OBJETIVOS ESTRATÉGICOS Objetivo 1:

2.5.2.1 Pressure relief

The original method for regulating compressors was to use a pressure relief valve to release excess air pressure into the atmosphere. The valve in its simplest design can be spring-loaded, whereby the spring tension determines the final pressure. Frequently a servo-valve controlled by a regulator is used instead. The pressure can then be easily controlled and the valve can also act as an off-load- ing valve when starting a compressor under pres- sure. Pressure relief creates a significant energy requirement, as the compressor must work con- tinuously against full counterpressure.

A variant, which is used on smaller compressors, is to unload the compressor by fully opening the valve so that the compressor works against atmo- spheric pressure. Power consumption is signifi- cantly lower using this variant method.

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tla s C op co A ir po w er N V, B el gi um , 2 01 5 2.5.2.5 Start/stop

Compressors below 5–10 kW are often controlled by completely stopping the electric motor when the pressure reaches an upper limit value and by restarting it when the pressure drops below the lower limit value. This method demands a large system buffer volume or large pressure differ- ence between the upper and lower limits, in order to minimize the heat load on the electric motor. This is an energy-efficient and effective regulation method, provided the number of starts is kept low.

2.5.2.6 Speed regulation

A combustion engine, gas turbine or frequency- controlled electric motor controls the compres- sor’s speed and, consequently, the flow rate. It is an efficient method for maintaining a steady outgoing pressure and lower energy consumption.

The regulation range varies with the type of compressor,and is largest for liquid-injected com- often used on process compressors where the gas

is unsuitable or too valuable to be released into the atmosphere.

2.5.2.4 Pressure relief with throttled inlet

This is the most common regulation method cur- rently in use. It combines a maximum regulation range (0-100%) with low energy consumption: only 15–30% of full load power with an off-loaded compressor (zero flow). The inlet valve is closed, but with a small opening used at the same time a blow-off valve opens and releases the discharge air from the compressor.

The compressor element therefore works with a vacuum in the inlet and low counterpressure. It is important that pressure relief be carried out

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2.5.2.3 Throttling the inlet

Throttling is a simple method to reduce flow by increasing the pressure ratio across the compres- sor, according to the induced under-pressure in the inlet. This method is, however, limited to a small regulation range. Liquid-injected compressors, which can overcome such a high pressure ratio, can be regulated down to 10% of maximum capac- ity. The throttling method creates a relatively high energy requirement, due to the high pressure ratio.

ference between loading and off-loading pressure limits and by the permitted number of unloading cycles per hour.

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Off-loading device for a piston compressor.

pressors. Frequently, speed regulation is combined with start-stop at low degrees of loading and pres- sure relief at standstill.

2.5.2.8 Suction valve unloading

Piston compressors can be effectively regulated by mechanically forcing the inlet valves into the open position. As a result, air is pumped out and into the cylinder with minimal energy loss that is often lower than 10% of the full-load shaft power. Double-acting piston compressors generally pro- vide multi-stage off-loading, whereby one cylinder at a time is off-loaded to better match the flow rate to the demand. An odd method used for process compressors is to allow the inlet valve to be open during a smaller or greater part of the piston stroke to provide nearly continuous flow rate control.

2.5.2.9 Load–unload–stop

This is the most common regulation method used for compressors with greater than 5 kW capacity, and it combines a large regulation range with low losses. It is, in practice, a combination of the start/ stop method and different off-loading systems. For additional information, please see 2.5.4.2.

2.5.2.7 Variable discharge port

The flow rate of screw compressors can be regu- lated by moving the position of the discharge port into the housing, in the rotors’ lengthways direc- tion, towards the inlet. However, this method generates high power consumption and is rather unusual.

tla s C op co A ir po w er N V, B el gi um , 2 01 5

tained pressure is common. Usage is limited to single-stage compressors, due to the complexity and increased costs.

Pressure relief:

The original method for regulating dynamic com- pressors was to use a pressure relief valve or blow- off valve to release excess compressed air into the atmosphere. In principle, this method works iden- tically as with pressure relief on a displacement compressor.

2.5.3.3 Load–unload–stop

Previously mentioned regulation methods can be combined to control the compressor unit. Two modes are commonly used:

- Modulating:

The excess flow is released into the atmo- sphere (or the inlet), but energy consumption is unchanged.

- Auto dual:

The flow turndown of the unit will be limited to the turndown of the intake valve and/or the outlet guide vanes for flows below the turndown limit. The regulation system fully closes the inlet valve at the same time as the compressor’s out- let is opened to the atmosphere (compare with the displacement compressor). The off-loading power is still relatively high, representing 20% of the full load power, depending on the design of the impeller etc.

2.5.3.4 Speed regulation

Speed regulation has a similar effect as the use of inlet guide vanes. Flow can be varied with constant pressure within the compressor turndown range. At higher powers, speed variation is less advanta- geous, due to the high cost of the required drive installation.

2.5.3.1 Inlet regulation

Throttling the inlet:

The inlet can be throttled on a dynamic compres- sor to continuously reduce the capacity of the com- pressor. The minimum flow is determined when the pressure ratio reaches the pump limit and the machine becomes unstable (surge).

The regulation range is determined by the design of the machine (e.g. the number of stages and the impeller design) but also to a large degree by external factors such as counterpressure, suction temperature, and coolant temperature. The mini- mum flow often varies between 60% and 85% of the maximum flow.

Inlet guide vanes:

Vanes arranged as radial blades in the intake cause the drawn-in gas to rotate while the flow is throttled. The method has the same impact as throttling, but offers a greater regulation range and features improved energy utilization. Regulation down to 50–70% of the design flow is a typical value. There is also the possibility of increasing slightly the capacity and pressure of the compres- sor to a certain degree, by turning the vanes in the opposite direction. This, however, may impair per- formance to a certain degree.

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