Customer solution heat exchangers SECESPOL for ORC technology

Customer solution heat exchangers SECESPOL for ORC technology

SECESPOL has a broad background for calculating Shell and Tube heat exchangers for all recommended TEMA constructions. These heat exchangers, besides mass-produced tubular exchangers (JAD, DNA, P-Line) and plate heat exchangers (L-Line, R-Line, LUNA, SafePLATE), cover demand for special products in chemical technology and power engineering.

One of the cases of conventional tubular heat exchangers is the heat recovery from the heat transfer oil circuit of the ORC - Organic Rankine cycle technology.

Project: Reconstruction of District heating system

Heat exchangers supplier: SECESPOL-CZ, s.r.o.

Heat exchangers heat load: 9000 kW, 900 kW

ORC technology uses biomass combustion to produce electricity and heat using organic substances instead of water vapor.

During the construction of this technology, a modern biomass combustion boiler with automatic fuel dispensing conveyor, manipulator for optimum storage in silos and flame temperature control was installed. The flame temperature is controlled by flue gas recirculation to minimize the formation of NOx. Boiler efficiency is increased by a heat recovery flue gas heat exchanger with preheating combustion air and by optimized combustion of fuel with precise oxygen dosing by lambda probing.

Heat transfer oils from the boiler are used to absorb large amount of energy without changing their physical state. In this case, the synthetic oil Therminol was used. This proven oil is made up of terpenyl-based molecules, high-molecular substances, which ensure that they remain in the liquid state at relatively low pressures. This results in high operating temperatures, improved oil transport properties and better heat transfer coefficients.

Distributed heat transfer oil from primary circuit is then used to heat up the silicone oil of the ORC module (secondary circuit). This oil is brought to saturation temperature, evaporated in the evaporator and further used within turbine to power generation. The secondary circuit oil (dimethyl-bis (trimethylsiloxy)silane) is a special polymeric derivative of silane (SiH4) with oxygen bonds, and is therefore a silicone based fluid with excellent thermal properties. Another advantage is that it does not carry harmful erosion processes like water vapor.

The subsequent condensation of the silicone oil vapors and its cooling forms the heat source of the tertiary circuit, which ensures the heating of water into the district heating distribution network. For this purpose, the SECESPOL heat exchangers were designed. Their design parameters on the oil side are TS = 345 ° C and PS = 16 bar. The heat exchange surface of these exchangers is made of carbon steel U-tubes. The heat exchangers are designed with tube side multi pass to maximize heat transfer values with subject to permitted pressure losses and external dimensions.

Design of heat exchangers for technical oils

Technical oils used as a service medium in heat exchangers completely change the concept of design of exchangers from the point of view of conventional steam-water applications. The transport oil at an operating temperature of 300 ° C can have diametrically different parameters compared to water at 80 ° C, from the B-JAC database, for example:

- about 7 times lower thermal conductivity

- about 40% less heat capacity

- about 15% higher dyn. viscosity

- More than 10 times higher molecular weight

In addition, these substances exhibit generally steeper nonlinear behavioral changes of physical parameters with temperature. This often underestimated fact must be carefully considered when designing heat exchangers and include corrections of thermal calculations. Advanced methods of calculating the heat exchange surface with gradual integration of the heat load along the exchanger surface with local media properties and the heat transfer coefficienta are necessary. The worse transport properties of the working substance may result in a multiple need for the heat transfer area at a given power, higher pressure losses, higher risk of fouling of heat exchangers due to lower velocities etc. Material selection can be affected by a considerable amount of chloride ions, exchangers are subject to conformity assessment for dangerous substances, which is not needed in case of use water as a working medium.

All these aspects justify, if not directly require, an individual approach to the design of heat exchangers for applications with thermal oils and other chemicals of unique properties. Separate chapters are multi-component mixtures and mixtures of unknown exact composition. SECESPOL has its shell and tube heat exchangers proven in practice.