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IRISH ENERGY CENTRE www.irish-energy.ie 10 G O O D P R AC T I C E C A S E ST U DY REFRIGERATION C ASE STUDIES As part of this scheme, the Irish Energy
Centre contracted Enviros March,
Manchester, U.K., to conduct
comprehensive energy audits of the
refrigeration facilities at Dairygold Co-op
Society, Mitchelstown, Co. Cork, and at
Glanbia Ingredients, Virginia, Co. Cavan. consumption and the publication of an
annual statement of their achievements.
The Irish Energy Centre provides support
to the membership by helping in the
preparation of annual statements and also
through workshops, seminars, news-
letters and site visits, and by providing
information on specific approaches to
energy efficiency. Refrigeration is used for a variety
of applications in industry but
primarily for cooling and freezing
of products, condensing vapours,
maintaining environmental
conditions and cold storage. It is a
major consumer of electricity and
in some sectors, particularly food,
drink and chemicals, it represents
a significant proportion of overall
site energy costs – up to 90% in
the case of some cold storage
facilities. There are many
opportunities to improve
efficiencies and save money.
Generally a cost reduction of 20%
may be achievable with a good
return on any investment made. The Annual Self Audit and Statement of
Energy Accounts Scheme is operated by
the Irish Energy Centre for the largest
energy consumers in the industrial sector
in Ireland. Participation in the scheme is
voluntary with member companies
committing to annual energy savings
targets, regular audits of their energy Dairygold Co-op Society, Mitchelstown, Co. Cork Figure 1: Schematic diagram of typical refrigeration system EVAPORATIVE CONDENSER SCREW COMPRESSOR LEVEL CONTROL LIQUID RECEIVER EXPANSION VALVE SHELL & TUBE EVAPORATOR GLYCOL
CHILLED WATER THE AUDIT The terms of reference of the audits in
both instances was to: 1 ) Examine the complete system loading including cooling, product, insulation and
auxiliary loads. 2 ) Review whole system and component design. 3 ) Audit metered electricity data and relate to factory production and weather conditions. 4 ) Propose energy efficiency opportunities in the above areas and in general operation /
maintenance procedures. 4 ) Provide advice and suggestions regarding energy savings, possible investment costs
and pay back periods. DAIRYGOLD CO-OP SOCIETY, MITCHELSTOWN The Dairygold food processing plant at
Mitchelstown, Co. Cork, is one of the
largest milk processors and ingredient
manufacturers in Ireland. Its products
include milk powders, spreads, casein, and
whey powder. Refrigeration is an essential
part of production at the plant, and it is
used for a wide range of cooling processes.
The refrigeration system costs 190,450
per annum to operate, accounting for
roughly 13% of the plant’s total electrical
energy consumption. The plant has a
comprehensive metering system that
monitors the system coefficients of
performance (COPs) of the refrigeration
system. The COPs typically ranges from 4-5
in summer time during peak production
periods to 1-2 in winter time when very
little milk is processed. System COP is
defined as the ratio of cooling achieved to
the total power to the system. builder/chillers and finally returned to the
main tank. The main tank supplies a
distribution header in the compressor
house, which supplies chilled water to 65
heat exchangers throughout the plant. Potential Savings The potential to reduce the operating
cost of the refrigeration system was
evaluated under the headings of cooling
load reduction, distribution system,
refrigeration system design and plant
operation. The audit revealed that cooling load could
be reduced by using softened water rather
than chilled water to carry out cooling of
the refrigeration compressor cylinder
head. This had been in operation on a
temporary basis but the soft water had
been run to drain after use. Savings of
about 4,400 per year could be achieved
if this soft water was returned to the
evaporative condenser sump and if the Dairygold Refrigeration System –
Description The Dairygold refrigeration system is an
ammonia system serving several
evaporators with individually controlled
evaporating temperatures. High-pressure
ammonia gas is generated in a central
compressor house that contains 13
compressors. The high-pressure, high-
temperature discharge gas is condensed
in 3 evaporative condensers mounted on
the roof of the compressor house. The condensed liquid refrigerant is stored
in a liquid receiver, which supplies 4
evaporator circuits, namely, the Baudelot
coolers, the Buco ice makers, the plate
heat exchanger and the glycol system. A
back-pressure valve is located on the inlet
to the condenser to ensure that sufficient
pressure / temperature is available to
remove ice from the Buco when it is
operating in the ice building mode. The glycol cooling circuit supplies 3 milk
pasteurisers and a few low temperature
cooling loads. The other 3 evaporator
circuits all form part of the chilled water
system. Chilled water is stored in a large
storage tank near the compressor house.
