Controlling backpressure is imperative to maintaining consistent starch cooking processes and ensuring a quality end product. Three conditions must be understood to ensure that adequate backpressure is being maintained.
Choked flow steam injection
The system backpressure at the discharge of the genuine Hydro-Thermal Jetcooker™ plays a vital role in affecting the velocity of steam injection in the starch slurry. Based on industry studies in the field of direct steam injection and the design of the Jetcooker, limiting the backpressure at the discharge, to less than or equal to 60% of the steam supply pressure (calculated on an absolute scale) ensures a sonic velocity for steam injection (also known as choked flow steam injection). Maintaining the recommended pressure differential between the system backpressure and steam pressure safeguards helps the steam that is injected into the starch slurry at sonic velocity. It goes through a phase of instantaneous condensation that provides a uniform temperature profile. The pressure differential is also critical in preserving the operational stability of the Jetcooker. Operating outside the choked flow steam injection condition may result in issues such as (1) incomplete steam condensation leading to steam hammer and mechanical vibrations and/or (2) reduction in steam capacity of the injection nozzle due to decrease in steam velocity.
System backpressure below the minimum required values
With most starch cooking applications utilizing high cook temperatures (exceeding boiling point) to obtain the desired properties from different grades of starches, it is imperative to maintain the minimum required backpressure through the system to prevent flashing at the cook temperatures. As a universal thermodynamic property, system pressure must be maintained above atmospheric pressure to achieve temperatures above the boiling point of the fluid. Insufficient backpressure for a given cook temperature above the boiling point leads to instantaneous flashing. As a result, issues such as two-phase flow patterns (creating steam hammer) or steam surging and inability to achieve expected cook temperatures resulting in incomplete cooking of the starch can occur.
Maintaining system backpressure
Most starch cooking systems that incorporate the Jetcooker are a hold tube design that consists of regulating valves to maintain adequate system backpressure. The backpressure regulating valve plays an essential role in the performance and stability of the system. The setting of the valve affects two major system characteristics: (1) maintaining choked flow steam injection through the Jetcooker, and (2) holding enough backpressure at the Jetcooker discharge and the hold tube section to prevent flashing, to produce a batch of uniformly cooked starch.
With the extended service life of the Jetcooker, components like the back pressure regulating valves are often not maintained as recommended. They can lead to system malfunctions such as steam surging, pressure spikes, hammering, and vibrations.
Using another method to create system back pressures such as baffle/orifice plates or manual hand valves can result in undesirable outcomes. The fixed position designs may generate fluctuations in system backpressure due to changes in process conditions (such as flow rates) or may be unable to maintain backpressure depending on changes in cook temperatures.
Another limitation of using fixed orifice designs or manual valves to maintain system backpressure is their inability to respond to a change in viscosity of the process fluid. A fixed valve position set to maintain the recommended system backpressure for cooked starch (high viscosity) would not provide the same backpressure results when running water (low viscosity) through the system during warm-up cycles.
This could result in problems related to flashing during the warm-up cycle. Similarly, a valve position set based on water service will create excessive back pressure when the system handles cooked starch at a higher viscosity.
Controlling backpressure will have a significant impact on the performance of a starch cooking system. The best way to ensure proper continuous output is to identify issues and pinpoint the cause of them quickly, reducing possible downtime.
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