Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2002-05-01
2003-06-24
Tanner, Harry B. (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S228400
Reexamination Certificate
active
06581399
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to an apparatus and method for controlling the operation of a centrifugal chiller refrigeration system. More particularly, the chiller control system of the present invention operates a centrifugal chiller which possesses both a magnetic bearing centrifugal compressor and an adjustable speed motor drive. Additionally, the present invention discloses the necessary components and control logic for use with the chiller control system.
A centrifugal chiller typically consists of the following components: one or more evaporators, compressors, condensers and expansion devices. In a chiller, the compressor acts as a vapor pump, where raising the pressure of the refrigerant from the evaporating pressure to the condensing pressure provides an active means of absorbing heat from a lower temperature environment and rejecting that heat to a higher temperature environment. As an active machine, the chiller requires an apparatus to control its operation.
In general terms, a centrifugal compressor for a chiller typically consists of the following components: inlet guide vanes, one or more impellers within a housing surrounded by one or more diffusers with collectors driven by some mechanical shaft means, such as for example, an electric motor. The mechanical shaft means is supported by one or more bearings of the rolling element, journal, or magnetic bearing type which accommodate both radial and axial loads. In variable speed electric chillers, the centrifugal compressor is supplied with electrical power through an adjustable speed motor drive which alters the frequency and/or voltage of the power to the motor to modulate the speed of the compressor.
The chiller control system for a centrifugal chiller typically performs one or more of the following functions: adjust inlet guide vane position and/or compressor speed to match the cooling capacity with the cooling load, monitor chiller operating conditions for unsafe operation and take appropriate action when encountered, display chiller operating conditions for user interpretation, and/or operate the chiller in response to a predefined schedule.
Chiller control systems of the microprocessor type typically consist of one or more of the following devices, a microprocessor which runs a control algorithm, sensors which acquire operating data from one or more points on the chiller, display devices for communicating information on chiller operating conditions and various devices for the input of information to the chiller control system. While these chiller control systems have performed adequately for centrifugal chillers consisting of compressors with rolling element bearings and/or journal bearings, they are inadequate for chillers with magnetic bearing centrifugal compressors.
Rolling element bearings are generally passive devices and, during normal operation, operate without the requirement of active control. The chiller control system does not typically provide active control of the rolling element bearings where, in this context, active implies continual adjustment of some bearing feature. Chiller control systems for centrifugal chillers which use rolling element bearings in the compressor may monitor the bearing temperature, at periodic intervals, as an indication of whether the machine is operating properly. An elevated temperature is used as an indication of a potential mechanical problem with the bearings. If the measured bearing temperature exceeds a predefined setpoint, the chiller control system may be programmed to stop the machine and alert the user.
In magnetic bearing centrifugal compressors, the compressor rotor is suspended on a magnetic field generated in the magnetic bearings. For definitional purposes, “magnetic bearings” are electromagnetic devices used for suspending a rotating body in a magnetic field without mechanical contact. The bearings can be further classified as active, indicating that some type of active control system is necessary to ensure stable levitation of the rotating body.
Distinct from other compressor types, a magnetic bearing centrifugal compressor uses magnetic bearings as the primary means for supporting the rotor structure. There is a clearance gap between the rotating and stationary components of the bearing that is measurable and controllable. For the magnetic bearings to operate properly, electrical power and proper operation of the magnetic bearing control electronics are required.
As described previously, existing chiller control systems for centrifugal chillers do not work adequately for centrifugal chillers with active magnetic bearing centrifugal compressors. The necessary control strategies are not provided by the controllers known in the art.
Specifically, these chiller control systems do not monitor the magnetic bearings for stable levitation which is required in order to prevent damage to the magnetic bearing centrifugal compressor. Existing chiller control systems may allow the compressor to turn at high speeds while the magnetic bearings are not stably levitated. When this occurs, the rotor does not spin about a fixed axis. Rather, the rotor spins on an axis contained within a small cylinder defined by the clearances between the compressor rotor assembly and the stationary compressor housing. The unconfined rotation of the compressor rotor assembly may generate large forces (due to the kinetic energy stored in the rotor at high speeds), and may thereby damage the magnetic bearings, the compressor rotor assembly, and compressor impeller, as well as the attached stationary compressor housing. In the event of a loss of active control of the compressor rotor, the rotor may contact the auxiliary bearings within the compressor.
Due to the disadvantages associated with chiller control systems known in the prior art for centrifugal chillers which have magnetic bearing centrifugal compressors, it should therefore be appreciated that there is a need for a chiller control system for a magnetic bearing centrifugal chiller.
In view of the foregoing, it is an object of the present invention to provide a chiller control system apparatus and method for controlling a centrifugal chiller which possesses a magnetic bearing centrifugal compressor and an adjustable frequency motor drive.
The function of a chiller control system is to operate a centrifugal chiller in such a manner as to meet the cooling load requirements. The chiller control system continuously monitors the cooling load and other chiller variables, and adjusts the operation of the chiller to match the cooling load. In sophisticated chiller control systems, in addition to matching cooling load, the control system seeks to operate the compressor in a manner that maximizes operating efficiency to reduce overall electrical power consumption.
While maximizing overall centrifugal chiller operating efficiency, the chiller control system must operate the magnetic bearing centrifugal compressor safely by avoiding compressor surge. Surge occurs when there are sudden reversals in the direction of fluid flow through the compressor impeller as the pressure difference across the impeller becomes too large. (Since additional static pressure rise occurs in the compressor diffuser as the fluid is decelerated, the pressure near the diffuser entrance may exceed the pressure at the impeller exit.)
When the impeller exit pressure drops below diffuser pressure, the fluid flow direction reverses and flows back into the compressor impeller, resulting in significantly increased stresses and a substantially increased vibration of the compressor rotor. The flow reversal causes the pressure at the impeller exit and within the diffuser to drop. When the pressure drops below the surge point, the flow again reverses direction and flows into the diffuser. A compressor operates in a surging condition when these sudden flow reversals are occurring. The flow reversals during surge damage the chiller equipment.
Prior experimental studies have shown that the maximum operating efficiency of a centrifugal compressor is
Benedict Scott M.
Cole Greg S.
Mainstream Engineering Corporation
Tanner Harry B.
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