Computational and experimental justification for increasing the performance of the regenerative heat exchanger in the steam generator blowdown system of the AES-2006 project (RU V-392M)

the performance of the


Design of the regenerative heat exchanger in the blowdown system
The regenerative heat exchanger (RHE) is part of the blowdown and drainage system of the steam generators (SGs) at Novovoronezh NPP-II 1 and 2 and is designed to cool the blowdown (boiler) water from the SGs, before it undergoes special water treatment , and to subsequently heat the purified blowdown water returning to the SGs in different operating modes of the unit (Lukasevich et al. 2004, Shchelik et al. 2006, Zhukov and Lukashov 2010, RB-002-16 2016, 2021. The heat exchanger is a vertical type apparatus, consisting of two removable covers, a body, nozzles, supports, sling devices, cylindrical multi-pass coils (located inside the body between two tube sheets). A general view of the heat exchanger is shown in Fig. 1.
The medium being cooled from the SGs (for purification) is supplied "from the bottom up" through the tubular space, and the heated purified medium (from SVO-5) is supplied "from the top down".
The main characteristics of the heat exchanger are presented in Table 1.
The main parameters of the heat exchanger are presented in Table 2.

Characteristics of the regenerative SG blowdown heat exchanger at NvNPP-II 1 based on the results of the commissioning work
The commissioning work of the SG blowdown system showed the low performance of the RHE system at NvNPP-II 1 , 2021. At nominal flow rates in the blowdown system, significantly low blowdown and feed water temperatures were observed before and after the RHE. A set of measures aimed at cleaning the tubular and intertubular space of the heat exchanger did not at all increase its performance. Table 3 presents the results of commissioning of the RHE in the SG blowdown system at NvNPP-II 1 (before upgrading). Since the SG blowdown system at NvNPP-II 1 and 2 is closed, the flow rates of the cooled (blowdown) medium are equal to those of the cooling (feed) medium. The measurement error was no more than ± 2.0% for the temperature and no more than ± 4.5% for the flow rate.  Temperature of the cooled medium at the RHE outlet, from special water treatment in the SG (intertubular space), °С

237-251
Medium pressure, MPa 6.9 Temperature of the cooled medium at the RHE inlet, from the SG (tubular space), °С 285 Temperature of the cooled medium at the RHE outlet, from the SG for special water treatment (tubular space), °С ≥ 100 Medium flow rate, t/h 140 Figure 1. Design of the regenerative heat exchanger in the SG blowdown and drainage system: 1 -supply of the cooled medium from the SG; 2 -supply of the cooled medium from the RHE to the blowdown aftercooler; 3 -supply of the purified coolant from SVO-5 for heating; 4 -return of the purified and heated coolant to the SG The measured temperature changes in the tubular and intertubular space of the RHE at NvNPP-II 1 for a flow rate of 60 t/h and 148.2 t/h are shown in Fig. 2a, b, respectively.
The design temperature changes in the tubular and intertubular space of the RHE are shown in Fig. 2c.

Mathematical simulation of heat exchanger operating modes
The authors of this article have previously put forward an assumption about the reasons for low temperatures before and after the RHE, namely, the low flow rate of the coolant in the intertubular and tubular space of the heat exchanger (0.34 m/s and 0.63 m/s) for the design flow rates of the medium in the system , 2021. In the design (current) scheme for connecting the RHE piping (the medium being cooled from the SG in the tubular space is supplied "from the bottom up", the heated medium in the intertubular space is supplied "from the top down"), the directions of forced and free (convection) movement of the medium are opposite. In the case of low flow rates of boiler water, this phenomenon significantly reduces the heat exchanger performance (Rassokhin 1987, Margulova 1994, Trunov et al. 2001, Andrushechko et al. 2010.
The calculations were carried out using the Ansys program (CFX thermohydraulic module) (Snegirev 2009, Bruyaka et al. 2010. The purpose of the calculation was to determine the coolant rate in the RHE tubes for the design scheme of the heat exchanger piping. To estimate the rate in the heat exchange tubes of the regenerative heat exchanger, a 3D model was built (Fig. 3) and a thermohydraulic calculation was carried out for the RHE nominal operating mode (flow rate = 140 m 3 /h). The RHE piping has a design configuration: the supply of hot water from the SG to the tubes is "bottom-up".
The calculation results showed that the average rate in the RHE tubes was no more than 0.47 m/s. The calculation, in which hot water from the SG to the RHE tubes is supplied "from the top down" (that is, the scheme for connecting the RHE piping proposed by the authors, opposite to the design scheme) showed that the average rate in the RHE tubes would be 0.7 m/s.
Thus, the assumption about the influence of the piping connection scheme on the RHE performance was confirmed.

