Background
GRE won the contract to design and manufacture 4 bespoke thermoregulating (heating and cooling units), each unit having 4 separate circuits – the 4 circuits supplying 14x drums (all linked together in configuration 2 + 4 + 4 + 4).
While general process (heating/cooling of process fluids) system principles and instrumentation requirements from previous similar heating/cooling systems provided some aspects of base knowledge, this project was different in terms of design requirements (sheer amount of rollers – within the process environment that had to be thermally controlled to a high degree of accuracy) that would need to be simultaneously heated/cooled depending on ever-changing process requirements) and its working parameters.
Compared to previous GRE projects, this cooling/heating system necessitated a new pressure control system, new flow rate monitoring principle, all new software, and used thermal oil with certain components at high temperature (150°C).
Project Challenges
The main challenges of these cooling/heating units included the introduction of an array unknown components and their characteristics, especially when used with thermal oil (which was selected by us as a cooling/heating medium, so that it could be run safely at just above atmospheric pressure) at 150°C (which would lead to temperature swings – and the resultant fatigue caused by continual expansion/contraction – of approximately 130°C). These chiller/heaters also required accurate and self-regulating outlet pressure control (due to the very high temperatures). Previous units had been controlled via basic software, whereas these units required a whole new functional design ethos and subsequent programming/testing, to ensure that they were both safe and effective. The method of flow rate monitoring also required a new approach, due again to the high temperature of the oil making previously used flow meters unsuitable.
Concept and solutions
Example 1 – flow through regulating valves : despite extensive research in selecting a suitable component, the pressure reducing valves were fitted to the unit but, during testing, were initially found to reduce the flow to the point of blocking the majority of the flow through the system and degrading the overall performance. After several unsuccessful attempts to devise a method to change the pressure setting of the valves, the valves were uninstalled, disassembled, and found to have built-in filters that were finer than the main system filters and were clogging up. Our technicians then modified, rebuilt and tested the newly modified units to remove the back pressure issues caused by the ‘too-fine’ filters. This particular example is a very noteworthy one, as even though the valves should have worked according to the manufacturer’s specifications, they still ended up having to be tested in a real-world environment and modified by our specialist engineers.
Example 2 – flow monitoring; our designers decided that the flow was to monitored by combination venturi/differential pressure flow meter (thermodynamic calculations/models/analysis software were used to design an in-house venturi assembly, which could be used with an off-the-shelf differential pressure transmitter, at the high temperatures that the system would be working at). The venturi was designed by GRE, to have a minimal pressure drop so it did not impede the system flow. The differential pressure transmitters that were initially selected were found to fail due to the start-up pressure spike of the pumps, even though the specifications for both pumps and differential pressure transmitters did not indicate that pressure spikes would be a problem. All differential pressure transmitters were removed, new units selected and then replaced, and the pipework from the venturi to the differential pressure redesigned.
