Energy Management in Plastics Processing - Part 9 Cooling Process Water
A series of energy efficiency worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics industry reduce costs through efficient use of energy. UK Government Environment and Energy Helpline 0800 585 794

Cooling process water

A reliable and consistent source of cooling water is essential for fast and repeatable plastics processing. Cooling process water and refrigeration plant use approximately 11% of all the energy consumed in manufacturing in the UK. However, energy and carbon savings of up to 25% are easily achievable without any technical risk, simply by implementing good practice and proven technology.

·         Tip: Cooling plant is generally reliable and is ignored unless there is a problem. Regular analysis of performance data will quickly detect any losses in efficiency.

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Savings of up to 25% are possible with little technical risk.

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Measures of performance: energy efficiency ratios (EERs)

Coefficient of performance (COP) is the ratio of the cooling capacity to the absorbed power of a compressor.

Coefficient of system performance (COSP) is the ratio of the cooling capacity to the absorbed power of the complete system. This includes the effect of the power consumption of fans and pumps as well as the compressor.

COP and COSP can be used to indicate the relative energy efficiency of the chiller or the system and to compare systems with one another.

The measurement of COP and COSP depend on the conditions used to asses them and should only be used for comparison when identical conditions are used.

Air blast cooling

Low ambient temperatures in the UK and the flow temperatures used in plastics processing mean that use of air blast cooling can reduce energy costs considerably. Air blast cooling pre-cools the return water from the process and significantly reduces both chiller loads and energy use. This can produce significant energy and carbon savings for low additional costs.

If the ambient temperature falls to 1ºC or more below the return water temperature, then the return water is diverted through the air blast cooler. The more the ambient temperature is below the return water temperature, the greater the air blast cooling effect. It is possible to switch off the main chiller when the ambient temperature is 3ºC below the return water temperature.

The cost savings generated by using air blast cooling give a typical payback period on the investment of less than 2 years and often as little as 12 months.

•  Tip: Chillers with new or retrofitted air blast cooling circuits can show large reductions in operating costs.

• Tip: Chillers with air blast cooling circuits have shorter compressor running times, lower maintenance costs and extended chiller life.

• Air blast coolers are available for capacities as low as 5 kW with no effective upper limit as units can be linked together to provide greater cooling capacity.

Air blast cooling is possible for most of the year in the UK

Chillers

Every conventional chiller is a compressor that pumps refrigerant; for every 100kW of cooling capacity, it will use about 30kWh of electricity. Even a small plastics processing site can need a 200kW chiller, with an operating cost of over £16,000/year. However, simple measures can often improve the energy efficiency of chillers significantly.

Cooling load

Eliminating or reducing cooling loads will reduce running costs and improve efficiency.

• Tip: Only supply cooling where needed.

• Tip: Use the maximum possible water temperature; a 1ºC rise in the supply temperature reduces the energy required by about 3%.

• Tip: Fit adequate insulation to minimise unnecessary loads on the chiller.

Systems

Minimising the load allows a better assessment of the system design and particularly how it responds to part loads.

• Tip: Optimise existing systems. Use the most suitable refrigerant and optimise the system for high part-load and winter efficiency. This is particularly important when additional chillers have been added to the system.

• Tip: Balance pumps and chillers and match them to the normal load (with controls to match a variable load).

• Tip: Check that pipework and pumps are sized correctly for current demands.

• Tip: Keep chillers well ventilated to provide good airflow over the condensers.

•  Tip: Use heat recovery to provide energy for space heating and hot water.

Components

Component selection is another important factor in energy efficient operation.

• Tip: Scroll and screw compressors are more efficient and can replace existing chillers.

• Tip: Avoid running chillers at low loads.

• Tip: Use large evaporators and condensers and avoid direct expansion evaporators if possible.

• Tip: Use variable speed drives (VSDs) for pumps and fans to match the output to process demands.

Operation and maintenance

Cooling systems often operate at low efficiency due to a lack of routine maintenance.

• Tip: Service chillers regularly and keep records of plant conditions.

• Tip: Clean evaporators, air blast coolers and heat exchanger surfaces regularly.

• Tip: Check flow/return temperatures and system flow rates to verify these are correct and optimised.

• Tip: It is a legal requirement to keep systems gas tight and to repair gas leaks.

• Tip: Design moulds and cooling baths or spray tanks to provide good heat transfer from the plastic to the cooling water.

• Tip: Set all systems components to turn off automatically when not in use.

The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.

Download the complete series as an Adobe Acrobat file.

Last edited: 22/10/07  

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