Heat Pump Heat Exchangers

Heat pumps for industrial settings are designed for high-capacity applications and often require intensive engineering to specify for a certain process. As heat pumps are capable of efficiently moving and re-using process heat, they are suited to supplying process heating via existing waste streams or ambient heat in settings where emission reductions are important. Chillers or condensers in industrial processes produce waste heat that can be used for process fluid heating via heat pumps in the pharmaceutical, chemical, food & beverage, pulp & paper, metal processing, and many other industries. The ability of heat pumps to pull heat from, or reject heat to, a reservoir has the additional benefit of reducing loads on chillers while allowing for precise control of temperature differentials between condensers and evaporators in industrial applications. Waste heat from industrial processes can be reused via heat pump systems to boost process efficiency and reduce dependency on external energy sources. Heat is moved from waste streams to other unit operations that can take advantage of the heating or cooling afforded by the system instead of requiring additional energy input. Industrial processes use a relatively wider variety of heat sources/sinks as well as process fluids relative to commercial and residential heat pump systems, often at more extreme temperatures and pressures.
Condensers and evaporators in industrial settings must be compatible with aggressive industrial process fluids at extreme (below 0°C or above 100°C) temperatures or pressures. For example, in sulfuric acid production processes, the heat pump system’s evaporator can be designed to reuse waste heat from 85°C sulfuric acid condensate streams and transfer that heat to a glycol mixture passing through the condenser. District heating networks ensure as little heat is wasted as possible by supplying heat from existing natural or industrial heat sources to nearby industrial operations, as well as residences and pools. Heat pumps are crucial in modern district heating systems that provide heat for local processes from industrial sources including sewage/wastewater facilities and data centres, as well as from nearby bodies of water such as rivers, seawater, etc., aquifers, or other geothermal sources.
Semi-welded plate heat exchangers are well-suited for use in industrial heat pump applications; they feature design pressures up to 62 bar (900 psi), available corrosion-resistant plates, and low hold-up volumes; all key features for use with saline heat sources and modern refrigerants such as CO₂ and ammonia. When both the refrigerant-side and fluid-side are corrosive, or operate at a high pressure and/or temperature, the gasket-free construction of fully welded plate heat exchangers is ideal. However, when the fluid-side is not corrosive and at moderate operating conditions, process designers may opt for semi-welded options that allow for mechanical cleaning and servicing of the fluid-side. Alfa Laval’s RefTightTM system is exclusive to semi-welded line and uses the following design principles to improve gasket lifespan reduce the likelihood of leaks:

• Laser welding outside of the area where gaskets sit to avoid rough weld surfaces at the sight of the seal.
• Smooth machined gasket grooves to ensure full contact for a strong and long-lasting seal.
• Groove gaskets with full support to prevent gasket deformation under high pressures.

For more information on Alfa Laval’s semi-welded line of plate heat exchangers, please click the box below:

Careful design of heat pump systems in terms of the desired temperature of process fluids is crucial to ensure system efficiency — for example, delivering temperatures above 100°C typically results in only about 50% of the efficiency (COP) compared to systems delivering temperatures below 90°C. Refrigerant choice is another key factor in efficient, high COP heat pump design; the following refrigerant parameters influence system efficiency and therefore the size of heat exchanger, refrigerant charge volume, and electrical demand:

• 1. Latent Heat of Vaporization: A higher latent heat results in greater heat transfer per unit mass of refrigerant since more energy is transferred during condensation/evaporation.
• 2. Operating Pressure: Refrigerants that boil and condense at moderate pressures alleviate the load of the compressor.
• 3. Heat Transfer Properties: High thermal conductivity and low viscosity enhance the refrigerant’s ability to transfer heat effectively in evaporators and condensers.
• 4. Small Molecule Structure: Refrigerants with lower molecular weight are generally easier to compress, reducing the compressor’s energy consumption.

