13.3 Screw Slide-Valve Unloading and Feedback
Key Takeaways
- Screw capacity control is continuous: a slide valve opens a bypass slot back to suction, and CIRO screens show that position as a percent where 100% means full slot (bypass covered, full displacement).
- Unloaded screws still need oil injection and oil cooling; low slide is not an idle state you can run without proving oil pressure and cooler flow.
- At low slide percent, internal bypass often makes kW/ton worse even though amps look nicely low—shed a machine rather than park several at 25–40%.
- High amps at 100% slide with a dirty condenser, high approach, or high suction is expected load, not a slide fault.
- Stuck-slide failures move neither amps nor suction when commanded; a false transmitter lets capacity change while the percent on the screen does not match amps—prove all three: command, feedback, and kW.
13.3 Screw Slide-Valve Unloading and Feedback
Reciprocating compressors unload in steps. Industrial screws unload continuously with a slide valve that uncovers a bypass slot back to suction, shortening how much of the rotor length is actually compressing. On CIRO operating screens the position is a percent. 100% means full slot—the bypass is covered and the machine is using its full displacement. Lower percentages open the slot. That single idea—percent is position, not a promise of proportional kW or proportional tons—is the core of this section.
What the slide is doing
Male and female rotors mesh and trap a volume of suction gas. The slide lives along the rotors. Pulled toward unload, it opens a path so part of the trapped gas returns to suction before the discharge port. Capacity falls smoothly. Hydraulic pistons, solenoids, and a 4–20 mA (or similar) position transmitter move and report that motion. Some packages also have a separate volume-index (Vi) adjustment to match built-in volume ratio to the actual pressure ratio. Do not confuse Vi with capacity percent. CIRO screens that show one number labeled slide or capacity are talking about the capacity slot, not Vi.
Because unloading is continuous, the controller can trim to a suction setpoint without the suction bounce of 25% recip steps. That is an advantage only if the feedback is true and the machine is not parked for hours at a miserable part-load efficiency.
Reading percent on the screen
Treat slide % as a field device reading you must cross-check against operating screens:
- Commanded position versus feedback %
- Feedback versus motor amps or kW at the suction and discharge on the same screen
- Suction trend after a step change in command
100% = full slot. If the screen says 100% and the motor is at the amp value you expect for those pressures, the slide is likely there. If the screen says 100% and amps are an unloaded trickle, the transmitter is lying high or the slide is stuck unloaded. If the screen says 40% and amps look like full load at those pressures, the slide is stuck loaded or the transmitter is lying low.
Never convert % to tons with a straight line. Internal bypass means mass flow and shaft power do not track percent one-for-one, especially below mid-range.
| Screen / field | Healthy full-slot picture | Unhealthy mismatch |
|---|---|---|
| Slide feedback | Near 100% when the plant needs all of this machine | 100% displayed while amps look unloaded, or 30% displayed while amps look like FLA at those P's |
| Motor amps / kW | Track the manufacturer's point at this suction and discharge | High amps blamed on the slide while SCT vs wet bulb is 25°F of approach |
| Suction | Moves when you step the command 15–20% | Command changes, suction and kW do not (stuck); or kW moves and % does not (transmitter) |
Unloaded screws still need oil
Oil on a screw is sealing, lubrication, and cooling. Unloading does not mean the machine is idling, skip oil. Injection ports still need oil. Bearings still need pressure. Thermosiphon loops still need a driving head (see floating head and thermosiphon oil coolers). Liquid-injection oil cooling still needs liquid at the injection valve. Shaft-driven or dedicated oil pumps must prove pressure before load; an unloaded machine that loses oil pressure is still a wreck.
At very low slide, discharge temperature and oil temperature can do unexpected things: less mass flow to carry heat, more internal leakage, more recirculated gas. Packages publish a minimum slide or minimum load. Running below it to avoid starting the lag machine can be worse than a clean start-stop with timers. Oil cooling load is not zero at 20% slide.
Part-load efficiency and bypass
The gas that slips back through the slot is compressed at least part way and then re-ingested. That bypass penalty is why kW/ton often worsens as slide % falls even though the amp needle looks nicely low. A plant with three screws at 30% is frequently less efficient than two at 70% or one at 90% plus a stopped lag machine. This is the efficiency link to sequencing: unload-and-stop beats forever-unloaded.
VFDs change the story. Slowing the motor reduces displacement without opening the same bypass slot, so part-load kW/ton is often better on speed control than on slide alone. Many packages use both: slide for a band, VFD for another. CIRO will still expect you to know that low slide % is not efficient operation just because current is low.
Do not call high amps at 100% a slide fault if the condenser is dirty
Full slot plus high condensing pressure is supposed to make high amps. Mass flow at a given suction is still large, and the pressure ratio is high. A dirty condenser, failed fan, scaled coil, or non-condensables will show: high SCT versus wet bulb, large approach, high amps, slide 100%. Rebuilding the slide transmitter will not clean the condenser.
Diagnostic order when amps look too high:
- Confirm slide feedback and a local position indication if present.
- Read condenser approach and fan %.
- Compare amps to the manufacturer's point at this suction and this discharge, not to FLA in the abstract. FLA is a motor limit, not the expected load at 20°F suction and 70°F condensing.
- Only then suspect slide mechanics.
High suction (warm rooms, post-defrost) also raises amps at 100%. That is load, not a slide.
Stuck slide versus false transmitter
Stuck slide (mechanical): hydraulic solenoid failed, piston seals bypassing, slide gummed, linkage broken. Command changes, amps and suction do not. Sometimes the actuator stem moves and the rotor slot does not; sometimes nothing moves. Local flags, hydraulic pressure gauges, and a flashlight on the indicator rod beat a single HMI number.
False transmitter (instrument): the slide moves and capacity changes, but the screen % is wrong—loose coupling, wrong scale (4 mA mapped to 0% versus 10%), damaged magnet or LVDT. Clue: amps and suction respond to a command change, but % does not match amps. Another clue: % jumps, amps do not.
A simple prove-out: step the command 20%, watch three things for several minutes: feedback %, motor kW, suction. All three should move in a consistent direction. Then reverse the step. If kW and suction move and % does not, chase the transmitter. If nothing moves, chase hydraulics and the slide itself. If % moves, kW is high, and approach is 25°F, chase the condenser.
Log the as-found: commanded %, feedback %, amps, suction psig, discharge psig, oil temperature. That set is what a CIRO-style item is really asking you to interpret.
On a CIRO-style screw package screen, the slide-valve reading is 100%. What does that position mean?
Three identical screws have been sitting at about 30% slide for hours on a light winter load. Why is kW/ton often worse than stopping two machines and running one near full slot?
A 300 HP thermosiphon screw shows slide 100%, motor amps near FLA, SCT 25°F above wet bulb, and condenser fans already at 100%. The night operator wants to rebuild the slide transmitter. What should you do first?
You command a screw from 80% toward 50%. Motor kW and suction both move as expected, but the HMI slide percent stays at 81%. What is the most likely diagnosis?