Refrigeration

Plate heat exchangers offer compact designs that are able to accommodate close temperature approaches, and there are options to support every refrigerant on the market. In both commercial and industrial refrigeration installations, the goal of the process is to cool a process fluid using a refrigerant, which is defined as a substance used in a refrigeration cycle to absorb heat and later release it to continue the cycle. The heat is absorbed from a relatively low-temperature environment and then released at a higher temperature to a recipient such as water or air. This process can be generalized using the diagram below:

Heat is absorbed from a target system to be cooled when the refrigerant is evaporated. Afterwards, the refrigerant is compressed, increasing its pressure and temperature; it is subsequently condensed, releasing heat to a designated recipient. Once it is expanded, it returns to its original temperature and vapour fraction and is ready to continue the cycle. Heat exchangers are used as both condensers and evaporators in this cycle to facilitate the transfer of heat to the desired target in at a controlled rate. The target system can be the air inside a refrigerator, the pipes below an ice rink, or glycol within an industrial process that needs to be cooled below the freezing temperature of pure water. There are two main types of evaporators: flooded (circulation) and dry expansion (DX). Flooded systems maintain liquid refrigerant throughout the evaporating side via make up using a separator that processes incoming two-phase mixture exiting the heat exchanger.  With the heating surface of the unit always wetted, convective forces are greater, facilitating a higher heat transfer coefficient and thus a smaller heat transfer area is required. These units are used in larger applications with a higher refrigerant volume, while dry expansion units (where the refrigerant is completely evaporated and often slightly superheated at the exit) tend to be used in smaller applications or when a separator is undesired, typically using brazed plate style units. Systems with flooded evaporator units may also require oil evaporators if system oil is soluble in the refrigerant. When plate heat exchangers are used as condensers, they are almost always liquid-cooled via water or glycol. There are many possible modifications to the basic refrigeration cycle pictured above, each of which is facilitated by further applications of heat exchangers; the following list briefly summarizes how heat exchangers are used in refrigeration cycles in addition to condensers and evaporators:

Subcoolers: A heat exchanger is used to further cool condensed refrigerant exiting the condenser from just below its condensation temperature to a designated temperature, often using sources such as well water. This allows for the evaporator to use a lower temperature refrigerant (or a lower vapour fraction after expansion), leading to an increase in cooling capacity for the refrigeration system.

Desuperheaters: As can be seen in the diagram above, there is a significant jump in temperature in the refrigerant between the end of compression and the start of condensation. This is valuable thermal energy that can be harnessed to heat process fluids in an industrial application or water for domestic use in a domestic application. Heat exchangers are used as desuperheaters to transfer the heat from the hot refrigerant in its vapour state to a target system.

Intercoolers: An additional desuperheater interjected between two compression cycles, cooling the vapour-phase refrigerant before it is further compressed. Intercoolers improve compression efficiency since less energy is required to compress a cooler (denser) refrigerant, while protecting system components from high temperatures by preventing the refrigerant from reaching the temperature it otherwise would reach in a single compression cycle to the target pressure.

Economizers: Liquid refrigerant exiting the condenser can be split into two streams, with one entering an expansion valve, cooling down and entering a two-phase state at an intermediate temperature and pressure. These two streams are then routed through the same heat exchanger, where the cold side refrigerant evaporates and further subcools the hot side liquid refrigerant. The evaporated stream then joins the compression cycle at an intermediate phase by mixing with the partially-condensed, evaporated refrigerant from the primary evaporation stage. Like a desuperheater, this mixing lowers the temperature of the vapour refrigerant entering the later stage of the compression cycle, improving efficiency and protecting equipment. The benefit in this case is that there is no dependency on an external cooling source, such as water. At the same time, the condensed refrigerant that underwent subcooling is now available to be used in the primary evaporation system at a lower temperature (or a lower vapour fraction after expansion), meaning that cooling capacity has increased. Note that while a vapour-liquid separator (flash economizer) could also be used to route a fraction of vapour refrigerant to an intermediate compression stage, this requires more physical space relative to a heat exchanger economizer.

Oil Coolers: Lubricating oil used in compressors must also be cooled, especially in larger industrial applications, where excessive heat can degrade oil and promote wear of internal components. Heat exchanger units are used to cool oil via a liquid phase cooling fluid (e.g. water), cold refrigerant, or relatively warmer condensate from another part of the cycle.

