Jung-In Yoon

Heat transfer enhancement with a surfactant on horizontal bundle tubes of an absorber

Abstract This paper is concerned with the enhancement of heat transfer by surfactants added to aqueous lithium bromide (LiBr) solutions. Three different kinds of tubes in horizontally stagged arrangement are tested with and without an additive of normal octyl alcohol. The test tubes are a bare tube, a floral tube and a hydrophilic tube. The additive mass concentration is about 0.05–5.5 wt%. The heat transfer coefficient is measured as a function of the solution flow rate in the range 0.01–0.034 kg m −1 s −1 . The experimental results are compared with/without surfactants. Among three kinds of tubes, the hydrophilic tube shows the highest permeability. It has 4–73% higher wetted area than that of the bare tube, and 10–22% higher than that of the floral tube. Without surfactants, the hydrophilic tube is in the range 10–35% higher heat transfer coefficient than that of the bare tube, and 5–25% higher than that of the floral tube. With surfactants, the increase of the heat transfer coefficient is about 35–90% for the bare tube, 40–70% for the floral tube, and 30–50% for the hydrophilic tube.

A study on the advanced performance of an absorption heater/chiller with a solution preheater using waste gas

Abstract A large amount of consumed energy is released to the environment as waste heat, which may be used directly in some applications for useful purposes. Thus, from the standpoint of energy conservation, it will be meaningful to investigate systems with waste heat utilization. It has long been indicated that absorption cycles may have good potential applications for enhancing energy conservation via waste heat recovery. As such they have often been identified as an appropriate subject for research and development. Along this line, this paper also examines the development of an absorption cycle with waste heat utilization. More specifically, this study investigates a double-effect LiBr–water absorption cycle which uses exhaust gases from the burner of the high-temperature generator to preheat the weak absorbent solution on its way from the absorber to the low-temperature generator. The overall performance of the absorption heater/chiller system is analyzed and discussed on the basis of experimental results.

Comparison of System Performances of Hot-gas Bypass and Compressor Variable Speed Control of Water Coolers for Machine Tools

Recently, the needs of system performances such as working speed and processing accuracy in machine tools have been increased. Especially, the speed increment generates harmful heat at both moving part of the machine tools and handicrafts. The heat is a main drawback to progress accuracy of the processing. Hence, a cooler system to control temperature is inevitable for the machine tools. In general, two representative control schemes, hot-gas bypass and variable speed control of a compressor, have been adopted in the water cooler system. In this paper, comparisons of system performances according to the control schemes in a cooler for machine tools were conducted in detail. Each proportional-integral feedback controller for the two different control systems is designed. The system performances, especially the temperature control accuracy and coefficient of performance which is a criterion of energy saving, were mainly analyzed through various experiments using 1RT water cooler system with different two types of control scheme. These evaluations will provide useful information to choose suitable water cooler system for the engineers who design controllers of the cooler system for machine tools.

Precise Temperature Control of Oil Coolers with Hot-gas Bypass Manner for Machine Tools Based on PI and Feedforward Control

ABSTRACT:Recently, the performances of speed and accuracy are enhanced in machine tools. The high speed of the machine tools usually causes harmful thermal displacements on the objects. To reduce the thermal displacements, machine tools generally adopt oil coolers with precise temperature control function. This study aims at precise control of oil outlet temperature in the oil coolers with hot-gas bypass manner based on PI control logic. The control system was designed for obtaining steady state error within ±0.1℃ and maximum overshoot with 0.8% even though abrupt disturbances are added to the system. We showed that the PI gains could be easily decided by numerical simulations using practical transfer function which got experiments. Also, transient characteristics could be improved significantly by reflecting the inlet temperature of an evaporator to the output of a controller feedforwardly considering periodic abrupt disturbances. Through some experiments, excellent control performances were established by the suggested control.Keywords:Hot-gas bypass(핫가스 바이패스), Oil cooler(오일쿨러), Electronic expansion valve(전자팽창밸브), PI control(PI 제어), Feedforward control(피드포워드 제어)

Efficiency of Cascade Refrigeration Cycle Using C3H8, N2O, and N2

This article presents a new natural gas liquefaction cycle that utilizes propane (C3H8), nitrogen monoxide (N2O), and nitrogen gas (N2) cycles. A liquefaction cycle with staged compression was designed and simulated using HYSYS software for improving cycle efficiency. This included a cascade cycle with a three-stage compression consisting of a C3H8, an N2O, and an N2 cycle. The new C3H8, N2O, and N2 liquefaction cycle is compared to the optimized cascade cycle using propane, ethylene, and methane. The compressor work, specific energy, and coefficient of performance (COP) of the cascade cycles were compared and analyzed. The COP of the new cascade cycle that utilizes three-stage compression process 3 is 25% higher than that of basic single-stage process 1. Also, the new liquefaction cycle requires less specific power for the same amount of liquefied natural gas (LNG) produced by optimized cascade cycle. For example, the specific power and COP of the new cascade cycle are respectively up to 16% less and 14% higher than those of the optimized cascade cycle under the same conditions of feed gas composition and liquefaction rate.