Returning chilled water from the site is
cooled in a separate, plate-type
evaporator before being remixed and
discharged to an inner tank in the main
chilled water system. The water is
pumped over a series of Baudelot chillers
and then over a series of Buco ice- GOOD PRATICE C ASE STUDY Refrigeration TABLE 1. SUMMARY OF AUDIT COSTS AND SAVINGS IDENTIFIED ANNUAL COST OF POSSIBLE ANNUAL SAVINGS AS A REFRIGERATION AUDIT ANNUAL SAVINGS PERCENTAGE SITE COST ( ) ( ) IDENTIFIED ( ) OF TOTAL COSTS DAIRYGOLD 190,450 15,500 87,500 46% GLANBIA INGREDIENTS 44,500 13,600 17,400 39% Dairygold Co-op Society, Mitchelstown Dairygold Co-op Society, Mitchelstown Figure 2. Line Diagram of Dairygold System water flow was controlled through a
solenoid/needle valve arrangement
that would only permit flow when the
compressors were operating. The distribution pumps at the plant
operate at full load much of the time. To
reduce the auxiliary loading during times
of low demand, six circuits were identified
for control by either installing variable
speed drives or by improved load
scheduling. The proposed measures
would achieve a saving of 38,100 with a 1 -year payback period. The present ammonia system is rather
complex and is not reliably meeting its
demand. The audit confirmed that the
previous practice of ice building in the
Buco system was not cost effective and
should be permanently discontinued.
Apart from the savings that this would
achieve in its own right, the decision to
discontinue ice making would also make it
possible to remove a number of valves
from the Buco refrigeration distribution
system. This would result in savings of 8,900 per annum by simplifying the operation of the plant and reducing
pressure drops. During the peak operating period, the
chilled water temperature often fails to
drop to its target of 2°C. The theoretical
maximum demand of the plant is currently 8.8 MW, but the temperature of the chilled water begins to rise once the load
exceeds 2.3 MW. The control system was
investigated to determine the reason for
the increase in chilled water temperature,
and ways of operating the plant were
examined to ensure that the theoretical
output is achieved. System observation
showed that the plate heat exchanger is
operating effectively but that the
operating temperatures of both the Buco
and the Baudelot evaporators are too
high, and as such, their performance was
inhibited. It was therefore suggested that
the pilot valves on the Buco and Baudelot
be adjusted to a setting of -3°C and
-4°C respectively to allow the evaporators
to perform at their design capacity. A number of other areas of plant
operation that could be improved upon
were identified. It was observed that it
would be better to operate a compressor
dedicated to the plate heat exchanger
whenever there is sufficient load. This
would save 1.90 per hour and would
increase system capacity. It was also seen
that the operation of all of the chilled
water systems as a common unit would
provide better control and some energy
savings. Improved control of condensing
temperature could save between 8,900
and 26,700 per annum, and improved
scheduling of the plant during winter time
could save between 5,100 and 9,500
per annum. circuit, with a Buco ice builder as
evaporator, a bank of 7 reciprocating
compressors and 3 evaporative
condensers. The Buco ice builder only works as an
ice builder during the night-rate hours.
During the day it works as a Baudelot
tank, pre-chilling return water
that is cooled by the ice stored in the
tank below. The 7 reciprocating compressors are
controlled by a Programmable Logic
Controller (PLC). The PLC maintains an
appropriate suction set point pressure
and prevents additional compressors
operating if the temperature of the water
leaving the ice tank is below 0.25°C.
