Usually the air circulation system of a Yankee hood dryer is of the cascade type, and it works according to the counter current principle. This means that the exhaust air from the dry section is used as make-up air in the wet section, the make-up air from heat recovery is supplied to the dry section, and the total humid air exhaust is taken from the wet section. The above gives lower specific heat consumption, and a lower electrical consumption, than a conventional parallel system.
There is a way to further reduce heat consumption, and this consists in using a Heat Recovery Steam Generator (HRSG) before the traditional Heat Exchanger, for combustion and make-up air. Thermal energy, in the exhaust high humidity air flow can be recovered in the HRSG to produce additional steam for the Yankee cylinder. This application has a much greater thermal efficiency than the conventional air system arrangement. The above is an excellent solution, because it allows to recover maximum possible energy from the exhaust air of the Yankee hood (sometimes it is not possible to heat the combustion air above certain limits because this air should also cool the burner, and usually the temperature must be below 200°C). The flow of saturated steam produced by the HRSG could be 25-35% of the steam requested by the Yankee dryer cylinder.
1) HRSH Design
The basic principles for selecting a HRSG are:
2) HRSG Configuration
A HRSG can be configured with gas flow in the horizontal or vertical direction.
A vertical gas flow unit permits an arrangement of equipment that occupies less floor space, but requires an extensively heavier steel structure. Horizontal gas flow units generally occupy a greater plan area, but affords better access for maintenance of HRSG parts. A HRSG with vertical gas flow usually employs horizontal tubes supported by tube sheets. Most of these applications require a circulating pump in the steam generating section of the HRSG, which ensures uniform distribution of water to multiple parallel steam generating circuits. Pump are usually sized to maintain a circulation ratio of 4 to 1 at the maximum steam conditions.
A HRSG with horizontal gas flow uses vertical tubes connected to headers at the top and bottom. The tube and header assemblies may be either top-supported or bottom-supported. Natural circulation in the steam generating sections provides high circulation ratios without the use of pumps.
In the paper mill usually it is not necessary to superheat steam so the HRSG can be composed by two heating sections: Economizer and Evaporator, single pressure type.
3) Typical construction features
Normally included in the scope of supply are:
Insulation is placed on the inside. A system of internal liner plates protects the insulation from the gas flow. All HRSG pressure parts are supported in ways that allow complete freedom for thermal expansion of the casing and supporting structure.
Payback time of the HRSG is very interesting, because the proven advantage in terms of energy savings is remarkable, and this goes to the benefit of world tissue manufacturers.
A.Celli Paper has always been very attentive to the issue of energy savings as part of its corporate philosophy, and the application of this system ensures the best results in this sense for our customers. For more info or specific evaluation of the dedicated applications, please mail us (firstname.lastname@example.org) at your convenience.
Copyright by A.Celli Paper S.p.A.