ASHRAE Journal - March 2012 - 94

refrigeration

Laurel and Hardy consider the complexities of the refrigeration cycle and temperature lift while moving a piano up the hill.

the temperature Lift
By andy Pearson, Ph.D., C.eng., Member ASHRAE

From “The Music Box” by Hal Roach Studios

T

his column is the first in a series that explores some of the concepts associated with refrigeration and heat pump systems. It is aimed

at people who are not immersed in refrigeration technology, and who perhaps don’t fully understand the jargon. I hope seasoned hands will find the series useful, too, even if only to help explain a point to their less experienced colleagues.
In the natural world heat flows, as a rule, between adjacent objects if one is warmer than the other. The flow of heat is always from the warmer object to the cooler one, because heat is a form of energy, and the energy is being more evenly distributed. It is a bit like a bathtub full of water; if you push the water up one end of the tub and then release it, the water will redistribute itself so that the level is equal across the whole tub. Left to its own devices, heat, like the bathwater, always finds a level. People who want to sound
94 ASHRAE Journal

smart call this entropy, but the name is not important; it is just smoothing out the bumps. Refrigerators and heat pumps are mysterious because they seem to break this rule. They keep a temperature difference between the cold end of the system (inside the fridge) and the hot end (down the back), which is like being able to keep all the bathwater down at one end of the tub. On closer inspection, it becomes apparent that the fridge is not, in fact, breaking the rules at all. To understand this,
ashrae.org

we need to appreciate the concept of temperature lift. To make stuff colder or to keep it cold, we need to draw heat energy out of it. This is done by placing the stuff in surroundings that are colder than the stuff. The heat energy flows from the warmer stuff to the colder surroundings. To get rid of that heat energy, we need to make the energy warmer than the outside of the refrigerator. This is done by working on the heat—putting more energy into it—so that it can flow to the outside. How we turn the cold energy into warmer energy will be covered later in the series, so don’t worry about that right now. Just consider the four temperature levels described. The stuff in the fridge is one temperature and the surroundings are colder. The heat is then lifted to a high temperature and the outside of the fridge is colder. To get the heat from the temperature of the stuff in the fridge up to the temperature outside the fridge, it is necessary first to drop the heat down to the temperature of the
March 2012



ASHRAE Journal - March 2012

Table of Contents for the Digital Edition of ASHRAE Journal - March 2012

ASHRAE Journal - March 2012
Contents
Commentary
Show Coverage
Meetings and Shows
Feature Articles
ASHRAE’s Best: 2012 Technology Awards
Total Energy Wheel Control in a Dedicated OA System
Optimizing Design & Control of Chilled Water Plants Part 4: Chiller & Cooling Tower Selection
Improving DHW System Performance
Technology Award Case Studies:
Ice Rink Uses CO2 System
Standing Columns
Building Sciences
Emerging Technologies
Refrigeration
Special Products
IAQ Applications
People
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - March 2012 - Intro
ASHRAE Journal - March 2012 - ASHRAE Journal - March 2012
ASHRAE Journal - March 2012 - Cover2
ASHRAE Journal - March 2012 - 1
ASHRAE Journal - March 2012 - 2
ASHRAE Journal - March 2012 - Contents
ASHRAE Journal - March 2012 - Commentary
ASHRAE Journal - March 2012 - 5
ASHRAE Journal - March 2012 - Show Coverage
ASHRAE Journal - March 2012 - 7
ASHRAE Journal - March 2012 - 8
ASHRAE Journal - March 2012 - 9
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ASHRAE Journal - March 2012 - 24
ASHRAE Journal - March 2012 - 25
ASHRAE Journal - March 2012 - Meetings and Shows
ASHRAE Journal - March 2012 - 27
ASHRAE Journal - March 2012 - ASHRAE’s Best: 2012 Technology Awards
ASHRAE Journal - March 2012 - 29
ASHRAE Journal - March 2012 - 30
ASHRAE Journal - March 2012 - 31
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ASHRAE Journal - March 2012 - 37
ASHRAE Journal - March 2012 - Ice Rink Uses CO2 System
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ASHRAE Journal - March 2012 - 43
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ASHRAE Journal - March 2012 - 45
ASHRAE Journal - March 2012 - Total Energy Wheel Control in a Dedicated OA System
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ASHRAE Journal - March 2012 - Optimizing Design & Control of Chilled Water Plants Part 4: Chiller & Cooling Tower Selection
ASHRAE Journal - March 2012 - 61
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ASHRAE Journal - March 2012 - 71
ASHRAE Journal - March 2012 - Improving DHW System Performance
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ASHRAE Journal - March 2012 - 76
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ASHRAE Journal - March 2012 - 78
ASHRAE Journal - March 2012 - 79
ASHRAE Journal - March 2012 - Building Sciences
ASHRAE Journal - March 2012 - 81
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ASHRAE Journal - March 2012 - 83
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ASHRAE Journal - March 2012 - 85
ASHRAE Journal - March 2012 - 86
ASHRAE Journal - March 2012 - 87
ASHRAE Journal - March 2012 - Emerging Technologies
ASHRAE Journal - March 2012 - 89
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ASHRAE Journal - March 2012 - 91
ASHRAE Journal - March 2012 - 92
ASHRAE Journal - March 2012 - 93
ASHRAE Journal - March 2012 - Refrigeration
ASHRAE Journal - March 2012 - 95
ASHRAE Journal - March 2012 - Special Products
ASHRAE Journal - March 2012 - 97
ASHRAE Journal - March 2012 - IAQ Applications
ASHRAE Journal - March 2012 - 99
ASHRAE Journal - March 2012 - People
ASHRAE Journal - March 2012 - 101
ASHRAE Journal - March 2012 - Products
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ASHRAE Journal - March 2012 - 108
ASHRAE Journal - March 2012 - 109
ASHRAE Journal - March 2012 - Classified Advertising
ASHRAE Journal - March 2012 - 111
ASHRAE Journal - March 2012 - Advertisers Index
ASHRAE Journal - March 2012 - Cover3
ASHRAE Journal - March 2012 - Cover4
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