دانلود رایگان مقاله انگلیسی مبدل های حرارتی برای استفاده از حرارت دما بالای نمک گرمایش زمین - اشپرینگر 2018

Abstract

The basic component of two-circuit geothermal systems is the heat exchanger. When used in geothermal power systems, conventional shell-and-tube and plate heat exchangers cause problems related to the cleaning of the latter from salt-deposition and corrosion products. Their lifetime does not exceed, as a rule, 1 year. To utilize the heat of high-temperature geothermal brines, a heat exchanger of the “tube-in-tube” type is proposed. A heat exchanger of this design has been operated for several years in Ternair geothermal steam field; in this heat exchanger, the thermal potential of the saline thermal water is transferred to the fresh water of the secondary circuit of the heating system for apartment houses. The reduction in the weight and size characteristics of the heat exchangers is a topical problem that can be solved with the help of heat transfer enhancers. To enhance the heat transfer process in the heat exchanger, longitudinal ribbing of the heat exchange surface is proposed. The increase in the heat exchange surface from the heat carrier side by ribbing results in an increase in the amount of the heat transferred from the heating agent. The heat exchanger is easy to manufacture and is assembled out of components comprised of two concentrically positioned tubes of a definite length, 3–6 m, serially connected with each other. The method for calculation of the impact of the number and the size of the longitudinal ribs on the heat transfer in the well heat exchanger is presented and a criterion for the selection of the optimal number and design parameters of the ribs is formulated. To prevent the corrosion and salt deposition in the heat exchanger, the use of an effective OEDFK (oxyethylidenediphosphonic acid) agent is proposed. This agent has a long-lasting corrosion-inhibiting and antiscaling effect, which is explained by the formation of a strongly adhesive chelate layer difficult to wash off the surface. The passivating OEDFK layer is restored by periodical pulsed introduction of the agent solution into the brine at the heat exchanger inlet.

CONCLUSIONS

(1) The tube-in-tube heat exchangers are most efficient for the pickup of the heat from thermal brines.

(2) Longitudinal ribbing increases the heat exchange surface and the amount of the heat transferred from the heating agent and reduces the weight and size characteristics of the heat exchanger.

(3) Longitudinal ribbing is more efficient at low fluid flow velocities with the optimal rib height ranges within 5–15 mm and the number of the ribs ranging from 16 to 32. With the increasing number of the ribs, the heat extraction is enhanced; the hydraulic resistance, however, increases. The thermal insulation of the outer heat exchanger surface allows reducing the environmental heat losses.