Ammonia (R717) is a highly-toxic, highly-flammable, and highly-corrosive refrigerant and as such mainly sees use in industrial applications where exposure risk to the general public is minimized. However, it is often considered to be one of, if not the very best refrigerant to work with from a thermodynamic perspective as it has a high latent heat of vaporization. This allows process and system designers to use a smaller charge of refrigerant, and therefore use smaller heat exchanger to reduce lifecycle costs and physical space demands. Ammonia has gained significant traction as a refrigerant meeting the strictest environmental regulations due to having no Global Warming Potential or Ozone Depletion Potential, as well as being widely available at relatively low cost. Welded or semi-welded plate heat exchangers, such as Alfa Laval’s Compabloc®, semi-welded, and AlfaNova lines are suitable for use with ammonia due to their gasket-free construction using stainless-steel plates. Natural refrigerants are becoming increasingly prevalent in heat pump design due to industrial regulations; choosing a natural refrigerant, and consequently heat exchangers that are optimized to work with them, is crucial to ensure the investment stays future-proof. With excellent heat transfer properties as noted above, ammonia is often specified as one of the most efficient refrigerants; however, systems must also be designed to accommodate the unique challenges of using ammonia.
Future trends in refrigerant selection will subsequently influence the design of heat exchangers used as condensers and evaporators in heat pump systems. As low-density refrigerants become the norm, heat exchanger models will require intelligently-designed distribution systems to ensure even distribution of refrigerants across plate surfaces. This is due to the fact that low-density refrigerants will by necessity need to flow at higher volumetric flow rates to provide equivalent heat transfer performance with respect to high-density refrigerants; larger port sizes for gas-phase refrigerant inlets will also be needed to compensate. Models such as the Alfa Laval AC900, as seen on the right, are specifically designed with the above-mentioned features to work in air conditioning and refrigeration applications as evaporators and condensers in chillers and heat pumps.

As urban planners and commercial building designers strive to become more sustainable and reduce energy costs, heat pumps are increasingly being turned to as a solution. As heat pumps efficiency transfer heat from a source, they reduce the strain on existing high-energy-consumption heating and cooling systems. Businesses often combine electric boilers with heat pumps to supplement heat during peak periods or when outside temperatures are too low to operate a heat pump efficiently. At the same time, waste heat can be recovered from buildings and transportation systems to improve operational efficiency via external energy storage. By using water or other materials to store excess heat generated in off-peak times, energy can be reused during peak periods to reduce overall energy consumption. When used on a wide scale, such systems reduce energy requirements and on grid usage demands over large areas such as urban centres.
The reduction of carbon emissions as well as integration of electricity-based energy sources are key factors in the redesign of existing buildings and planning of future commercial facilities. To that end, heat pumps are replacing energy-intensive and fossil-fuel-dependant chillers and boilers in space heating and cooling, domestic hot water, and hydronic system applications in universities, shopping centres, hospitals and retirement facilities, hotels, high-rise offices and condominiums, restaurants, supermarkets, and virtually all other commercial facilities. Heat pump systems are also used in residential applications, where plate heat exchangers are used to support heat pump systems for farms and other large properties.
The use of efficient heat exchangers within heat pump systems in commercial applications lowers heating and cooling costs and allows for electricity-based heating from renewable energy sources while providing the flexibility to reduce dependencies on traditional commercial boiler and air conditioning systems. Replacing boilers in residential and commercial buildings on a global scale would amount to an estimated 230 TW⋅h in energy savings annually; the selection of the right plate heat exchangers for heat pump applications will therefore allow consumers and commercial facility operators to reap the benefits of reduced operating costs with relatively short payback times while contributing positively to local emissions and sustainability goals. Alfa Laval heat exchangers have advanced design features, such as integrated subcooling and state-of-the-art sealing systems to lead the commercial heat pump heat exchanger market. Alfa Laval Copper Brazed plate heat exchangers are widely available with low lead times and are ideal for use as evaporators and condensers in small and medium-scale commercial heat pump applications due to their compact size, versatility in design, and combability with most low-GWP refrigerants including CO₂ and hydrocarbons. All Alfa Laval brazed plate heat exchanger models benefit from maintenance-free design and high thermal efficiency — this makes them ideal for hydronic heating and domestic hot water applications, as well as other heat pump applications that do not use fluids corrosive to copper. The fully stainless-steel construction of Alfa Laval AlfaNova fusion bonded heat exchangers, which is the only offering of its kind on the market, allows it take advantage of ammonia and other corrosive refrigerants’ thermodynamic properties where copper-based models cannot. Alfa Laval’s brazed and fusion bonded heat exchangers are available with various design features, including a wide range of plate counts, connection standards, pressure vessel approvals, as well as asymmetric channels and accessories such as legs and insulating jackets.

