At the heart of the tissue making process is the Yankee dryer where critical properties are given to the tissue being produced. Yankee steam systems are used to deliver heat to Yankee dryer and evacuate the condensed and non-condensed steam.
Yankee steam systems share many of the same design principles as steam systems found in related industries and applications. They differ, however, in one major aspect. Yankee steam systems operate as a closed-loop system (see graphic below) where non-condensed steam, referred to as blow-through steam, is removed with the condensate formed inside the Yankee dryer, recompressed, and then directed back into the Yankee dryer. This creates a particularly important energy balance that must be considered to achieve maximum system efficiency.
The mass of steam that flows into the Yankee dryer is equal to the mass of condensate and blow-through steam that leaves the dryer. The latent heat of the steam that condenses in the dryer goes into heating and drying the product. With an efficient and properly sized thermocompressor, all of the blow-through steam is re-compressed by high-pressure motive steam and circulated back into the dryer, along with any make-up steam that might be required.
Computational fluid dynamic (CFD) analysis helps optimize the thermocompressor design. This ensures the amount of motive steam required does not exceed the amount of condensate leaving the system. The CFD analysis also verifies the thermocompressor has the capacity to handle all of the blow-through steam as suction flow. If the system is not optimized, it is possible to vent blow-through steam to atmosphere if the thermocompressor cannot handle the maximum amount of blow-through steam being recirculated back into the steam system.
When looking to upgrade your existing thermocompressor equipment, there are several items to consider when requesting a new unit:
When providing the range of operating conditions, it is important to define the actual operating range to the best extent possible. It may seem natural to ask for as wide of a range as possible, but this may have a negative impact on efficiency. Kadant Johnson can help with accurately assessing the different operating conditions to get the most out of your system while keeping potential future conditions in mind.
The thermocompressor can also be optimized based on a weighted average. For example, if 80% of the time the machine runs at one condition it can be optimized most effectively for that scenario, while still meeting other conditions that fall within the sizing guidelines.
Thermocompressor optimization using CFD software ensures users will achieve the greatest operating efficiency with the flexibility to tune the device to specific operating conditions.