10 January 2015
The use of a refrigeration system as part of the manufacturing or business process is widespread. Depending on which industry sector your business works within, the power consumption of the refrigeration system will usually represent a large portion of your site’s utilities overheads.
Whilst there have always been opportunities to cut these costs, the recent and continually increasing costs of providing this service has made the practice of conducting thorough energy audits a must. The primary focus of the audit described here is the cost of the power consumed.
Here are 10 key areas which should figure in a power saving audit of a refrigeration plant:
1. System Monitoring and Controls
As detailed throughout this section, any sizeable system should have a Scada or Building Management System (BMS) installed to optimise control and energy efficiency. This not only provides “at a glance” operational data to the Plant Engineer, but allows the degree of automated control required to provide energy saving opportunities that manual intervention would not be able to match nor sustain.
A. Screw compressor efficiency
Despite the advances in capacity slide control, large screw compressors perform less efficiently when used at part load operation and work best at full load. The percentage by which power consumption increases is relative to the percentage load as the loading decreases. Typically, a screw compressor will use 65% of maximum power at only 40% load capacity when capacity slide control is used.
A plant will have non-productive periods when the screw compressors are not ideally matched to the lower refrigeration load. This can lead to cases where two compressors are running at part loads, when the same duty could be accomplished by one compressor running at near full load.
The Scada or BMS can be programmed to match the best combination of compressors during peak and off peak periods. As few as possible should be running and as close to maximum load as possible.
Piston compressors show superior energy efficiency when working at part load operations leaving the screw compressors free to satisfy the base load.
The correct staging and control of the compressors can yield savings of up to 15%.
B. Chilled and Frozen Room Temperature Control
During production periods there is a necessity to maintain pre-set chilled and frozen room temperatures within a narrow range. However, outside of these periods, when the finished product and the work in progress are safely stored away in chill / frozen stores and no product or materials are exposed, there is no obvious need to maintain these same temperatures.
An audit of the room temperatures combined with Scada or BMS control will allow finer management of overall and individual room temperatures. Co-ordinating the timing and degree of room temperature shifts to match the production periods will provide optimised energy use and process efficiency.
C. Evaporator Defrost Timing Control
Generally the defrost times for the production area evaporators are set at fixed intervals throughout the week with little regard to actual need i.e it is not necessary to defrost at the same intervals required for production when an area is inactive due to non-production.
Evaporator defrosting will generally use hot gas or electric elements to introduce heat into the evaporator and effect the defrost. If there is only one evaporator, the room temperature can rise significantly and the heat will then have to be removed when the unit starts back up introducing unnecessary load and instability to the system.
The Scada or BMS can be programmed to increase the defrost intervals out of hours so as to reduce energy usage.
Savings can be up to 2% of the system total.
D. Variable head pressure control
The head pressure setting on the condensers is usually fixed at a level determined during commissioning and is rarely changed once the system is settled. There is a direct relationship between condenser head pressure and compressor power consumption. The higher it is the greater the power consumption. This setting does not necessarily have to be fixed and significant power savings are possible by controlling this head pressure setting as a function of the ambient wet bulb temperature prevailing at that time.
Subject to the minimum system requirements the Scada or BMS can monitor the wet bulb temperature and adjust the head pressure setting according to a pre-established algorithm.
Savings of between 9 and 12% on the power bill are possible by use of this method.
2. Condenser Efficiency
Maintenance of maximum condenser efficiency will also yield power savings though the minimisation of head pressure and reduced usage of the air blast fans.
Areas for assessment are:
A. Location of condensers
Condensers should be located in an area away from other buildings so they do not compete for the same air,
They should also be a suitable distance from other condensers as well as any hot air/ exhaust outlets from the plant.
B. Installation of automatic air bleeds
A build-up of air and non–condensable elements inside the coils can significantly affect the condenser performance. Each condenser or group of condensers should be fitted with an automatic air bleed at a high point of the system. This will ensure that air is regularly removed in small quantities rather than by a manual method where large quantities may have to be removed all at once.
One significantly negative consequence of manually removing large quantities of air (in the case of ammonia systems) is the smell of ammonia which may cause distress to site personnel.
C. Use of VFD control on condenser fan motors
The fitting of a Variable Frequency Drive (VFD) will yield power savings as the fans will only run at the speed required to maintain the head pressure in the system, resulting in more stable head pressure.
This area alone can save 2 to 3% of the refrigeration power bill.
3. VFD speed control on screw compressors
Compared to the earlier example of a screw compressor with capacity slide control to manage part load performance, by comparison at 40% load the VFD controlled screw compressor will only consume 40% of maximum power (compared to 65% with the capacity slide control).
With the VFD the relationship between load and power consumption is virtually linear in the range 20 to 80%. The secondary advantage of the VFD is that it will significantly reduce the starting current required.
4. High Efficiency motors
All motors should be rated to Eff1 for fridge compressor duty. Also consider the replacement of rewound motors that will have a lower efficiency than new ones.
