06-04-2012, 01:28 PM
Refrigeration
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Refrigeration - an introduction to the basics
This Danfoss publication must be regarded as a supplement to the comprehensive literature on refrigeration
that is available today and which is primarily aimed at readers with a professional relationship to the
refrigeration industry/trade e.g. refrigeration engineers and installers.
The contents of this book are intended to interest those who are not engaged every day with refrigeration
plant but who wish to extend their knowledge on the basic principles of appliances they see every day.
When compiling the material for the booklet a deliberate attempt was made to provide a thorough description
of the elementary principles involved together with an explanation in everyday language of the practical
design of the individual components.
Introduction
The job of a refrigeration plant is to cool articles
or substances down to, and maintain them at a
temperature lower than the ambient temperature.
Refrigeration can be defined as a process
that removes heat.
The oldest and most well-known among refrigerants
are ice, water, and air. In the beginning, the
sole purpose was to conserve food. The Chinese
were the first to find out that ice increased the life
and improved the taste of drinks and for centuries
Eskimos have conserved food by freezing it.
Fundamental terms
On an international level, agreement has been
reached on the use of the Systeme International
d’Unités - often referred to as the SI-system. For a
number of countries the implementation of the
SI-system is still an on-going process.
In this booklet the SI-system will be the primary
unit system used. However, in many parts of the
refrigeration community it is still practice to use
metric units or other alternative units. Therefore,
the practically used alternative units will be given
in parenthesis where needed.
The table shows the SI-units and the other often
used alternative units for the quantities that are
used in this booklet.
Heat, work, energy and power
Heat and work are both forms of energy that can
be transferred between objects or systems. The
transfer of heat is closely connected to the temperature
(or temperature difference) that exists
between two or more objects. By itself heat is always
transferred from an object with high temperature
to objects with lower temperatures.
Heating of water in a pot on a stove is a good
everyday example of the transfer of heat. The
stove plate becomes hot and heat is transferred
from the plate through the bottom of the pot and
to the water. The transfer of heat to the water
causes the temperature of the water to rise. In
other words, heating an object is the same as transferring
energy (heat) to the object.
The practical build-up of a refrigeration plant
Figure B gives details of momentary temperatures
in a refrigeration plant. At the compressor outlet
the pressure is 7.6 bar and the temperature is 60 °C
because of the presence of superheated gas. The
temperature in the upper part of the condenser
will quickly fall to saturation temperature, which
at the pressure concerned will be 34 °C, because
superheat is removed and condensation begins.
Pressure at the receiver outlet will remain more or
less the same, while subcooling of the liquid begins
because the temperature has fallen by 2 °C
to 32 °C.