Packaged units
13.1 INTRODUCTION
Factory-built
selt-contained systems are very familiar in the form of domestic appliance, and retail units such as vending
machines, drinking water chillers and display cabinets and counters. Although
larger systems may need to be finally assembled in situ, groups of components
can be conveniently delivered as pre-assamled units, and it is the exeption for
a system to be costum built from individual components, exept in the case of
large industrial installation.
The
main benefit of packaging is reduction in cost because a high proportion of the
total cost of a refrigerationor air-conditioning system is made up of work
which can be carried more quicly, efficiently and under better control within a
factory rather than on the installation site. There are
other advantages as well including.
1.
Correct selection and balance of
components
2.
Factory control of system
cleanliness , leak tightness, and in some cases, charging
3.
Inspection and testing of the
complete unit before it leaves the factory
4.
Delivery to the site complete and
in working order , so avoiding site dilays for materials
Simplified site installation,
with a minimum of disruption,
inconvenience and cost.
A possible
disadventage is that the size range of available packages may be limited, but
bespoke packaged are frequently possible. There is a risk of misapplication ,
and care should be taken understand the funtionality of the packaged and it is
limitations. Comprehensive application data is normally available for or
marketed packaged unit to allow designer or sales engineers to make the correct
selection for their purposes. Error in application stem mainly from a lack of
understending of the requirement and a tedency to purchase at the lowest price
without the protection of a clear sfecification. Once the application is fully
understood , assessed , and sfecified the possibility of error is reduced.
Tranport
application use specialized systems, and these are covered in chapter 17, as
are refrigerated display cabinets. Heat pump units are in chapter 27.
13.2 CONDENCING UNITS
A condencing unit is single package comprising the
compressor, the condencerreither air-or water-cooled) mounted on the a base
plate or frame. And all connecting piping. Together with the becessary wiring
and control to make the set fungtional (figure 13.1) condencing units
generally include a liquid receiver and
are ready for site connection to an evaporator. Accsessories such as pressure
cut-out switches, liquid line drier, sight glass and fan speed control may be
included, and the specification should be cerefully checked. They are built in
sizes from less than a kilowatt to more then 100 kw cooling capacity and may
incorporate more than one compressor. Air-cooled types may have two or more fans.
Sometimes
Figure 13.1 air-cooled condensing unit, 2-cyl semi-hermatic
compressor, suitable for several refrigerants (bitzer)
Figure 13.2 outdoor air-cooled condencing unit, scroll
compressor, suitable for several refrogerants(emerson climate technologies)
The condenser is separate in which case the packaged part is
termed a compressor unit. Units intended for outdoor application have
weatherproof housing may be suitable for either wall or floor mounting
(figure13.2) .
The
condensor is matched to the compressor, and cooling capacity data refer to air
or water temperature onto the condenser
as detailed in chapter 6. Since compressor and condensing unit do not
include and evaporator, they are not complete system and are not charged with
refrigerant, but have holding charge of dry nitrogen , or a little of the
refrigerant gas to maintain a slight positive pressure for transit. Suction and
liquid interconnecting lines and
electrical connection is installed on site. Ratings for condencing unit refer to entering temperature of the
condencing medium –air or water ; traditionally presented as curves (figure 13.3)
Figure 13.3 typical
condencing unit rating curves .
And now most commonlly found from manufacture ‘ computer
selection programmes. The standatrd condition for rating condencing unit are
given in european standard EN 13215, 2000. An estimate of condencing unit
capacity at an ambient condition different to the one provided can be made
graphically as indicated in cahapter 10.
Some
manufacturers of air-cooled condencing unit offter a range of condenser size
for each compressor, and thes eshould be
closely compared in terms of higher duty
and lower running cost. A small condenser will result in a higher condencing
temperature and therefore a drop in the refrigerating effect , since the liquid
enters the expansion valve hotter, and decrease in volume pumped due to the
lower volumetric efficiency. There will also be an increase in the drive motor
power.
13.3 Compressor packs
This term is used to describe an assembly of several
compressor mounted on a frame . complete with liquid receiver ,suction and
discarge headers, oil separation and oil oil return piping and controls (figures
13.4 and 13.5). they are widely used in centralized supermarket system and the
compressor serving the low – temperature evaporators and those serving the
chill loads may be mounted on the same frame and piped accordingly. Compressor packs
provide an efficient means of dealing with a wide span of refrigeration load.
There can be up to 10 small compressor ,either scroll or semi – hermetic , and this offers a large
number of capacity steps by switching compressors . alternatively a smaller
number of uneven compressor may be used. Compressor packs are usually custom
built to individual supermarket requirements although manufacturers will probably use
standardized design and contruction
methods.