Four types of plate heat exchangers with similar thermal and hydraulic properties are used in refrigeration applications: brazed and fusion bonded gasket-free heat exchangers, as well as semi-welded and fully welded plate-and-frame heat exchangers. The distinction lies in presence of gaskets, plate materials, corrosion resistance, and brazing materials for gasket-free units. Brazed/bonded plate units feature stainless steel internal constructions with either copper brazing or fusion bonding using stainless steel. In contrast, welded units are available with titanium plates and other speciality alloys. While semi-welded heat exchangers face limitations due to their gaskets, the liquid side can be opened for inspection and cleaning, which is not possible with fully welded or brazed plate units. Brazed plate units are more compact options with standardized constructions that offer lower capital costs, while welded units feature more flexibility in terms of customization and are available in larger sizes for virtually all industrial applications. The following synopsis outlines various styles of Alfa Laval heat exchangers as well as their special features for refrigeration applications — for more information on a certain product line, please click the respective product title or model name:

Brazed Plate Exchangers:

Alfa Laval CB, CBH, and ACH lines of copper brazed heat exchangers are compact and maintenance-free solutions for evaporation, condensing, and other applications in refrigeration cycles. This style of heat exchanger is the most common in commercial and small-capacity industrial refrigeration applications and utilize copper brazed construction. In addition to suitability with standard refrigerants, they are capable of handling low GWP (Global Warming Potential) refrigerants, including natural refrigerants like CO₂ and hydrocarbons. Depending on the model, there are brazed plate units specifically optimized to handle transcritical CO₂ systems, propane, and synthetic refrigerants, among other alternative refrigerants (excluding ammonia). They can be rated for up to 130 bar as CO₂ gas coolers, and are available with asymmetric channels for propane and high-density refrigerants where a low refrigerant charge is desirable.

AlfaNova Fusion Bonded Plate Heat Exchangers:

Built with a unique fusion bonding method, AlfaNova models use a 100% stainless steel construction, making it the only model of its kind on the market. It has an advantage over traditionally brazed units in cases where copper reacts negatively with one of the fluids, such as the case of ammonia as a refrigerant. AlfaNova models are equally compact and efficient relative to standard brazed plate heat exchangers and are effective solutions for cooling of sensitive liquids such as clean water or hygienic media.

Semi-Welded Plate Heat Exchangers:

For larger scale industrial refrigeration and heat pump applications, compact semi-welded heat exchangers feature high efficiency and reliability in scaled-up processes. Semi-welded units have welded ports on one fluid side to support high pressures and chemically difficult fluids seen in industrial processes while maintaining cleanability on the gasketed side. These units excel in cascade systems operating with CO₂ as the secondary medium for freezing applications with reduced ammonia charge, as well as in general ammonia-based applications. In addition, they are capable of being used for any of the other applications listed on this page.

After the 2016 Paris Agreement, there has been greater international focus on the use of natural refrigerants as an alternative to synthetic refrigerants such HFC (hydrofluorocarbon) variants, which have a greater contribution to global warming. Synthetic refrigerants include moderately high GWP HFC variants with high densities such as R32 (difluoromethane), as well as low density variants like HFO-based (hydrofluoroolefins) refrigerants. While some higher density variants have a lower GWP than refrigerants like R410A, they are still relatively high compared to target GWP level in certain global markets, while also presenting flammability risks. Lower density synthetic refrigerants can have GWP levels around 0.1% of comparable HFC refrigerants, but in addition to flammability risks, HFOs have reactive carbon-carbon bonds that make handling them potentially difficult. Synthetic refrigerants of both density levels feature options that have zero ozone-depletion potential.

The term ‘natural refrigerants’ refers to:
hydrocarbons like propane (R290), isobutane (R600a), and propene (R1270), as well as ammonia (R717), CO₂ (R744), air, and water. While each of these has extremely low GWP, they each carry trade-offs that require proper design considerations in order to use them. Hydrocarbons are generally widely available at low cost, but are more flammable than synthetic refrigerants. Ammonia can accommodate refrigeration applications at a variety of temperatures and capacities, but is toxic and flammable (though it will not burn without a supporting flame). The industrial trend surrounding ammonia is to apply a secondary cooling system to the ammonia evaporator leading to the target cooling area — this isolates the ammonia to an area such as a machine room while a safer secondary fluid directly interacts with the target fluid/space. CO₂ is neither flammable or inherently toxic, but still poses a risk to human health and is not as efficient as other natural refrigerants. In addition, CO₂ must be used in high pressure systems, requiring heat exchangers capable of handling said pressures, as well as other design considerations such as pressure-relief valves to protect systems and people. So-called transcritical systems condense CO₂ above its critical point (where there is no distinction between fluid and vapour phases) of 31°C, meaning the condenser essentially acts as a gas cooler.

Modern heat pump systems commonly use R32 as a substitute for the previous standard of R410A; however, R32 itself can be replaced with propane or CO₂ for domestic applications, while ammonia and HFO blends are seeing use in district heating network heat pump applications. Indeed, most previously common HFC refrigerants such as R134a (1,1,1,2-Tetrafluoroethane), R407A, and R410A have equivalent replacements within the categories of natural refrigerants, or synthetic HFOs and HFO-HFC blends. Air conditioning applications are also seeing increasing use of ammonia in large central A/C systems and CO₂ in combined refrigeration and air conditioning systems for supermarkets.

With the advantages and drawbacks of natural refrigerants in mind, a well-designed modern heat exchanger makes it possible to maximize the advantages of a given refrigerant and stay ahead of competitors in a developing refrigeration market.