Lecture 19: Illustrations: Heat exchangers, throttling devices System...

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ME 200 –Thermodynamics I Spring 2016

Lecture 19: Illustrations: Heat exchangers, throttling devices

System Integration

Yong LiShanghai Jiao Tong University

Institute of Refrigeration and Cryogenics800 Dong Chuan Road Shanghai, 200240, P. R. China

Email : liyo@sjtu.edu.cnPhone: 86-21-34206056; Fax: 86-21-34206056

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Lecture Contents Last lecture---Open system at steady state (Control Volume system)

» Mass balance Energy balance

This lecture---Illustrations» Heat exchangers, throttling devices» System Integration

nozzles, diffusers,

turbinescompressorspumps

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Throttling Devices (Throttling Valve) Throttling valve::: a steady-flow engineering device used to

produce a significant pressure drop along with (usually) a large drop in temperature» In a throttling valve, enthalpy remains constant» No work device» Heat transfer almost always negligible

– small area; less time available» PE and KE changes usually negligible

Concepts

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Throttling Devices Analysis

» For ideal gas T may remain the same » For real fluid T could increase/decrease; » For refrigerants the p causes T

0 0 0 00

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EXAMPLE 1 Expansion of Refrigerant-134a in a Refrigerator

Refrigerant-134a enters the capillary tube of a refrigerator as saturated liquid at 0.8 MPa and is throttled to a pressure of 0.12 MPa. Determine the quality of the refrigerant at the final state and the temperature drop during this process.

Assumptions 1) Qcv=0 2) KE=0

hf <h2 < hg

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Heat Exchanger

Heat exchanger:::a steady-flow engineering device used to carry out heat transfer between two fluid streams without or with direct mixing» No work device» Heat transfer occurs between fluids within the device;» Heat transfer from the heat exchanger to its surroundings usually

negligible» PE and KE changes usually negligible

Concepts

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Common Heat Exchanger (HX) Types

Cross-flow HX.

Tube-within-tube counter flow HX.

Tube-within-tube parallel flow HX

Flate plate HX.

Tube-in-shell

Finned Tube HX

Direct contact HX.

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0 0 0 0 00 0

0

1 1 2 2 1 1i i i i e em h m h m h

1 2 1i i em m m

1 1 1i em m m 2 2 2i em m m

1 1 2 2 1 1 2 2i i i i e e e em h m h m h m h 1 1 1 1 2 2 2 2( )e e i i e e i im h m h m h m h

1 1 1 2 2 2( ) ( )e i e im h h m h h

Direct contact HX.

counter flow HX.

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Example 2 Power Plant Condenser Known: Steam is condensed at steady state by

interacting with a separate liquid water stream.

Find:

1) The mass flow rate ratio of the cooling water to steam

2) The energy transfer rate

Assumptions:1. CV in steady-state.2. Q syssur＝ 03. KE=0， PE=04. states 2, 3, and 4,h hf (T)

Analysis:

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Example 2 continued

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Given:

Example 3: Direct contact Heat exchanger

Assumptions:» Steady-flow process» W= 0» Q = 0» KE=PE=0

Find: The ratio of the mass flow rates of the feedwater and the superheated vapor

Superheated steam 1000 kPa

Water1000 kPa

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Given:

Example 3: Direct contact Heat exchanger

Find: The ratio of the mass flow rates of the feedwater and the superheated vapor

Superheated steam 1000 kPa

Water1000 kPa

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Steady Flow Devices Summary

Always apply mass balance and energy balance with any applicable assumptions

Nozzles are used to V at the expense of p ; h from inlet to exit

Diffusers are used to increase p at the expense of V; h from inlet to exit

Turbines produce work output via expansion of working fluid; h and p from inlet to exit

Compressors need work input for compression of working fluid; h and p from inlet to exit

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Steady Flow Systems Summary (contd.)

Difference between compressors (gas) and pumps (incompressible liquid) is the type of working fluid

Throttling valves have large p and h remains constant; » For ideal gas T may remain the same » For real fluid T could increase/decrease; » For refrigerants the p causes T

Heat exchangers have either direct or indirect heat transfer between fluid streams

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Throttling Valve and Heat Exchanger

Throttle valves are used for flow control or for providing temperature drop in refrigeration cycles

Heat exchangers are used in many applications to modify the state of fluid as required

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System Integration

System integration ::: combine components to achieve some overall objective, subject to constraints such as minimum total cost, minimum space….

Vapor compressionrefrigeration system

Steam power cycle

Concepts

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Known: A system consisting of a heat-recovery steam generator and a turbine with operating data.

Find:

1) Power developed by the turbine, in kJ/s.

2) Turbine inlet temperature, in oC.

Example 4 Waste Heat Recovery System

Assumptions:1. CV in steady-state.2. Q syssur＝ 03. KE=0， PE=04. Combustion productsare modeled as air asan ideal gas

Analysis:

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Example 4 continuedFind:

1) Power developed by the turbine

2) Turbine inlet temperature T4.