Operating parameters of the SG blowdown system after upgrading of the RHE piping at Novovoronezh NPP-II 1
The proposed option for rerouting the RHE piping was initially accepted for implementation at NvNPP-II 2 during the installation of the blowdown system, and after the expected result was confirmed during the commissioning of the system, it was applied in April 2020 at NvNPP-II 1 (Fig. 4).
Changing the direction of the flow of the cooled and heated medium increases the heat exchanger performance (Figs 5-7), and also contributes to the sludge washing out of the heat exchange tubes into the lower chamber.
Figs 5-7 present the results of a comparative analysis with the design data of the RHE operation (temperature drops Δt in the tubular and intertubular spaces depending on the flow rates) of the SG blowdown system at NvNPP-II 1 before upgrading and NvNPP-II 1 and 2 after upgrading. The temperature and flow rates of the medium were measured by standard technological (design) means (including thermocouples and orifice gages); the measure-ment error was no more than ± 2.0% for the temperature and no more than ± 4.5% for the flow rate. Based on Figs 5-7, it can be concluded that with the current RHE piping configuration at NvNPP-II 1 (after upgrading), the RHE parameters fully correspond to both the calculated (design) values and the RHE parameters at NvNPP-II 2.  . RHE piping of the SG blowdown system: a. before upgrading at NvNPP-II 1; b. after upgrading in 2020 at NvNPP-II 1 and 2 (1 -supply of the medium being cooled from the SG into the RHE; 2 -supply of the cooled medium from the RHE to the blowdown aftercooler and further to SVO-5; 3 -supply of the purified coolant from SVO-5 to the RHE for heating; 4 -return of the purified and heated coolant from the RHE to the SG) The results of testing the new RHE connection configuration at NvNPP-II 1 showed that the RHE of this power unit provides the temperature difference in the tubular and intertubular space as close as possible to the design (calculated) data.

Conclusions
The commissioning of the SG blowdown system at NvNPP-II 1 revealed insufficient RHE performance in terms of ensuring temperature differences in the tubular and intertubular space.
Based on the results of the commissioning of the RHE in the SG blowdown system at NvNPP-II 1, according to the proposals made by the authors of this article (with a confirmed thermohydraulic calculation of the heat exchanger), with the recommendations of the developers of the system and the RHE manufacturer taken into account, the RHE piping at NvNPP-II 2 was rerouted so that the medium being cooled from the SG in the tubular space is supplied "from the top down" and the heated medium in the tubular space is supplied "from the bottom up" (forced and convective movement of the medium coincide).
The results of testing this RHE connection configuration at NvNPP-II 2 showed that the RHE of this power unit provides the temperature difference in the tubular and intertubular space as close as possible to the design (calculated) data.
• The test results of the new RHE connection configuration at NvNPP-II 1 showed as follows: • The RHE also provides temperature differences in the tubular and intertubular space as close as possible to the design (calculated) data; and • The expected (calculated) result from the RHE upgrading was confirmed. • Changing the RHE connection scheme made it possible as follows: • To reduce the temperature difference between the feed water pipelines and the blowdown water return pipes to the SG; • To reduce the heat load on the blowdown aftercooler and, as a result, reduce the heat load on the intermediate circuit system of normal operation consumers; and • To ensure the design modes of operation of the SG blowdown system and the intermediate circuit system of normal operation consumers.
According to the test results, it was decided to modernize the RHE piping at NvNPP-II 1 as well.