| OCR Text |
Show OPTltll O{I(tTOa D BOILER CONTROL LOOP run nov S(NS( TRIM S(I FIG. 3 Schematic representation of a microprocessor-based fuel/air trim system for boilers. FIG. 4 Boiler trim controller modulator. Figure 4 shows an interface unit. The principal function of this unit is to provide suitable isolation between the computer and the boiler/furnace electrical system. This isolation is accomplished with OPT022 devices. The unit also houses the power supplies for the signal conditioning and computer I/O connector boards . Since 201 this is a prototype controller, a process control computer is used that has significantly more capability than that required to implement the control scheme . The computer is a MACSYM 120 manufactured by IBM to specifications set by Analog Devices, Inc . In essence, it is an industrially hardened IBM personal computer. The detector signal is processed by the computer for subsequent use in the control algorithm described below . A signal generated by an optical detector located on the modulator motor shaft is used to determine the modulator frequency. This signal is a necessary input for the phase sensitive detection routine. The modulator, which is shown in Figure 5, consists of a magnetically driven butterfly valve . The magnetic FIG . 5 Magneticall y driven f uel flow modulator for industrial systems . drive removes the need for shaft seals. The diameter of the butterfly disk is determined by the flow velocities and the plumbing constraints of the fuel system . The design is predicated on standard plumbing components with a wide range of diameters. Considerable effort has been taken to ensure the fail-safe operation of the modulator. In the event of a power failure or computer malfunction, the modulator is automatically locked in the full open position. Unlike in an analog system, the modulator does not have to operate continuously, so that the overall reliability of the system is better. The fuel flow rate is monitored by a potentiometer attached to the main fuel flow control linkage. The final major tie point to the boiler/furnace is a variable length jack shaft control link which is used to trim the position of the air damper. SOFTWARE - A complete description of the software is beyond the scope of this paper; however, a general outline of the control algorithm is given in Figure 6. Some of the routines not included are those that control the modulator, startup, housekeeping and flame detection. After the flame is ignited , the fuel flow is stabilized, and the |