Efficiency enhancement of the ocean thermal energy conversion system with a vapor–vapor ejector

In this article, 20 kW ocean thermal energy conversion with a vapor–vapor ejector is newly proposed. As a vapor–vapor ejector is installed in the system, the pressure difference between the turbine inlet and outlet increases. Therefore, the amount of the working fluid required for the total turbine work of 20 kW is less than when no vapor–vapor ejector is installed. Therefore, installing a vapor–vapor ejector in the system decreases the evaporation capacity and the pump work. The performance analysis considered the outlet pressure of the high-stage turbine, the mass flow ratio of the working fluid at the outlet of a separator just after the high-stage turbine, and the nozzle diameters of the vapor–vapor ejector. As the outlet pressure of high-stage turbine becomes lower, the turbine gross power of high-stage turbine and system efficiency increase although lower outlet pressure of high-stage turbine results in lower ejector performance. Similarly, in terms of mass flow ratio, the highest system efficiency was shown at mass flow ratio of 0.4 at the outlet of a separator just after the high-stage turbine. On the other hand, the performance of the ejector at mass flow ratio of 0.5 at the outlet of a separator was largest. When the nozzle diameters of the vapor–vapor ejector are properly designed, the vapor–vapor ejector shows the highest performance. After the optimization of the operation parameters, system efficiency of the proposed ocean thermal energy conversion power cycle was 2.47%, relatively 15% higher than that of the basic ocean thermal energy conversion power cycle (2.2%).

A Numerical Analysis on Heat Transfer Between the Air and the Liquid in a Hybrid Solar Collector

A hybrid solar collector, in this study, means the flat plate solar collector that has a fin-and-tube heat exchanger combining with a heat pump system. This collector can get the thermal energy from the ambient air for heating the circulated water in the collector when solar radiation is not enough. Thus, this collector can supply thermal energy to evaporator of the heat pump using heated water. For a foundation design of this collector, the numerical analysis was conducted for confirming the heat gain of water by the ambient air according to the length and the installation angle of fins in the fin-and-tube heat exchanger of solar collector. As a result, much of heat gain of water was obtained on the higher fin length with 30o of installation angle, but the pressure drop of air side increased adversely with heat transfer rate. Thus an area goodness factor considering both heat transfer enhancement and pressure drop of air side was also investigated. As a result, 50 mm of the fin length with 0o of installation angle has a good heat transfer enhancement than that of the other installation conditions compare with increment of pressure drop. However, mostly, higher heat transfer rate was obtained when the area goodness factor was low. So, it is needed to be considered whether the condition, maximum value of area goodness factor satisfying the heat transfer rate, can be expected or not.

Performance Characteristics of a Closed-Circuit Cooling Tower With Multiple Paths

The performance of a closed-circuit wet cooling tower (CWCT) with multiple paths having a rated capacity of 9 kW has been studied experimentally. When the CWCT has to operate with a partial load, the required quantity of cooling water reduces and thereby the velocity of the process fluid inside the tubes decreases. The velocity of the process fluid can be increased by installing blocking tubes in the heat exchanger. The test section in this experiment has multiple paths that have been used as the inlet for cooling water that flows from the top part of the heat exchanger. The heat exchanger consists of eight rows and 12 columns and the tubes are in a staggered arrangement. Heat and mass transfer coefficients and temperature drops were calculated with several variations including multiple paths. The results obtained from this study were compared with those reported and found to conform well. The investigation indicates that a CWCT operating with two paths has higher heat and mass transfer coefficients than with one path.

Performance Analysis of Brine Chiller Refrigeration Cycles

Comparison analysis of vapor-injection, two-stage compression and cascade refrigeration systems is carried out in this paper. R22 and R404A are employed for working fluid. Analysis conditions have been set based on brine chiller for semi-conductor manufacturing process. Results are summarized as follow: (1) Vapor-injection system has the highest coefficient of performance (COP) among systems. (2) Vapor-injection and cascade systems have the optimal intermediate pressure in terms of COP while that of two-stage compression system increases along with it (3) The optimal intermediate pressure is varied by change degree of compression works in both levels of compressor.

Experimental characteristics of a storage tank on a harvest-type ice storage system

This paper is concerned with the development of a new method for making and separating ice and saving floated ice by installing an evaporation plate at underwater within a storage tank. In a conventional harvest-type ice storage system, a tank saves ice by separating a formed ice from an installed evaporation plate, which is located above an ice storage tank as an ice storage system. A new harvest-type method shows better heat transfer efficiency than a convectional method. It is because the evaporation panel is directly contacted with water in a storage tank. Also, at a conventional system a circulating pump, a circulating water distributor and a piping are installed, but these components are not necessary in a new method. In this study two kinds of ice storage systems are experimentally investigated to study the thermal characteristics of ice storage tanks. The results show the applicable possibility and performance enhancement of a new type.