Annunciator Screw Compressor Loading

Overview

A screw compressor is a type of compressor that uses rotating 'screws' that continuously rotate and force air from larger to smaller air pockets. A capacity (or load) slide valve controls how much air is bypassed around the compressor, thus directly controlling load. Some compressors also have a "Vi" slider, which controls the ratio of air volume in the compressor from beginning to end of the load cycle. The annunciator can control the capacity slider, and optionally the Vi slider as well.

Screw compressor sliders are moved by energizing air solenoids that, while energized, force the slider to move one direction or another. Each slider will have two solenoids- one for each direction. If the annunciator is only controlling the load slider, then two outputs and solenoids are used (load/unload). If the annunciator is also controlling the Vi slider, than two additional outputs will be required for four total.

Optionally, the slider(s) may have a feedback sensor that gives the position of the slider as a 4-20 mA reading. The actual reading at each end stop is within the 4-20 range, but not necessarily 4 or 20 mA. This can be calibrated as described in the feedback section below.

The annunciator has the ability to load and unload a screw compressor based on sensor conditions. One - Four sensors can be used to control this loading. The sequence of operations is as follows:

1. The annunciator enters normal engine running state with the screw loading in 'auto', OR after previously running in 'manual' the 'auto' mode is selected
2. A load delay counts down
3. The annunciator enters normal load / unload loop:
4. If one or more sensors call for unload, the screw will be unloaded
5. If the primary sensor calls for load and the other sensors are in deadband, the screw will be loaded
6. Otherwise, the screw will not change
7. Once enough pulses in a row have been sent, the annunciator will stop trying to move the load valve

The primary (first) sensor is the main sensor that controls loading and unloading. Which sensor is used will depend on field requirements. For example, if the unit can run fully loaded, then the primary load sensor can be manifold pressure, with other secondary sensors only for unloading in unsafe operating conditions (e.g. low suction). As another example, if a certain suction pressure needs to be maintained then the primary load sensor will be suction, and other secondary sensors can be used to trigger other minor unload conditions.

Setup

The screw load setup is reached from the annunciator home screen by selecting "Annunciator Setup" -> "Outputs Setup" -> "Screw Loading". The setup consists of two or three pages.