An introduction to commercial and industrial refrigeration systems T

COVER STORY
SKILLS WORKSHOP
HVAC&R SKILLS WORKSHOP
MODULE 35
PULLOUT
PULLOUT
An introduction to
commercial and industrial
refrigeration systems
Produce case
Although refrigeration was invented for the
preservation of food, there are many other
commercial uses to which it has been applied.
This article will highlight those applications
and the equipment used.
Freezing and
chilling of products
In order to achieve an acceptable freezing
or chilling performance, the designer needs
techniques to enable the accurate prediction of
freezing and chilling rates and also product heat
loads during these processes.
Factors affecting
freezing and chilling
• the initial temperature of the product
During freezing or chilling, the temperature of
the product changes with time. The variation of
temperature depends on a number of factors:
• the surface heat transfer conditions between
the product and the cooling medium
• the size of the product
• the shape of the product
• the thermal properties of the product
(a) density
(b) thermal conductivity
(c) specific heat
(d) freezing temperature
(e) latent heat content
• the temperature of the cooling medium
The exact physical analysis of freezing and
chilling situations is a complex mathematical
exercise, but approximate freezing and chilling
methods that have been developed from
research are available.
Freezing or chilling of solids
Freezers used are generally of two major types
– air blast freezers and plate freezers. Cryogenic
tunnels can be considered as a variation on
an air blast freezer, however, these are only
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SKILLS WORKSHOP
of mobile cool and cold rooms transporting
perishable goods throughout the world, not
all of them necessarily refrigerated. In some
instances the insulation and the frozen product
may provide sufficient protection for the time
required to effect delivery.
Cool room
A room refrigerated to chill and store a product
without freezing it. Their size can vary from a
small room to suit a retail butcher’s shop to quite
large premises where bulk storage of product
is required for extended periods, e.g. fruit.
A common method of construction utilises
prefabricated panels consisting of two sheets of
painted steel or aluminium bonded to each side
of a suitable, impermeable, self extinguishing,
insulating material
The meat preparation and packaging room in
a supermarket is usually a ‘cool’ room with the
operating temperature level a compromise
between product and personnel requirements.
Industrial blast chiller.
appropriate for very short term solutions or
very high value products due to their extreme
carbon footprint when compared with vapour
compression refrigeration systems.
The techniques to predict freezing rates,
freezing times and heat loads are similar for
both types. Air blast chillers are widely used for
chilling products.
One method uses a modified Plank’s equation
which takes account of the six factors listed in
the previous sub-clause plus the thermal centre
or mass average temperature at the end of the
freezing process.
Of the factors listed above all but the last two
are generally fixed. The role of the designer is
to manipulate these to optimise the design in
terms of cost and energy efficiency.
Liquid chilling
There are many important situations requiring
the chilling of liquids including milk cooling,
production of chilled water used in processing,
cooling of beer and cooling of secondary
refrigerants (glycol or brine).
The ways that cooling is achieved can be
divided into two major types:
• continuous use of in-line heat exchangers
• batch use of external heat exchangers,
jackets or cooling coils to chill a reservoir
of liquid
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As for solids chilling, in both cases the heat
flow depends on the overall resistance to heat
transfer, the area across which heat transfer can
occur and the mean temperature difference
between the liquid and the cooling medium
Q = U A ∆ tav
where:
Q = heat flow (W)
U = o
verall heat transfer
coefficient (W/m2°C)
A = heat transfer area (m2)
ean temperature difference
Dtav = m
between the cooling medium
and the liquid (°C)
Refrigeration plant
A term used to describe collectively the items
of equipment used to compress, condense,
expand and evaporate the refrigerant medium
used in the system.
The following items relate specifically
to refrigeration:
Cold storage
The trade or process of preserving perishables
by refrigeration. Although this discussion
concentrates on static installations, it should
be remembered that there are many types
Refrigerated enclosures
and cabinets
Amongst the smallest refrigerated cabinets is
the domestic refrigerator.
In commercial applications, the merchandising
and display cabinets are usually purpose built
for the specific task.
Typical examples include:
• Drink cabinet – usually vertical with glass
doors, multi shelved and incorporating a
self contained air cooled refrigeration system
rejecting the heat into the immediate
environment.
• Dairy cabinet – usually vertical, open and
multi shelved. Refrigeration is usually
provided by a remote (either water or air
cooled) condensing unit.
