The efficiency of an air compressor is most commonly measured in ‘scfm’ delivered at ‘full-load input power’. The ‘scfm’ measures the volume of air under Standard conditions of temperature, pressure and humidity. The density of air reduces with an increase in the air temperature. Thus, as the ambient air temperature reduces the mass of air contained in a unit volume i.e. density increases.
Reducing inlet temperature or increasing the air density is equivalent to increasing the mass flow though the volume remains constant. Thus, though the input power nearly remains the same, the mass flow from the compressor will increase.
This mass flow increase effect is less pronounced for lubricant-injected, rotary-screw compressors because here the incoming air mixes with the higher temperature lubricant. Conversely, as the temperature of intake air increases, the air density decreases and mass flow and pressure capability of the compressor decreases. The resulting reduction in capacity is often met by operating additional compressors, thereby increasing energy consumption.
Typically, the air inside the utility room is warm because of the machine operations. Thus, the inlet air to the compressor should be taken from a point cooler point outside the building.
When inlet air is cooler, it is also denser. As a result, mass flow and pressure capability increase with decreasing intake air temperatures, particularly in centrifugal compressors. This mass flow increase effect is less pronounced for lubricant-injected, rotary-screw compressors because the incoming air mixes with the higher temperature lubricant. Conversely, as the temperature of intake air increases, the air density decreases, and mass flow and pressure capability decrease. The resulting reduction in capacity is often addressed by operating additional compressors, thus increasing energy consumption.
By a rule of thumb, every 5° C drop in air temperature at the inlet of an air compressor reduces the energy consumption by 1%. Thus, the air inlet for compressors should be taken from a cooler point.
Impact of Inlet air temperature in Centrifugal air compressors:
Passive Cooling: Passive cooling is based on evaporation of water in the inlet of the compressor. Due to evaporation, the inlet air is humidified, and the latent heat of evaporation is absorbed from the inlet air. As a result, the inlet air is cooled. The effective cooling capacity is limited by the humidity, because the evaporation process only takes place as long as the air is not saturated. Since the cooling effect of passive cooling (maximum temperature drop) substantially depends on the humidity of the inlet air, the power savings differ at different relative humidities.
Active Cooling: To overcome the limitation of passive cooling, in which the inlet air can only be cooled to the wet bulb temperature, active cooling can be implemented. Active cooling requires external power to achieve the desired cooling temperature, therefore, it usually involves higher system complexity, space requirement, investment and operating costs than passive cooling. Despite these drawbacks, the active cooling provides also several advantages. Above all, the cooling effect is independent of weather conditions. Constant inlet conditions can be ensured, so that it allows an optimal and stable operation efficiency during the year. Mechanical and absorption refrigeration systems are commonly used techniques for active cooling.
Mechanical Refrigeration System: A mechanical refrigeration system uses a circulating refrigerant as a medium, which absorbs and removes heat from the inlet air by means of a heat exchanger and subsequently rejects that heat elsewhere. Typically, the evaporator is directly installed in the inlet of compressor as a heat exchanger and the inlet air can be cooled down to 3–4 °K higher than the refrigerant temperature. The refrigerant vapour is compressed by using a centrifugal, screw, or reciprocating compressor, which are mostly driven by electric motor. Consequently, the electrical power consumption of the mechanical refrigeration system is high. However, the mechanical refrigeration system Copyright© 2016 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station has a high coefficient of performance (COP), which can be up to 5.
Absorption Refrigeration System: The absorption refrigeration system utilizes waste heat instead of electricity as energy source. This ability provides an energy savings opportunity if waste heat is available. In the absorption cycle, LiBr and water is the preferred refrigerant and in combination an absorbent agent due to their chemical stability and operational safety. A conventional system produces chilled water at temperatures up to 2°C as cooling media, so it is possible to use direct contact air-cooler to achieve a smaller temperature difference (about 2°C) between chilled water and cooled air, compared to the indirect contact air-cooler. Besides the conventional system various types of absorption cycles at different levels of system complexity and efficiency exist. A single stage system will have a COP of 0.7– 0.8 and a double-effect unit a COP of 1.4. Unlike the mechanical refrigeration system, the absorption refrigeration system does not lose efficiency at part load and provides higher operational flexibility. Absorption systems have typically higher investment costs and space requirements, but lower operating and maintenance costs than mechanical refrigeration systems.
Inlet air chilling system:
- An inlet chilling system cools the compressor air intake, increasing air density and thus engine output.
- The inlet air can be cooled via water-cooled chillers or air-cooled chillers. Water-cooled chillers are more efficient but require a continuous supply of makeup water.
The inlet chilling system is skid-mounted and works as follows:
- First, the combustion air passes through an inlet screen and barrier filter to prevent debris from entering the engine or fouling the coils.
- The chiller coils utilize a water and glycol mixture to cool the air to the optimal temperature for the performance of the engine. The chiller coils can also be used as the heating coils in an anti-icing system during the winter periods.
- The air is then scrubbed using a drift eliminator to remove airborne condensation and keep it from entering the engine.