend the majority of their hours at part load, not full load, so this is where the real energy bill is decided. This guide explains what a screw compressor economizer is, why part-load control determines whether it pays off, and what to check on a used package before you buy.

What an economizer actually does

In a screw compressor, the gas is compressed continuously as the rotors mesh, and the pressure rises progressively from the suction end toward the discharge end. Because any pressure between suction and discharge exists somewhere along the rotors, a port can be located at an optimized intermediate point in the casing. That port is the economizer, sometimes called a side port. It functions as a secondary suction inlet into which additional gas can be drawn at that intermediate pressure.

The gas comes from a subcooling device, either a flash economizer vessel or a shell-and-coil subcooler. Liquid refrigerant on its way to the evaporator is subcooled there, and the flash gas produced in the process is routed to the economizer port rather than back to the main suction. Subcooling the liquid increases the refrigeration effect per pound at the evaporator, which raises capacity, and because the flash gas re-enters partway up the compression process instead of at full suction pressure, the efficiency gain comes at a lower energy cost than compressing that gas from suction would. The net effect is a single-stage screw compressor achieving something close to the benefit of two-stage compression from one machine.

Shell-and-coil subcoolers are slightly less efficient than flash-type economizers, because the liquid leaving the coil has to stay above the flashed liquid temperature that cools it. They are, however, well suited to systems with remote or elevated evaporators, since liquid can be supplied to those evaporators at essentially condensing pressure. Knowing which arrangement a used package uses tells you something about the system it came from and how it will behave in yours.

Why part-load control decides whether the economizer pays off

This is the part that separates a good used-screw purchase from a disappointing one. The economizer’s benefit is tied to how the compressor unloads.

Slide valve unloading

Nearly all rotary screw compressors use a slide valve to unload. The slide valve moves along the length of the rotors and shortens the effective compression length, which reduces capacity. It is infinitely adjustable and gives reasonable suction pressure control, typically down to around 25 percent of full capacity. The drawback is that unloading with a slide valve carries a power penalty: as the machine unloads, power does not fall in proportion to capacity, so part-load efficiency degrades. It gets worse at higher compression ratios, meaning lower suction or higher discharge pressures make the penalty larger.

The economizer interacts badly with slide-valve unloading. As the slide moves to unload, it eventually passes the economizer port’s position along the rotors, and the port loses its intermediate-pressure relationship. In practice, economized screw compressors typically lose economizer operation at roughly 75 percent slide position, and below that point the machine runs non-economized. So a compressor that shows an attractive full-load efficiency number can be running without any economizer benefit for much of its actual operating life, because much of that life is spent below 75 percent load. Some designs mitigate this by making the side port part of the slide valve mechanism so the port moves with the slide, but that is a design feature you have to confirm, not assume.

Variable-frequency drive control

A VFD changes the picture. When capacity is regulated by varying compressor speed instead of moving the slide, the slide can be held in the fully loaded position while the drive reduces speed to match the load. Because the slide stays put, the economizer port keeps its intermediate-pressure relationship to suction, and the economizer stays effective across the load range rather than dropping out at part load. That is why VFD-equipped screw packages can hold their efficiency advantage down to low load, while slide-valve-only machines lose it. Most screw compressors can run down to roughly 50 percent speed as rated by the factory, and combining VFD speed control with slide-valve trim for the very bottom of the range is a common approach.

For a used buyer, the takeaway is direct: a screw package that includes a working VFD is worth materially more in real operating cost than the same compressor with slide-valve control only, especially if your plant spends most of its hours at part load. If the VFD is not included, budget for one and factor that into the comparison.

What to inspect on a used economized screw package

Beyond the standard used-compressor checks of operating hours, last overhaul, oil analysis history, and motor and starter condition, an economized screw package has specific items that determine whether the efficiency you are paying for is actually there.

• Economizer port and piping. Confirm the port exists and is plumbed to a subcooler or flash economizer, and identify whether it is a flash type or a shell-and-coil subcooler. A port that was capped or never connected means you are buying a non-economized machine regardless of the model designation.
• Slide valve and actuator. Check that the slide valve moves through its full range and that the hydraulic actuator and its oil supply are functional. A sticking or worn slide valve undermines both capacity control and, on designs where the port moves with the slide, economizer behavior.
• Capacity control type. Establish whether the machine relies on slide-valve unloading only, or includes VFD speed control. This single fact drives the part-load efficiency you can expect.
• Variable Vi capability. Some screw compressors offer a variable volume ratio that can be adjusted with loading. Where present and functional, it extends the efficient operating range. Confirm whether the used unit has it and whether it still operates.
• VFD condition and rating. If a drive is included, verify its rating matches the motor, check its condition, and confirm it is suitable for the installation. A drive that needs replacement changes the economics of the deal.
• Subcooler vessel condition. If the package includes its economizer vessel, inspect it as you would any pressure vessel, and confirm the ASME documentation.

Why part-load hours dominate the energy bill

It is worth being concrete about why this matters so much financially. Industrial refrigeration load is rarely constant. It rises and falls with ambient conditions, production schedules, product pull-down, and door openings, and the design peak that sets the compressor size occurs only during a small fraction of the year. The rest of the time, which is most of the time, the plant runs below design load. That means the part-load region is where the compressor spends the overwhelming majority of its operating hours, and therefore where it consumes the overwhelming majority of its lifetime energy.

A compressor selected or bought purely on its full-load efficiency number is optimized for the condition it will rarely see. Two machines that match at full load can diverge sharply once you weight their performance by the hours actually spent at each load point. This is precisely why the economizer’s part-load behavior and the presence of a VFD are not fine print. They are the terms that determine what the machine costs to run over its life, and on a used purchase they are terms you can verify before you buy rather than discover after installation.

Matching the machine to your duty

The economizer earns more at higher compression ratios, so it is most valuable on low-temperature duty where the lift between suction and discharge is large. If you are buying for a high-temperature application with a modest lift, the economizer benefit is smaller and the part-load control question is correspondingly less critical. Conversely, for low-temperature cold storage or freezing duty that runs long hours at part load, a used economized screw package with working VFD control is often the single most cost-effective way to add efficient capacity. Define your suction and discharge conditions and your expected load profile first, then evaluate used candidates against that duty rather than against nameplate capacity alone.

Talk to Refrigeration Equipment Pros

Refrigeration Equipment Pros supplies surplus and used industrial refrigeration equipment to buyers across North America, Latin America, and the Caribbean. Browse current inventory at refrigerationequipment.net, and if you are decommissioning a plant or have surplus units to move, visit our Sell To Us page. To discuss a specific requirement with our team, call or text 201-805-1441.

Sources

• ‘Using Screw Compressors with Economizers to Improve Efficiency’ – IIAR Condenser (iiarcondenser.org).
• ‘Using VFDs in Refrigeration and Cold Storage Applications’ – Danfoss (danfoss.com): economizer operation lost near 75% slide position; operation to ~50% speed.
• ‘A Refrigeration Screw Compressor Package’ – Enerflex technical paper (flash vs shell-and-coil subcooler tradeoff).

Screw compressor capacity-control and vapor-injection technical literature (Purdue ICEC proceedings; ScienceDirect) for slide-valve range and part-load behavior.