5. Review of suction pipework
The fact that the suction pipe sizing was correct when the plant was built does not mean it is still the case. As many plants evolve and grow so the vapour load demands on the suction pipework increase to an extent that the pressure drop in the pipework becomes excessive. The will increase power consumption as the compressors have to be run at a lower suction pressure to maintain the correct pressure at the point of use for the temperature requirements.
6. Review of refrigeration pipework insulation
It is common to see the older pipework insulation sweating profusely in high humidity conditions or even frozen on the surface. This is because the insulation is waterlogged and has effectively failed. Inadequate insulation will affect the efficiency of the system and so raise power consumption.
7. Power Factor (PF) Correction
Whilst not directly associated with refrigeration the recent changes in the way Queensland Energy customers are billed for their power use means that businesses will be financially disadvantaged by having a poor power factor.
Since the refrigeration power requirements will form a large part of the site power costs any savings due to the installation of PF correction will have benefits in the cost of running it.
Typically the financial disadvantage of having a site PF of 0.85 compared to a corrected PF of 0.95 is nearly 12%.
8. Heat Recovery
A screw compressor will feature an oil cooler generally in the form of a shell and tube heat exchanger. This would normally be cooled via a closed circuit loop with an evaporative condenser or, in some cases, by the refrigerant itself. The heat liberated from the compressor oil is significant and this could be utilised as part of a hot water system or boiler hotwell top up etc.
Approximately 1-2% of the motor power input can be recovered as low grade heat by this means.
9. Multi-staging – High Stage / Low stage vs Single stage
In a plant where the system runs at generally the same temperature range there would be little point in considering multi-staging.
However, where there are significantly different temperature requirements such as chilling and freezer storage, running a single stage system would not be efficient as the complete system would have to be set up for the lowest temperature.
This would not only consume power unnecessarily but would make control of the higher temperature applications more difficult. Splitting the system up into at least two stages would improve the system efficiency.
10. General on-site maintenance
A. Regular inspection and maintenance of hot gas valves
Hot gas valves (where fitted) should be inspected on a regular basis for gas tightness when closed. This will likely involve a re-kit at regular intervals. A passing or stuck hot gas valve will constantly bleed hot gas back into the vapour system and reduce evaporator performance and system efficiency.
B. Regular inspection and cleaning of in line strainers
A blocked strainer will affect the flow of refrigerants to the process; these should be inspected and cleaned regularly.
C. Regular inspection and maintenance of freezer door seals and coldstore curtains
These should be inspected on a regular basis to ensure the minimum of air leakage into busy traffic areas where the moisture ingress will rapidly freeze onto evaporator coils and reduce their performance.
D. Regular cleaning of evaporative and air blast condenser coils
The scheduled cleaning of the coils will help to keep the cooling efficiency high and the head pressures under control.
E. Regular cleaning of evaporators
Shopfloor evaporators should be regularly cleaned, particularly in areas where powders or cardboard / polystyrene packaging are handled. This will ensure the efficient operation of the equipment.
F. Regular leak inspection on Hydrofluorocarbon (HFC) plants
Check for leaks on flexible lines. Ensure that all lines are securely fastened and there is no opportunity for flexible lines to rub on the compressor body. Replace all corroded metal parts on the refrigeration circuit.
2 other important areas for consideration:
1. Systems using HFC as the refrigerant
The recent sharp increase in the cost of many HFC based refrigerants will continue as the supplier base becomes even scarcer. This makes the potential running costs of any system using HFCs very high going forward as a small leak or complete loss of charge will result in a large cost to recharge the refrigerant.
Ammonia has no ozone damaging potential and so has not been increased in price because of this. The use of ammonia in future smaller system projects should be considered to reduce the costs associated with some refrigerants.
For existing HFC systems, it is not simply a matter of draining out the HFC and pumping in ammonia in its place, there are many differences between the two systems which would make this an impossible option in most cases.
The table below shows the current price and industry predicted prices for some common HFC Refrigerants in the future.
2. Cryogenic vs Mechanical Refrigeration
Some food plants will use liquid nitrogen tunnel freezers or freezer cabinets as a means of providing rapid freezing of their products. The use of liquid cryogens should be reviewed and compared to the mechanical (i.e. ammonia) centralized system alternative.
The cost of the cryogen is high and together with the rental of the system components (e.g. receiver, freezer tunnel), adds up to a process that would have been cost effective to install initially but expensive to run in the long term.
A review of the long term costs of the cryogenic system versus that of the mechanical alternative utilising, for example, a spiral freezer in place of the tunnel, and batch freezers in place of the freezer cabinets, would be advisable.
Manufacturers are in business to make a profit. The ever rising costs of labour and raw materials, fuel and electrical power, the carbon tax, make the achievement of a business profit a continual challenge. The refrigeration system is a significant overhead to the business and should be treated in the same way as any other overhead i.e. continually scrutinised.
An audit will give the reassurance that nothing else can be done to improve the system, or the more likely outcome that there is plenty that can be done to reduce the running costs, much of it at a low investment to the business with a favourable return.
Once the identified changes have been put in place, the profit generated will be realised year after year with minimal effort.
ABOUT THE AUTHOR
Martin Bevis is a Senior Process Engineer at Wiley and can be contacted on 1300 385 988 or email email@example.com
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