|The heat pump technology developed by Vilter over more than 20 years offers a number of benefits to the food processing industry Credit: Emerson|
A novel heat pump system based on a single-screw compressor is cutting water heating costs in the food processing industry and helping users meet their sustainability goals.
It seems odd that one part of an industrial process would create heat as a by-product and then vent it into the atmosphere, while another part of the same process burns fossil fuel to raise the temperature of water. In reality this is what the food processing industry commonly does.
A typical food processing plant refrigerates large quantities of its products to keep them fresh, in that way generating heat, and burns, say, gas in boilers to create large amounts of hot water for cleaning and sanitation.
But an ammonia-based, high-temperature heat pump system that employs a single-screw high-pressure compressor is helping several food processing applications around the world to cut the waste of this energy, while reducing the consumption of carbon-based fuel. Each of these three projects required the system to provide the highest coefficient of performance (COP) possible, a technology solution with low annual operating and maintenance costs, and the use of a non-ozone depleting refrigerant with zero global warming impact.
According to Emerson, which with its project partner Star Refrigeration, provided the solution, the system’s use of ammonia refridgerant gives each application a better performance compared with competing technologies when it comes to consumed resources. Emerson says this is possible because of the higher temperatures used.
The system also met each project’s environmental requirements and kept operating and maintenance costs low. Its compressor from Vilter, which became part of Emerson in 2009, raises the temperature of water for boilers by super-heating exhaust heat from a refridgeration system.
Savings in each case arise because the balanced radial and axial force of the compressor means stress on the unit’s bearings is reduced, and minimising maintainance and operation costs. The compressor also delivered a performance unachievable with any other type of compressor, according to Vilter.
Nestlé’s plant in Halifax, UK, is one of these applications. Here the food processing company relies on large refrigeration systems for chocolate manufacturing, storage and distribution. Refrigeration is essential to cool chocolate while heat is needed to separate it from the shaping moulds.
Nestlé had been using one central coal-fired steam generation plant for its requirements but wanted to capture the waste heat to replace the need for additional gas-fired equipment. It also wanted the new system to help cut overall energy demand.
One of the few refrigerants that could meet all of the efficiency and environmental requirements was ammonia, an efficient refrigerant (designated as R-717), commonly used by the food and beverage industry for process cooling and refrigeration.
However, it has not been commonly used in industrial heat pump applications, where high temperatures are required. And, according to Sam Gladis, Vilter’s Business Director, the International Energy Agency’s Heat Pump Centre recently deemed such an application impossible and said there were no suitable high-pressure compressors available to make using ammonia a reality in high-temperature industrial heat pumps. Emerson begged to differ.
In the Nestlé project the ammonia heat pump has a dual purpose. The system delivers chilled glycol at 0°C and hot water at 60ºC using waste heat. Heat can be taken from the 0°C process glycol and lifted to 60ºC in one stage. Nestlé’s site has, since the commissioning of the system in May 2010, been raising the temperature of 64,000 litres of water each day to 60ºC. And this hot water is delivered far more efficiently than from the company’s previous coal-fired steam generator, says Emerson.
Nestlé’s system won the ‘Industrial and Commercial Project of the Year’ title at the 2010 RAC awards in Britain as it cut process utility costs at the site by more than $394,000 per year. By reducing gas combustion it has also cut CO2 emissions by more than 500 tonnes per year, and water consumption has fallen by around 40%.
|The heat pump technology is also heating the hot water for the Norwegian town of Drammen’s new districy heating network Credit: Emerson|
Another application of Emerson’s heat pump technology is at the Kraft Foods plant in Davenport, Iowa, US.
Like many food processing plants, the company had been paying for electrical energy to remove heat from spaces refrigerated by an ammonia-based system and venting that heat to the atmosphere. It was also paying for natural gas to heat the hot water it used to clean the plant hygienically.
Kraft Foods determined that if the rejected heat could be captured and used to provide water heating, substantial energy would be saved.
‘The plant installed high-efficiency boilers and invested in capturing and recovering boiler stack heat,’ says Gladis.
The highest pressures and temperatures in the refrigeration system were in the compressor discharge gas. These provided the best source for heat to be transferred to the sanitation clean-up water. The problem was that ammonia at typical condensing pressures condenses at temperatures as low as 24°C to 35°C, although the liquid does at the same time have a high heat energy.