This saves energy, as, when the tank is
below 0.25°C, the return temperature is
also low, and ice tends to build on the
plates to no useful purpose. Three
compressors make ice at night under
manual control. The 3 condensers are manually controlled
according to observed conditions,
but it seems that automatic control of the
condensers could save energy. GLANBIA INGREDIENTS, VIRGINIA, CO. CAVAN Glanbia Ingredients, Virginia is a
subsidiary of Glanbia plc, which is an
Irish-based international food company,
servicing a worldwide market. The
company was formed in 1997 through the
merger of Avonmore Foods plc and
Waterford Foods plc and it has
manufacturing operations in Europe and
North America. It is the largest dairy
processor in Ireland and the U.K.
combined. The Virginia plant produces
cream base and cream for Baileys Original
Irish Cream Liqueur and milk powders. Energy consumption at the site is very well
monitored. The refrigeration plant costs 44,500 per annum to operate, which accounts for approximately 13% of the
plant’s total electrical energy costs. The
audit revealed that there is potential for
energy cost savings through improved
control of the refrigeration plant. Glanbia Ingredients Refrigeration
System – Description The main chilled water system at the
Glanbia site consists of a single stage GOOD PRATICE C ASE STUDY Refrigeration Dairygold Co-op Society, Mitchelstown Figure 3. Line Diagram of Glanbia System Potential Savings The potential to reduce the operating cost
of the refrigeration system was evaluated
under the headings of cooling load
reduction, refrigeration system design and
plant operation. The main cooling loads
on the plant are 4 pasteurisers in the
cream plant, the Gilbey plant pasteuriser
and chilled water Contherms. The main
findings of the audit were as follows: » The load on the refrigeration system could be significantly reduced if the 4 milk and
cream pasteurisers were modified to a more
optimal design – such as using lake water for
cooling plates and utilising additional heat
recovery. This would result in a saving of 7,600 per annum, however the additional costs associated with retrofitting may prove
to be uneconomic. » If the proposed flat-rate electricity rate is accepted, there would be no advantage to
building ice at night. The cost of building ice
would increase to the point where it ceases could be significantly reduced and plant
performance improved. Although the
recommendations made in these two case
studies are site specific, the principles
could be broadly applied to other
refrigeration plants. For example, both
audits revealed the significant savings
potential of; » The use of Variable Speed Drives. » Optimising control systems to ensure that the overall system operates at the most
appropriate temperature and pressure set
points for the system load. » Reducing unnecessary cooling loads. Modifications may be required to achieve
these savings and may result in some
investment, but the simple payback on such
investments would be generally good. Various electricity tariffs may affect the
way a plant is operated and this can
be seen especially in the process of ice
building at night time. Although not
resulting in energy savings, cost
savings may be achieved by appropriate
tariff structures. In these case studies, the practice of ice-
building was found to be uneconomical
and the consultants recommended that it
be discontinued. The case studies show that, not only can
an audit identify significant energy cost
savings, it can also identify measures that
will improve the capacity of the system
and simplify operation of the plant. to be economical. If the plant is to operate
without building ice, another evaporator
would therefore be required. This would cost 51,000 , but would reduce the annual operating cost of the plant from a projected 61,000 to 33,000. » A new control system for the plant is proposed. It is estimated that this system
would save roughly 15% of annual energy
usage. The control system should be capable
of monitoring the system to enable
alteration of temperature and pressure set
points to maintain system efficiency and
ensure optimum utilisation and sequencing
of system compressors. » The system would improve the plant’s ability to provide process cooling when the ice
begins to disappear towards the end of the
day. CONCLUSION The energy audits at Dairygold and
Glanbia identified ways in which the
operating costs of the refrigeration plants GOOD PRATICE C ASE STUDY Refrigeration The Irish Energy Centre is funded by the Irish Government under the National
Development Plan 2000 – 2006 and part financed by the European Union. For further information on energy saving technologies and initiatives, contact:
Irish Energy Centre, Glasnevin, Dublin 9, Ireland. Tel: 01.836 9080. Fax: 01.837 2848. IRISH ENERGY CENTRE www.irish-energy.ie 120000 100000 80000 60000 40000 20000 0 Aug 99 Sep 99 Oct 99 KW No v 99 de c 99 Jan 00 Feb 00 Mar 00 Apr 00 Ma y 00 Jun 00 Jul 00 Glanbia Ingredients, Virginia, Co. Cavan Graph 1. Refrigeration Energy Consumption at Glanbia Ingredients, Virgina