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Related Concept Videos

Hess's Law03:40

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There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.
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Mechanisms of Heat Transfer01:14

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Heat transfer between the human body and its environment occurs through four main mechanisms: conduction, convection, radiation, and evaporation.
Conduction, accounting for approximately 3% of body heat loss at rest, is the process of exchanging heat between molecules of two materials in direct contact. This can result in both heat loss and gain. For instance, when the body is submerged in water, which conducts heat 20 times more effectively than air, it can either lose or gain significant...
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Mechanism of heat transfer01:19

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Understanding heat transfer mechanisms is essential for understanding how our bodies maintain balance in different environmental conditions. When the environment is thermoneutral, the body is in a state of balance, neither using nor releasing energy to maintain its core temperature. However, when the environment is not thermoneutral, the body employs four heat transfer mechanisms to maintain homeostasis: conduction, convection, evaporation, and radiation. These mechanisms facilitate heat...
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When a substance—isolated from its environment—is subjected to heat changes, corresponding changes in temperature and phase of the substance is observed; this is graphically represented by heating and cooling curves.
For instance, the addition of heat raises the temperature of a solid; the amount of heat absorbed depends on the heat capacity of the solid (q = mcsolidΔT). According to thermochemistry, the relation between the amount of heat absorbed or released by a substance, q, and its...
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Refrigerators and Heat Pumps01:07

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Refrigerators or heat pumps are heat engines operating in a reverse direction. For a refrigerator, the focus is on removing heat from a specific area, whereas, for a heat pump, the focus is on dumping heat into one particular area. A refrigerator (or heat pump) absorbs heat Qc from the cold reservoir at Kelvin temperature Tc and discards heat Qh to the hot reservoir at Kelvin temperature Th, while work W is done on the engine’s working substance.
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Just as interesting as the effects of heat transfer on a system are the methods by which the heat transfer occur. Whenever there is a temperature difference, heat transfer occurs. It may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice box. So many processes involve heat transfer that it is hard to imagine a situation where no heat transfer occurs. Yet, every heat transfer takes place by only three methods: conduction, convection, and radiation.
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Heat Transfer Analysis between R744 and HFOs inside Plate Heat Exchangers.

Anas F A Elbarghthi1, Mohammad Yousef Hdaib1, Václav Dvořák1

  • 1Department of Applied Mechanics, Faculty of Mechanical Engineering, Technical University of Liberec, Stdentská 1402/2, 46117 Liberec, Czech Republic.

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|August 26, 2022
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Summary

Carbon dioxide (R744) is a viable hot stream for plate heat exchangers (PHE), especially with higher inlet pressure and lower superheating. This study investigated R744 with newer HFO refrigerants to optimize heat transfer.

Keywords:
R744heat transfer analysisplate heat exchanger local analysispressure drop analysis

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Area of Science:

  • Thermodynamics and Heat Transfer
  • Refrigeration and Air Conditioning Engineering
  • Chemical Engineering

Background:

  • Plate heat exchangers (PHE) are critical in various applications, necessitating exploration of novel heat sources.
  • R744 exhibits favorable thermophysical properties for high-temperature PHE applications due to its smooth property variations at supercritical conditions.

Purpose of the Study:

  • To evaluate the performance of R744 as a hot stream in PHEs using advanced refrigerants.
  • To analyze heat transfer coefficients, pressure drop, and effectiveness under varying operational parameters.

Main Methods:

  • A local design approach was developed using MATLAB and the NIST database for real gases.
  • Three-phase flow (liquid, two-phase, gas) of R744 and HFOs (R1234yf, R1234ze(E), R1234ze(Z), R1233zd(E)) was simulated.
  • A two-step analysis examined the influence of the number of plates, hot stream pressure, and cold stream superheating.

Main Results:

  • Insignificant performance variations were observed for PHEs with more than 40 plates.
  • Increasing R744 pressure from 10 to 12 MPa significantly enhanced two-phase convection coefficients for all tested HFOs.
  • Higher cold stream superheating temperature differences reduced two-phase convection coefficients.

Conclusions:

  • R744 is confirmed as a suitable driving heat source for PHEs.
  • Optimal performance is achieved at higher R744 inlet pressures and lower cold stream superheating differences.
  • The study provides valuable data for designing efficient PHE systems utilizing R744 and HFOs.