Compressors capacity control devices

Most packaged water chillers are large enough to have capacity control devices in the compressor. The main control thermostat will unload the compressor as the water temperature approaches a lower safe limit, so as to keep the water as cold as possible without risk of freeze damage. 

When no capacity control or unloading device is provided, it is nec-essaiy to provide bypasses between the inlet and discharge in order that the compressor can be started against no load (see Fig. 10-93). 

Control Devices In many installations the use of gas is intermittent, and some means of controlling the output of the compressor is therefore necessary. In other cases constant output is required despite variations in discharge pressure, and the control device must operate to maintain a constant compressor speed. Compressor capacity, speed, or pressure may be varied in accordance with requirements. The nature of the control device will depend on the function to be regulated. Regulation of pressure, volume, temperature, or some other factor determines the type of regulation required and the type of the compressor driver. 

Reciprocating compressors (for air supply to air-diy a plant of small capacity) these are to be procured when substantially constant low volumetric capacity is required at higher discharge pressures. Vendor may be asked to indicate the type of capacity control being provided (variable frequency drive or by aity other means) as well as pulsation suppression device for the discharge side. This may be in the form of dampening pots. 

To ensure a more reliable temperature control and the formation of a cylindrical layer of cold gas in each section of the reactor, several collectors for its introduction can be mounted, with thermocouple 7 and control device 8 ahead of each to adjust the volume of supplied cold gas. As the cold gas, the discharge gas or initial natural gas or any combination thereof can be used. Furfhermore, the oxidation by-products can be pumped from the vat of column 11 by pump 12 into mixer 14, from which, along with the circulation gas (boosted by compressor 13) or initial gas, they move in the cooling gas flow to final oxidation. Making use of the method for temperature control of the process by supplying additional amount of cold gas, the AMTEK-engineering company has developed a technical project of a plant with a capacity of 5000 tons of methanol per year. 

A compressor is typically a specially designed device, and comes with far less surplus capacity than other process components. As a result compressors merit great care in specification of flow, inlet pressure, and discharge pressure. Similarly, the control system and equipment need to be carefully matched to provide turndown with maximum efficiency. 

Induction motors are the most frequent in use because of their simple and rugged construction, and simple installation and control. They are constant speed devices available as 3600 (two-pole), 1800, 1200, and 900 rpm (eight-pole). Two speed models with special windings with 2 1 speed ratios are sometimes used with agitators, centrifugal pumps and compressors and fans for air coolers and cooling towers. Capacities up to 20,000 HP are made. With speed... 

UNLOADER - A device in or on the compressor for equalizing high-side and low-side pressures for a brief time during starting and for controlling compressor capacity by rendering one or more cylinders ineffective. 

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