The concept has been extended to factory – built rooftop
units where the compressor pack instralled within the overall condenser housing
(figure 13.6)
Figure 13.4 : semi – hermetic compressor pack
Figure 13.5 scroll compressor pack( hubbard)
Figure 13.6 scroll compressor pack within rooftop condenser
(hubbard)
13.4Chillers and air cooling packages(artic circle)
Chillers and air cooling packages
These are true packaged units in
the sense that all the parts of the refrigeration system and its control are
factory assembled and tested in the complete state. There are four
configurations:
Air cooling,air –cooled
Air cooling,water – cooled
Liquid cooling,air – cooled
Liquid cooling, water – cooled
— Air cooling packages are limited by the volume of cooled air
that needs to be in close proximity to the machinery. Example of fully packaged
air coolers are room air conditioners of the integral ‘through the wall’
type(chapter 25) and refrigerated vehicle units (chapter 17).
— Liquid cooling packages . frequently termed ‘ chiler ‘ .are self – contained system and
may be either air –cooled or water –cooled.(figure 13.7 and 13.8). because
chilled water is a highly flexible and
and efficient means of distributing cooling in many types of building ,the range of chillers
available on the market is very large. They can be found with compressors of
all types: reciprocating, scroll,screw and centrifugal together whit heat
exchangers of the shell and tube type or plate heat exchangers. Two refrigerant
circuits are commonly used because this gives more capacity flexibillity and
offers some redundancy.
— Air conditioning is the main application, but they are also used
for process cooling towers and this improves the chillers’ overall
thermodinamic effectiveness as compared to air-cooled chillers because heat
rejection is at or near the ambient air’s wet-bulb temperature rather than the
higher, sometimes much higher, dry-bulb temperature. Many chillers are designed
to work in reversed mode to provide heating when required.
Figure 13.7 :air – cooled water chiller ,nominal 600 kw
capacity with two r134a refrigerant circuits,single shell and tube evaporator ,
and capacity controlled semi – hermetic screw compressor (airedale)
Figure 13.8 :water – cooled water
chiller ,nominal 250 kw capacity with two r407c refrigerant circuits,brazed
plate heat exchangers (1 evaporator,2 condensors), and multiple scroll
compressors(airedale)
— Chillerratings, in
accordance whit EN14511, refer to the entering and leaving chilled fluid
temperatures and either the ambient
air-condition(air-cooled) or the cooling water entering and leaving
temperatures(water-cooled).
13.5
To avoid the constraint of having
all parts in one package,the evaporator
be split from tyhr condensor and the compressor going with either.
The
units is designed as a complate system but
the two parts are located separately and connected on site.On some small
units,flexsible refrigrant piping may be provided.largersplit packages must be
piped on site by normal methods,and then processed and charged as an open
plant.the height of a split unit evaporator above the condensor may be limited
and the supplier’s instructions should be folowed.see also multi-splits .
Evaporator
sets,as supplied as part of a split package or for apllication with a
condensing unit,are of three main types:
1.Air-conditioning,having the
air-colling coil with drip tray under,expansion valve,fan and motor,air
filters,inlet and outlet grilles.They may also include dampers and duct
connections for return and fresh air,heaters,humidifiers and various controls.
2.cold store evaporators ,having
the coil with drip tray under,fans,aand possibly the expansion valve.
3.cold store evaporators for use
blow + below 2˚C with defrost elements.
13.6 TESTING OF PACKAGED UNITS
Manufactures test procedures for
packged units may include some of the following:
1.
Safety pressure tests and leak
tests should always be include(see also section 11.3)
2.
rating test,from a representative unit which
forms the basis for thepublished capacity and application leaflets.
3.
Type test, which verify the
product as designed will function properlywhen applied within the required operating
envelope , and fail safe when subjected to certain abnormal operating modes.
4.
Rating check test on a proportion of
production units , to verify that standards are being maintained.
5.
Function test on all production
units , to verify correct operation of components .
6.
A short – running test at normal conditions
to check for reliability of operation plus ,possibly, an approximate capacity check.
7.
Run at maximum operating
conditions.
Factory test procedures can be costly, but are much more
efficient than performing the work in the field. Records will be kept of all
such test and , in the case of larger
units, manufacturers will, if asked , provide a certified copy of the test on
the equipment supplied .
13.7
other packages
A very large variety of smaller self – contained
refrigeration and air – conditioning packeges are made, mainly for the cunumer
durable market and small domestic application . they include:
·
Domestic refrigerators and
freezers .
·
Retail display cold and
freezer cabinets and counters .
·
Cooling trays for bottles (
beer, soft drinks, wines)
·
Instantaneous draught beer
coolers . these usually comprise a tank of constantly chilled water , through
which the beverage flows in stainless stel piping.
·
Ice makers – cubes and
flakes.
·
Cooled vending machines.
·
Soft ice – cream freezers.
·
Dehumidifiers, in which air
is passed first over the evaporator to remove moisture and then over the
condenser to re – heat and lower the humidity
( see figure 26.1)
·
Drinking water chillers.
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