Case study: Boiler system design to constraints
A boiler system design was requested for a large food manufacturer that required 140,000 lb/h of saturated steam. Saturated steam is steam supplied at the vapor/water saturation point for a given operating pressure. In this case the saturated steam at 90 psig was at 331 F and the 175 psig steam was at 377 F. Operations required that the full boiler capacity was 90 psig of saturated steam, but lower quantities of steam were required at 175 psig of steam for daily process plus 90 psig steam for building heat requirements. The full steam capacity is needed only quarterly for building sanitation requirements. Available plant space was ample, but limited in ceiling height.The system design was to include a new condensate collection system, makeup water treatment system, deaerator, and feedwater pumps.
Initially, a designer might look at two or three fully modulating boilers producing steam at 175 psig in a configuration that includes a pressure reducing station to reduce steam down to 90 psig for the building heat systems. This scenario would have the boiler system producing all of the steam at 175 psig, only to have a large portion reduced down to the 90 psig heating load.
The final system configuration was designed using 10 modular boilers, each producing 14,000lb/h of steam. Three of the boilers were designed to provide the necessary 175 psig steam. The remaining boilers produce steam at 90 psig. The modular boilers run in high- or low-fire mode and sequenced off of a boiler control system that monitors both the 175 and 90 psig steam requirements. The higher pressure boilers run first with any steam not used by the process letdown to 95 psig. This operates the one, two, or three boilers at high fire depending on required load. The excess steam produced at 175 psig is let down to 95 psig and feeds the building heat system. Any additional steam required is provided by the seven 90 psig boilers.
The number of boilers required to run is dependent on load and controlled by the boiler control system. The high-pressure boilers run at high fire, with one boiler modulating between high and low fire to maintain the header pressure at the desired setting. System optimization has been performed by the boiler manufacturer and controls provider, bringing the average efficiency above 85% for the system.
The system described above has met the expectations of the food manufacturer and is an example of designing the system to meet the needs of the facility and the constraints of the system, building, and operations.
Bradley A. Pankow is a principal mechanical engineer and project manager for Stanley Consultants, focusing on educational and institutional steam, hot water, and chilled water generation and distribution systems. Prior to joining Stanley Consultants, he was factory manager and operations manager with H.J. Heinz; a lead mechanical engineer for Raytheon Engineers and Constructors; and engineering manager for Rovanco Piping Systems Inc.