Compressor and Expander
Introduction
Compressors are similar to pumps in that they increase the pressure of a fluid.
Links
It is possible to model a series of compressors which are linked together on the same physical shaft. There can be one turbine in the link to provide the total power, or it can be specified via a negative value in the Total Power Loss. Modeling a linked train is usually very dependent on a good set of curves in each compressor. A linked set of units are typically all set to use curves (on the Specifications page) with no other specifications typically required in each.
Settings
If the compressor is in reciprocating mode, rather than centrifugal mode, the settings are available on this page. Positive displacement units do not make use of characteristic or flow restriction curves.
Curves
Since startup and shutdown can be modeled as well, a healthy set of curves covering the range of speeds is typically a wise idea if curves are used.
The curve page is only available for centrifugal units.
Flow Limits
The flow limits page is available when modeling a centrifugal unit, but not when modeling a reciprocating unit. A simple stonewall or surge curve can be provided, usually two points in each suffices. These two curves do form part of the simultaneous solution. A warning will be displayed if the compressor approached the surging or stonewall operating limits. Stonewalling typically means that the flow rate will not increase any further.
Surging, where the compressor can not add enough energy to overcome system resistance, is modeling by adjusting the duty in the equations continually.
Some modelers choose to not use the surge and stonewall curves, but rather calculate their own values and display a warning in the user interface.
Nozzles
The compressor elevation is used to determine static head contributions.
Inertia
Modeling inertia can be helpful in realistic compressor modeling and tends to make it easier to model transients. Both inertia and friction result in a small duty loss.
Performance
The values displayed here typically match those of steady state, except that the duty loss due to inertia and friction is applied in dynamics mode only.
Specs
This determines what equations the compressor or expander contributes to the simultaneous equation network. By default the duty and polytropic efficiency are used as specifications. If curves are used as specification, then the compressor speed is typically used as a specification. However, if the compressor is linked to other compressors, then no speed specification is used and the speed is determined via the linked units.
The the Reciprocating option is checked, then positive displacement setting can be accessed on the Design, Settings view.
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