Capacity control devices

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

The use of speed control and/or a recycle valve is covered in Chapter 10. Our discussion in this chapter will concentrate on cylinder inlet valve unloaders and changing cylinder clearance. 

Inlet valve unloaders are used to deactivate a cylinder end and reduce its capacity to zero. Two of the more common types of unloaders are depressor-type unloaders and plug-type unloaders. Depressor-type unloaders hold the inlet valve open during both the suction and discharge 

The compressor manufacturer must be consulted if the cylinder is to be run single acting with the frame end unloaded. Many times rod load reversal and proper lubrication may not be achieved while running single acting with the frame end unloaded. 

When a cylinder end is deactivated, the pulsation levels in the piping system can increase significantly. If a cylinder may be operated with 

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Clearance pockets and other types of capacity control devices are available to suit any application. 

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). 

The electric arc furnace process accounted for about 25% of the 1982 U.S. steelmaking capacity (14). Most of the raw material used for the process is steel scrap. Pollutants generated by the electric furnace process are primarily particulate matter and CO. The furnaces are hooded, and the gas stream containing the particulate matter is collected, cooled, and passed to a bag-house for cleaning. Venturi scrubbers and ESPs are used as control devices at some mills. Charging and tapping emissions are also collected by hoods and ducted to the particulate matter control device. 

A refrigeration system will be designed to have a maximum duty to balance a calculated maximum load, and for much of its life may work at some lower load. Such variations require capacity reduction devices, originally by speed control (when steam driven) or in the form of bypass ports in the cylinder walls. 

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. 

Investment costs in electrodialysis with bipolar membranes Investment costs include nondepreciable items such as land and depreciable items such as the electrodialysis stacks, pumps, electrical equipment, and monitoring and control devices. The investment costs are determined mainly by the required membrane area for a certain plant capacity. The required membrane area for a given capacity plant can be calculated from the current density in a stack that is in electrodialysis with a bipolar membrane not limited by concentration-polarization effects. The required membrane area for a given plant capacity is given by ... 

Table I summarizes the locations, capacities, and pollution-control devices of the coal-fired plants included in the library. In the library each data set includes a brief text giving details of the plant s characteristics, the sampling method and date, the analytical techniques used, and any other useful details.

Plant Location Poll. Control Device Capacity (MM) Study Type3... 

Next consider company plans for supplies of low sulfur fuels to estimate the order of magnitude of added supplies of low sulfur fuels that would be required in 1975 if control devices were not in operation at the electric generating units. The National Coal Policy Conference in 1971 estimated that there would be 300 million tons of new mine capacity by 1975, of which 75 million tons would be in low sulfur coal. Assume two-thirds or 50 million tons could be dedicated to the electric utilities. This quantity added to the 45 million tons of low sulfur coal, which already meets the standards, yields 94 million tons of naturally occurring low sulfur coal that could be available in 1975. Subtracting 94 million tons from the total requirement of 425 million tons leaves 331 million tons of high sulfur coal which will be burned in utilities with devices. Or in the absence of devices, this quantity of coal must be processed to low sulfur standards. 

The experience reported above represents 95-97% sulfur to sulfuric acid conversion efficiencies. Sulfuric acid plants of 200 tonne/day capacity are common and some large singe-train plants are now operating on the scale of 4,000 tonne/day [17], so even these high conversion efficiencies are not enough to avoid local emission problems. Thus, the U.K. has a requirement of 99.5% containment of the sulfur burned as a feedstock, while the U.S.A. s 99.7% sulfur dioxide to sulfur trioxide conversion efficiency [56]. With modern process modifications and emission control devices in place, these requirements are being met. 

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