Techology Spotlight: Industrial Heat Pumps

Industrial heat pumps can significantly reduce fossil fuel consumption, energy costs and greenhouse gas emissions in a variety of drying, washing, evaporating and distilling processes. Industrial heat pumps can also be used to produce steam and provide process-water heating and cooling.  Industries that can benefit from this technology include food and beverage processing, pulp and paper, forest products, textiles and chemicals. 

The two main types of heat pumps are closed-cycle mechanical heat pumps and absorption heat pumps. Absorption heat pumps compress the working fluid thermally, rather than mechanically, and use the ability of liquids or salts to absorb the vapor of the working fluid to achieve a temperature lift. Closed-cycle mechanical heat pumps, the focus of this column, mechanically compress a working fluid, typically a refrigerant, to achieve a temperature lift. 

Industrial heat pumps are most commonly used to recover heat from a waste stream, such as exhaust from process equipment (e.g., a dryer).  The waste stream is often both warm and humid. The heat pump can recover both the heat associated with the waste stream's temperature ("sensible heat") and the heat associated with its humidity ("latent heat").  The recovered heat is then used to heat the supply air to the process, for example, or to provide heat to another process.

In a retrofit of fossil-fuel fired equipment, an industrial heat pump will increase electricity consumption due to the heat pump's compressors and fans, while reducing or eliminating fossil fuel use. Heat recovery results in the overall improvement of energy efficiency.  Industrial heat pumps are most cost effective in regions with low electrical costs compared to fossil fuel costs.  In industrial applications, simple paybacks of two to five years are typical.

Re-emergence of an existing technology

Many industrial heat pumps were installed in the 1980s.  However, the Federally mandated phase-out of ozone-damaging chlorofluorocarbons in the 1990s necessitated the development of new refrigerants capable of operating at the higher temperatures needed for typical applications. Today, new environmentally-sound refrigerants have allowed industrial heat pumps to reemerge in a range of applications.  Some older units continue in operation today and are being retrofitted from R22 refrigerant to the ozone-safe R134a.

Closed-cycle mechanical heat pump examples

Closed-cycle mechanical heat pumps range in size from relatively small—on the order of 25 hp (20 kW)—to as much as 13,500 hp. One project uses a 25-hp unit to dry flour so it can be pneumatically conveyed.  In another, an apple drying facility will use a 1,300-hp system to dry apples from 80 percent moisture content to 20 percent.  A 3,000-hp heat pump provides process cooling for a yeast factory.  Perhaps the largest closed-cycle mechanical heat pumps in the world are the six 13,500-hp units that use sea water as the heat source for district heating in Stockholm, Sweden.

In the United States, the size of an industrial heat pump is conventionally expressed in terms of electrical power requirements in horsepower.  In Europe, the measurement is kilowatts or megawatts of output (heating or cooling capacity). One horsepower is equivalent to 2500 Btu/h of electrical input, which corresponds to 15,000 Btu/h of cooling or heating capacity with a coefficient of performance (COP) of 6. One kW of capacity is equivalent to 3,412 Btu/h of cooling or heating capacity and about 0.22 hp of electrical input with a COP of 6.

Temperature and COP

Closed-cycle mechanical heat pumps can achieve maximum temperatures of 220º F with temperature rises of as much as 100º F. To achieve greater temperature rises, two-stage systems can be used.  The COP of the heat pump improves as the temperature difference between the heat source and the heat sink decreases.  Very high COPs can be achieved with small temperature differences, as shown in Table 1. For best efficiency, heat from the waste stream can first be recovered passively with a heat exchanger followed by heat recovery with a heat pump.

Avoiding corrosion, fouling

When drying acidic products, such as apples and oak, or if using caustic cleaning products, the heat pump's exhaust heat exchanger may require stainless steel tubes with aluminum fins coated with a protective material such as ElectroFin's "e-coat." Sticky exhausts can be handled by incorporating a wash cycle to periodically clean heat exchanger surfaces.  During the wash-down cycle, which might last a few minutes once a day or so, auxiliary heat can be used to maintain temperature.  

Other considerations

Warm water that is condensed out of the exhaust can be recovered for other uses.  If condensed water is not reused, its addition to the waste water stream must be accounted for.  When installing a large unit on a roof, structural issues must be considered.  The increased electrical demand of the compressor may increase electric demand charges and may require upgrade of the electrical service.

More Information

• "About heat pumps," IEA Heat Pump Centre.

• Industrial Heat Pumps for Steam and Fuel Savings, U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy.

• Industrial Heat Pumps: A Means to Mitigate Global Industrial Emissions, IEA Heat Pump Centre, 1995.

• "Stockholm /Yeast Factory project: 6 MW heat pump and industrial process cooling," Skandinavisk Termoekonomi AB.

Resources

About heat pumps

Industrial Heat Pumps for Steam and Fuel Savings

Industrial Heat Pumps: A Means to Mitigate Global Industrial Emissions

Stockholm /Yeast Factory project: 6 MW heat pump and industrial process

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