Alfa Laval plate heat exchangers are used in industrial and commercial applications throughout the world to achieve energy-efficient operation and minimize negative environmental impact; the following are some examples:

• At Alfa Laval’s flagship plate heat exchanger factory in Lund, Sweden, a heat pump solution using semi-welded units as the evaporators and condensers is implemented to reuse waste process heat. Heat pump implementation with a 40 kg hold-up volume of ammonia reduces energy consumption of the process from 3,700 MW/year to 1,850 MW/year with a total energy cost savings of 55%.
• The European Patent Office in Vienna underwent a modernization process to use heat pump systems in conjunction with geothermal heat sources rather than air heating. The system can be reversed to provide cooling in the Summer and operates using ammonia as the refrigerant; the condensers and evaporators are Alfa Laval’s semi-welded heat exchangers, and fusion-bonded plate heat exchangers are used as desuperheaters to discharge hot gas for domestic hot water heating.
• Alfa Laval CB65 brazed plate heat exchangers are integrated as evaporators and condensers in residential propane heat pump systems in collaboration with German heat pump manufacturer WOLF. By using non-toxic R-290 as the refrigerant, the efficiency of air-to-water heat pumps (COP = 5.7) is accessible for standard residential applications. Alfa Laval’s anti-freezing technology in brazed plate models also greatly reduces the risk of freezing water, allowing for use of sub-freezing temperature refrigerants.
• 14 Alfa Laval semi-welded plate heat exchangers are used as condensers and evaporators in an ammonia heat pump network delivering 33 MW of heating to the 20 Ha Fenland Greenhouse complex in Cambridge, United Kingdom. The system uses 4 heat pumps installed in parallel to recover heat from irrigation water in ponds to provide hot water for heating the greenhouse.
• The Ejby Mølle Wastewater Treatment Plant in Odense, Denmark uses a heat pump system to recover heat from treated wastewater before it is discharged into the Odense River, and subsequently distributes it to a district heating system. The system uses semi-welded plate heat exchangers as evaporators and condensers, as well as subcoolers and oil coolers to provide 20 MW of heating to the district heat network, the equivalent of the annual heat consumption of 5,000 households.

In addition to traditional electrically-driven heat pumps that use electricity to power a compressor circulating refrigerants through the refrigeration cycle, thermally-driven heat pumps exist as a promising alternative is the. Such heat pumps use heat, often sourced from combustion such as gas burners or internal combustion engines, to drive an absorption process rather than a compressor. In these processes, refrigerant mixes with the absorbent and is subsequently pumped to a generator where the aforementioned heat source boils the refrigerant to separate it from the absorbent before it enters the condenser. Where combustion sources are already installed or thermal energy is otherwise cheaply available, thermally-powered heat pumps are an attractive option to reduce consumption of electricity that would otherwise be more costly to generate and supply to the system. Alfa Laval heat exchangers are designed to operate effectively in this space based on design versatility that makes them compatible with a wide range of absorbents and absorbent-refrigeration mixes.
The widespread implementation of heat pumps with efficient heat exchangers allows cities and industries to take advantage of available renewable heat sources or reused generated heat from local processes in order to heat and cool homes, commercial facilities, and industrial processes within sustainable urban and suburban developments. As heat pumps are implemented with greater frequency around the globe, it will be crucial to design systems and select optimal heat exchangers in order to save financial and material resources to sustainably grow and operate commercial and industrial processes.