• Island case – normally open top, used for
displaying packaged frozen food. In some
applications sections of casing are brought
together extending the cabinet to more
than 15 metres long. Refrigeration is usually
provided by a remote (either water or air
cooled) condensing unit.
• Closed display case – used for displaying
fresh meat and fish, refrigeration is usually
provided by a remote (either water or air
cooled) condensing unit.
• Fruit and vegetable case – usually single
deck, sloping towards the front. Refrigeration
is usually provided by a remote (either water
or air cooled) condensing unit.
• Cryogenic container – for the long term
storage of medical and pharmaceutical
products at very low temperature, e.g.
animal and human semen.
COVER STORY
HVAC&R SKILLS WORKSHOP
MODULE 35
PULLOUT
Of the above, the drink, dairy and island
styles are commonly referred to as ‘reach in
merchandisers’.
Plate freezer
A contact freezer in which the refrigerated
surface is a flat metal plate.
Cold room and freezer room
A room refrigerated to freeze and store a
product. Commonly found in the meat and fish
industries, especially for export.
PULLOUT
Construction may be as for cool rooms or from
PIR insulation between two concrete skins
which provides very good thermal stability.
This alternate form of construction has found
appeal in the wine industry to encase large
fermentation and storage tanks.
Chest freezer
Access to this type is by raising a lid, also
commonly referred to as a deep freeze.
Well freezer
This type of cabinet may be open or closed
top depending on usage. It is commonly used
in supermarkets, ice cream parlours and some
restaurants for displaying and serving from large
ice cream containers.
Quick freezing
The process of freezing a food or other product
at a rapid rate of temperature reduction to
produce a desired crystalline structure in the
frozen product. This is generally appropriate for
small products such as peas, corn or chips.
Freezing tunnel
An elongated enclosure provided with rapid
cold air circulation for the freezing of foodstuffs.
Used in the processed meat and vegetable
industry. Product is usually automatically
conveyed through the freezing tunnel.
Horizontal plate freezer.
evaporator coils of refrigerating units. Common
defrost methods are electric element and
reverse cycle.
Intercooler
An intercooler is the intermediate vessel
between the high stage and low stage in
a compound system. The purpose of the
intercooler is to cool the discharge gas of the
low-stage compressor, as well as subcooling
the liquid before it is fed to the low stage
evaporators.
Baudelot cooler
Forced draft cooler
Used almost exclusively in applications where
air is used as a secondary medium to maintain
or cool the product.
Frost heave
The term used to describe the rising of a cold
room floor due to the expansion of freezing
water in the supporting soil. It can also occur in
cold room walls that are poorly or inadequately
vapour sealed.
Defrosting
This is the planned process of removing or
preventing the accumulation of frost (ice) on
Used in industrial applications where it is desired
to cool the fluid near to freezing point e.g. a milk
or beverage cooler. The fluid is circulated over
the outside of the open heat exchanger thus
facilitating the cleaning of surface.
Liquid overfeed system
(liquid recirculation)
Cascade refrigerating system
One having two or more separate refrigerant
circuits, each with a compressor, condenser
expansion device and evaporator, where the
evaporator of one circuit cools the condenser
of another (lower temperature) circuit.
Cascade refrigerating systems have become
popular in supermarket systems using CO2 for
the lower temperature circuit and a HFC in
the higher temperature circuit. This has meant
that only CO2 is circulated to the evaporators
in the cabinets (liquid recirculation to the dairy
cases and direct expansion to the freezers)
and the HFC circuit has a very small amount
of refrigerant in it. The cost of lost refrigerant
and damage to the environment are both
dramatically reduced. n
DA08 HVAC&R – An
Introduction manual
T H E A U S T R A L I A N I N S T I T U T E O F A I R C O N D I T I O N I N G A N D H E AT I N G
A system feeding an evaporator with refrigerant
at a rate greater than the amount evaporated
which means there is a mixture of gas and liquid
leaving the evaporator along the suction line.
The gas returning from the evaporator will be
saturated and not superheated.
Next month’s workshop — Solar air conditioning
DA08
HVAC&R AN INTRODUCTION
APPLICATION MANUAL
Australian Institute of
Refrigeration, Air Conditioning
and Heating
Information in this
month’s Skills Workshop
has been compiled from
AIRAH’s DA08 HVAC&R An
Introduction manual.
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