Heat Transfer and Pressure Drop in Main Heat Exchangers of a Thermal Oil ORC-Unit (Review)

Abstract

The purpose of the review is to find the best currently available correlations for calculating heat transfer and pressure drop in the main heat-transfer equipment items in organic Rankine cycle (ORC) units. The search is limited to the designs of apparatuses, which are the best ones in the opinion of the authors of this paper, for a conventional two-circuit ORC-unit, where thermal oil cools a heat source in the first circuit and transfers heat to refrigerant in the vapor generator (hereinafter referred to as the evaporator). Besides the evaporator, the second circuit of the unit includes a “refrigerant–water” or “refrigerant–air” condenser and a regenerative heat exchanger which heats up liquid refrigerant upstream of the evaporator with the exhaust vapor of the turbine (or expander). The criteria are presented for selecting working fluids for such units depending on the heat source temperature. The working fluids that have found the widest application at each temperature level (such as cyclopentane, benzene, toluene, MM, MDM, R1233zd, R245fa, R601, R601a, RC318, R134a) are listed, and their characteristics and thermodynamic properties are presented at specified condensation (25°C) and boiling (200, 120, and 70°C) points. The analysis of these data, including information on the proposed working fluids, has yielded nominal parameters of ORC-units. Thousands of fundamental and engineering works are devoted to the study of boiling and condensation processes, the interest in which has been growing over the past 10–15 years. The development of new energy conversion technologies and the appearance of new working fluids, materials, and methods of surface treatment has given a second wind. This paper reviews correlations for heat-transfer coefficients and hydraulic resistance factors in apparatuses with refrigerant boiling in round tubes, condensation in tubes and channels and in the shell side (on tube bundles), and heating and cooling of single-phase refrigerant in tubes and channels. The correlations for engineering calculation of the main heat-transfer equipment of ORC-units, which are the most convenient ones in the authors’ opinion, are presented.