Stonewall: At some point, as the discharge falls and the air flow through the increases at full load, the physical limitations will not allow more air through the stages – this point is known as stonewall. At greater turndown, it will be close to or at surge. For example, 15% turndown means the unit can run at 85% flow or higher, as equipped without hitting surge. Turndown is the percentage below full load flow the compressor can run without experiencing surge. This is an oversimplification of the surge action, however, each unit has a rise to surge limit or maximum pressure. Each centrifugal compressor has a maximum pressure it can reach for specific inlet conditions which will cause the air flow to reverse and surge, shutting off the compressor to avoid damage from the vibrations. Rise to Surge: As more compressed air is produced than needed, the centrifugal compressor must unload, or deliver less air to avoid over pressure. The actual volume of inlet air to be compressed will vary for a period of time with the inlet condition of pressure and temperature. Once an impeller is designed and a speed set, the energy that a pound of air will absorb in passing through the impeller is established.Ī centrifugal compressor will deliver a pound of air with a constant expenditure of energy - winter or summer. What are Turndown, Stonewall, and Rise to Surge? Pressures either in psig or bar (only using psia to convert from icfm/acfm to scfm)įigure 2.Understanding Centrifugal Manufacturer Operating Curves Actual machine specific performance is covered later in this document. Lowering the pressure will often allow more flow at the same or similar power input. Increasing the discharge pressure will normally leave the effect of raising the weight of the compressed air stream through the stages which will result in less flow of usable air (scfm) often at or near the same input power. This is also the case of discharge pressure with a fixed wheel, or impeller/diffuser/speed, compressor stage. The actual net effect of any of these conditions is dependent on the actual performance curve and aerodynamic characteristics of the design. Increasing the cooling water temperatures will again have the same “lightening” effect on the compressed air through the stages and power requirements as the previous conditions. Higher inlet pressure will have the opposite effect. Reducing the inlet pressure (altitude, negative compressor room pressure, dirty/poorly sized inlet filter) will lighten the compressed air flow (cfm) that travels through the stages also resulting in less usable air (scfm) at a reduce input power requirement. Colder temperatures will produce the opposite effect. Increasing the inlet temperature will lighten the total fixed air flow and deliver less usable air to the user (scfm) and reduce the input power requirement. Effect of inlet air temp on discharge pressureįigure 1b. Ignoring part loads controls anything that will increase or decrease the weight of the air going through the stages to final flow, and pressure will have a direct impact on input power.įigure 1a. The power requirement in this type of compression process, when the internal design parts are not considered, is basically dependent on the weight of the air going through the machine. Another term for this process is mass flow – the power requirement to deliver the rated cfm of flow at the rated pressure (psig) is determined by the weight of the air (some manufacturers also use the term “density”). The process of dynamic compression, as applied in a centrifugal compressor operating stage, is velocity and kinetic energy converted to pressure and temperature as the flow is restricted. The operating performance curve is shaped by the selected individual internal components and affected by operating conditions such as inlet pressure, inlet temperature, and cooling water temperature.
#COMPRESSOR PERFORMANCE CURVES PDF HOW TO#
Part II reviews typical operating performance curves and how to interpret them.Ī centrifugal air compressor operates over a range of flows and discharge pressures. Part I explained necessary terminology to understand centrifugal operations.
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