Conventional heat exchangers would have allowed the transfer of this energy to city water to raise it to the 63-85°C range requirement of wash-down water, but such a system would only provide limited pre-heating of the cold water supply.
If the refrigeration system compressor discharge gas, which was at the high pressure of 1340 kPa, could be fed directly into the suction of a heat pump compressor and be raised in pressure to 3200 kPa to 5600 kPa, then condensing this higher-pressure ammonia with cold water in a heat exchanger would capture signifcantly larger quantities of heat energy than heat reclaim, and elevate the cold water supply from 15°C up to the required temperature of 63°C.
|Emerson and its project partners have proved that amonia can be used in high-temperature industrial heat pump applications Credit: Emerson|
Another challenge was the requirement of the plant to have the wash-down water classified as potable. Local codes prohibit potable water to be in direct heat exchange with ammonia. Using such an ammonia heat pump system would require a secondary loop, thereby lowering the efficiency of heat transfer.
Given the need for sustainable projects to clear the same internal rate of return hurdle as non-sustainable projects, Emerson says it challenge was to define and justify the project. Project costs would include tapping into the ammonia refrigeration system, adding and installing a custom ammonia heat pump system, and employing electricity to operate the high-pressure ammonia heat pump system.
Kraft’s system now delivers hot water at 63°C using heat extracted from refrigeration. The inherent high-pressure capability of the single screw compressor allows for full acceptance of the highest operating discharge pressure from the host system, even greater than 12,400 kPa guage, without the risk of rotor deflection and excessive bearing thrust loads, which can result in accelerated degradation or the loss of efficiency because of over-compression.
In winter, the incoming cold water temperature is at 15.6°C, meaning that the estimated heating capacity of the heat pump for this season would have to be an estimated 1682 kW, providing 30.7 m3/hour of continuous water flow at 62.8°C. The heat pump was estimated to provide an average year-round heating capacity of 2056 kW, heating 38.6 m3/hour of water from 16.9°C to 62.8°C.
Since commissioning, the plant has been heating in the region of 773 litres of water per minute, which is delivered far more efficiently than from the natural gas hot water heater, says Emerson.
Over the courseof a year the ammonia heat pump solution cuts heat energy costs by over $250,000 and saves 53 million litres of water because of the reduced load on the evaporative condensers.
A Chilean project has also employed the Emerson’s heat pump system, but this time in the poultry processing business, where energy costs are high.
Chilean poultry processor Agrosuper had relied on boilers to heat up to 1300 litres of well water per minute for its sanitation needs. At the same time, it was removing heat from the poultry being processed and discharging it to the environment.
Agrosuper realised that if it could capture the waste heat from processing, it might be able to use it to heat the water.
With the new system, heat taken from the company’s -10ºC refrigeration load is lifted to 52°C in one stage. Since commissioning in January 2012 the facility has been heating 1300 litres of water per minute, which the solution provider Emerson says is delivered far more efficiently compared with the previous boiler. The ammonia heat pump solution has reduced heat energy costs by 72% and saves 57 million litres of water per year.
Apart from food processing applications, the same heat pump technology is also heating Drammen, a town in Norway that has turned to the frigid North Sea as a renewable energy source.
This community of 60,000 people on the Drammen Fjord near the capital city of Oslo, needed hot water at 90°C for a new district heating system for homes and businesses.Again, the requirements were the highest possible COP, low operating and maintenance costs, and no ozone or global warming impact.
‘Installing heat pumps to extract heat from water or air is increasingly popular in Europe,’ says Gladis, ‘largely because the heat they deliver far exceeds the energy they consume, greatly reducing the reliance on fossil fuels and the need for additional renewable energy sources.’
Emerson, once again working with Star Refrigeration and project partners in Norway – Norsk Kulde and Drammen Fjernevarme – provided the Vilter heat pump that uses ammonia and that, it says, has resulted in a performance improvement estimated to be more than 15% higher than one using R-134A, a refrigerant with a high global warming potential.
Again, the use of high temperatures allowed superior performance from consumed resources than competing technologies, says Emerson.
The company has also applied the system in a US-based residential solution in which the compressor helps condition air temperature instead of heating water.