【Q&A Question 041】May 31, 2016

Adjustment of rotational speed, adjustment of clearance chamber, adjustment of bypass, adjustment by pressing the intake valve

1. Load regulation； 2. Bypass adjustment ； 3. Stepless adjustment ； 4. Clearance adjustment

1. Speed adjustment 2. Clearance chamber adjustment 3. Bypass adjustment 4. Forced intake valve adjustment

One reverse one, two reverses one for return adjustment

For volume control in reciprocating compressors, methods such as backflow regulation, adjustment of the inlet valve (usually at 25%, 50%, 75%, 100%), clearance adjustment, and HOERBIGER’s stepless volume control system (HydrCOM) are commonly used.

Typically, compressor users select a compressor based on the maximum volumetric flow rate required by the device or system. However, the actual operating conditions of the compressor change according to the requirements of the process flow or the gas-consuming equipment. When the gas consumption is less than the compressor’s exhaust volume, it is necessary to adjust the compressor’s gas output so that its exhaust volume meets the requirements of the gas consumption, while maintaining stable pressure in the pipeline system. The common methods for adjusting air volume in reciprocating compressors include the following: speed control. Speed control involves adjusting the exhaust volume by changing the speed of the compressor. The advantages of this type of regulation are continuous gas flow, lower specific power consumption, constant pressure ratios across all stages of the compressor, and no need for specialized control mechanisms on the compressor ； However, it is only widely used in compressors driven by internal combustion engines and turbines. If the drive is an electric motor, an inverter is required. Due to the high cost of high-power, high-voltage inverters as well as the need for extensive maintenance and repair work, this method is rarely used in reciprocating compressors driven by electric motors at present. Furthermore, variable speed control may have adverse effects on the operation of the compressor, such as valve flutter, increased component wear, heightened vibration, and inadequate lubrication, which also limits the widespread use of this method. Clearance chamber adjustment: On the compressor’s cylinders, in addition to the fixed clearance volume, there is an additional chamber. During adjustment, this chamber is connected to the working chamber of the cylinder, thereby increasing the clearance volume, reducing the volume coefficient, and lowering the displacement. This is the principle behind clearance chamber adjustment. Depending on the method of connecting to the auxiliary volume, it is further divided into continuous, stepped, or intermittent regulation methods, which are commonly used in large-scale process compressors. The main drawback of this adjustment method is that it is usually done manually and has a slow response time; it generally needs to be used in combination with other adjustment methods. Although the method of connecting variable compensation clearance volumes can in principle achieve adjustment within the 0% to 100% range, the system has poor reliability, many vulnerable components, and is difficult to maintain. Bypass regulation: The exhaust pipe is connected to the intake pipe through a bypass pipeline and a bypass valve; to carry out regulation, simply open the bypass valve, and part of the exhaust gas will return to the intake pipe. This adjustment method is relatively flexible and simple to implement; together with an automatic control system, it also offers high precision. However, since all the compression work required to compress the excess gas is lost, its economic efficiency is low. Therefore, this method is suitable for occasional adjustments or those involving small amplitude adjustments. Adjustment by pressing the intake valve: Based on the length of time the intake valve is pressed, this method is divided into two types – full-stroke pressing of the intake valve and partial-stroke pressing of the intake valve. In the case of full-stroke opening of the intake valve for adjustment, during the intake phase, gas is drawn into the cylinder; during the compression phase, with the intake valve fully open, the gas that was drawn in is pushed back out of the cylinder. Assuming a compressor has a single-stage double-acting cylinder, if the intake valve on only one side of the piston is opened, the air volume is reduced by 50%. If both sides are opened at the same time, the exhaust volume becomes zero. Therefore, this compressor can achieve three levels of regulation: 0%, 50%, and 100% of the original air volume. It can be seen that the adjustment range for fully pressing open the intake valve throughout the entire stroke is large, making it suitable for rough adjustment. The principle of adjusting by partially opening the intake valve is similar to that of fully opening it, but it controls the amount of return gas by regulating the timing at which the intake valve closes during compression, thereby enabling continuous adjustment of the gas volume. Since the compression work decreases almost proportionally to the exhaust volume, it also offers high operational efficiency.

Adjust load, activate cross-line function, stepless adjustment, clearance adjustment

1. Inverter adjustment, 2. Outlet bypass adjustment

Unloader, bypass circuit, stepless air volume adjustment, clearance chamber adjustment, adjustable speed.

　　1. Speed control Speed control involves adjusting the exhaust volume by changing the speed of the compressor. The advantages of this type of regulation include a constant air flow rate, lower power consumption, unchanged pressure ratios across the various stages of the compressor, and no need for specialized control mechanisms on the compressor. However, it is only widely used in compressors driven by internal combustion engines or steam turbines. If the drive is an electric motor, then an inverter is required. Due to the high cost of high-power, high-voltage inverters, as well as the extensive maintenance needed, this method is rarely used in reciprocating compressors driven by electric motors. Furthermore, variable speed regulation may have adverse effects on the operation of the compressor, such as valve flutter, increased component wear, heightened vibration, and insufficient lubrication, which also limits the widespread use of this method. 　　2. Clearance chamber adjustment: On the compressor’s cylinders, in addition to the fixed clearance volume, there is another cavity. During adjustment, this cavity is connected to the working chamber of the cylinder, thereby increasing the clearance volume, reducing the volume coefficient, and lowering the exhaust volume. This is the principle behind clearance chamber adjustment. Depending on the method of connecting to the auxiliary volume, it is further divided into continuous, stepped, and intermittent regulation methods, which are commonly used in large-scale process compressors. The main drawback of this adjustment method is that it is usually done manually and has a slow response time; it generally needs to be used in combination with other adjustment methods. Although the method of connecting variable compensating clearance volumes can in principle achieve adjustment within the 0% to 100% range, the system has poor reliability, many vulnerable components, and is difficult to maintain. 　　3. Bypass regulation: The exhaust pipe is connected to the intake pipe through a bypass pipeline and a bypass valve; to adjust it, simply open the bypass valve, and part of the exhaust gas will return to the intake pipeline. This adjustment method is relatively flexible and easy to implement; together with an automatic control system, it also offers high precision. However, since all the compression work required to compress the excess gas is lost, its economic efficiency is low. Therefore, this method is suitable for occasional adjustments or those involving small amplitude adjustments. 　　4. Adjusting by pressing the intake valve 　　Based on the length of time the intake valve is pressed, this method is divided into two types: fully pressing the intake valve throughout its stroke and partially pressing it during part of its stroke. In the case of full-stroke opening of the intake valve for adjustment, during the intake phase, gas is drawn into the cylinder; during the compression phase, with the intake valve fully open, the gas that has been drawn in is expelled from the cylinder. Assuming a compressor has a single-stage double-acting cylinder, if the intake valve on only one side of the piston is opened, the air volume is reduced by 50%. If both sides are opened simultaneously, the exhaust volume becomes zero. Therefore, this compressor can achieve three levels of regulation for air volume: 0%, 50%, and 100%. It can be seen that the adjustment range for fully pressing open the intake valve throughout the entire stroke is large, making it suitable for rough adjustment. The principle of adjusting by partially opening the intake valve is similar to that of fully opening it; however, by controlling the timing at which the intake valve closes during the compression stroke, it is possible to regulate the amount of gas returning, thus enabling continuous adjustment of the gas flow. Since the compression work decreases almost proportionally to the volume of exhaust gas, this approach also offers high operational efficiency.