I am looking at an ammonia refrigeration unit and trying to understand how loading and unloading solenoid valves work? and also how to modulate the compressor motor over a period of time.


2 Answers 2


Many compressors are driven by induction motors for efficiency, which means that they do not have the starting torque required to get going against the full back pressure of the system. This is true of both air compressors and refrigeration compressors.

Air compressors usually have a check valve between the compressor itself and the reservoir tank and an automatic valve that vents the line between the compressor and the check valve to atmosphere whenever the compressor is stopped.

In small refrigerators, the system is simple enough that the pressure equalizes through the expansion valve within a few seconds or minutes of the compressor shutting off without any other intervention. You just have to make sure that you don't try to restart the compressor too soon after its last shutdown.

In larger refrigeration systems, explicit solenoid valves are used to isolate the compressor from the rest of the system and to equalize the pressure between input and output so that it can start up.

With regard to "modulation", nearly all refrigeration systems use "bang-bang" controllers — either full on or full off, with the duty cycle modified as needed in order to maintain a particular temperature. It's rare to need anything more sophisticated than that.


Load/Unload and Modulation are different control schemes for rotary-screw compressors (either for air, ammonia or other gases). Both methods use a slider valve to reduce the capacity of the compressor.

Modulation controls this slider valve to maintain a specific system pressure, it is however very inefficient when compared with modern VFD controls. Load/Unload uses the same slider valve, but operates with the valve all the way closed or all the way open. In this fashion, the compressor is operating at its maximum effiency when loaded, and wasting less power than when it is idle.

I am actually having a contractor convert one of our 350HP Quincy air compressors from modulation to load/unload on Monday. The air system already has VFD compressors for keeping the pressure stable. Switching the compressor over to load/unload will increase its overall efficiency.

Load/unload is much better than start/stop. While it does use more power, it greatly reduces wear and tear on the compressor and supporting electrical components.

This image shows the slider vavle (Image Source):
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The rotary-screw wiki article explains load/unload and modulation operation:

In a load/unload control scheme, the compressor remains continuously powered. However, when the demand for compressed air is satisfied or reduced, instead of disconnecting power to the compressor, a device known as a slide valve is activated. This device uncovers part of the rotor and proportionately reduces capacity of the machine down to typically 25% of the compressors capability, thereby unloading the compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air-compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual-control or auto-dual scheme.

Instead of starting and stopping the compressor, a slide valve as described above modulates capacity to the demand. While this yields a consistent discharge pressure over a wide range of demand, overall power consumption may be higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition.
Due to the limited adjustment in compressor power consumption relative to compressed-air output capacity, modulation is a generally inefficient method of control when compared to variable-speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal-combustion engine and operated without the presence of a compressed-air receiver), modulation is suitable.

This Youtube video shows the slider valve in operation.


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