Heat exchangers are a crucial piece of any application that involves heat transfer. In residential buildings heat exchangers can be found in radiant floor applications, pools, or domestic drinking water applications. In commercial buildings, heat exchangers are used for free cooling applications, chiller and heater loops and district energy. In Industrial and Process applications, heat exchangers are found in areas such as steam plants transferring heat from steam to water. They are also used in the refrigeration cycle as evaporators and condensers in chillers to save space and improve efficiency.

There are three main heat exchanger types used in HVAC applications: plate and frame, brazed plate and shell and tube.

The construction of a plate and frame is a series of plates and gaskets held together by two headers; a fixed header and mobile header. Plates hang in between a top guide bar and a bottom guide bar. On the edges of the plates the gaskets serve as spacers and seal the liquid in the heat exchangers when the plates are tightened together.

This is commonly known as a single pass arrangement and is the preferred arrangement all the piping is one side of the unit. Should you need to inspect, repair or clean or change plates you do not have to undo the piping. The piping stays in place and the mobile head slides on the top and bottom guide bar, providing access to every plate in the plate pack.

A plate and frame heat exchanger is designed for pressures up to 450 psi, temperature goes from -10 deg. F to as high as 300 deg. F, or potentially higher given special gaskets. It is excellent for low to moderately high flow rates. It is ideally suited for close approach, limited space, low holdup volume system, low pressure liquid to liquid application.

Plate and frame heat exchangers support a large flow-rate with connections up to 20’’. Plate gaps can range from 2mm for high heat transfer applications, up to11mm for applications involving more viscous fluids or fluids containing particles.

Both brazed plate heat exchangers and plate and frame designs use counterflow between the plates to achieve high heat transfer. This means that the two media flow in opposite directions across the plates. Carefully designed plates channel the fluid and create turbulence to maximize heat transfer. Because the heat transfer rate is high, systems can use close approach temperatures with temperature crosses. As a result, the sizing of a plate and frame heat exchanger is generally smaller than a comparable shell and tube heat exchanger.

In Brazed plate heat exchangers, The hot side and cold side fluids also run in opposite directions and are separated by corrugated pressed plates. The plates however are brazed together to provide a permanently sealed assembly. The heat transfer rate is very high allowing for close approach temperatures. The brazed plate does not require a modular frame and is generally more compact than plate and frame heat exchangers. Because of the brazed construction, standard pressures and temperature ratings are very high, typically 650 psi and higher.

A shell and tube exchanger consists of a number of tubes mounted inside a cylindrical shell. The hot side and cold side fluids run in a mixed flow configuration, where the heat transfer rate is lower than the plate and frame heat exchangers, and approach temperatures are limited. Shell and tube heat exchangers can handle high temperatures. The flexibility of the design allows for large tube diameters and shell diameters. This means that compared to plate and frame and braised plate heat exchangers, shell and tube heat exchangers can be designed to handle fluids with larger particles. A standard shell and tube heat exchanger has lower levels of heat transfer but is very good for applications using dissimilar media, such as water and steam or water and oil.

Importantly, an installation of a shell and tube heat exchanger must reserve (and protect) sufficient space to allow the shell to be removed for any service or replacement of the tube bundle. No one wants to incur the cost of punching a hole in a wall to accommodate a simple maintenance task.

A key difference between the three main designs is that plate and frame heat exchangers offer flexibility for expansion. Shall and tube heat exchangers cannot be easily altered or expanded because of the limitations of the shell. Similarly, brazed plate heat exchangers cannot be expanded, because of the manufacturing process. If system conditions, including building load, call for changes to heat exchanger performance, plates can be added or removed as needed.

A heat exchanger is often a large portion of the overall cost of an HVAC system, because the majority of the heat exchanger is stainless steel. As with any component in an HVAC system, sizing is an important aspect of ensuring long term performance and efficiency.

Undersized heat exchangers can lead to insufficient cooling or heating in the building. So, heat exchangers are often oversized. But oversizing of heat exchangers can also lead to problems, including higher up-front costs, increased use of floorspace, and increased fouling.

A key aspect to understand in heat exchanger sizing is the three-way relationship between size, performance and cost. Larger sizes of plate and frame heat exchangers with a larger available surface area, offer better performance in heat transfer. This allows a large heat exchanger to accommodate a small temperature approach (the difference between water temperature entering and leaving the heat exchanger) - perhaps as small as 2°. The tradeoff here is that the small temperature approach removes some of the burden on other components in the HVAC system, but the large size of heat exchanger can be very expensive. At the other end of the continuum, a small heat exchanger, with a smaller available surface area, offers reduced performance in heat transfer, so it requires a greater approach temperature - perhaps 3°. The smaller heat exchanger has a lower installed cost, but the reduced heat transfer performance places a greater load on other components in the HVAC system. Operating costs will likely be higher.

The goal in properly sizing a plate and frame heat exchanger is to find the optimum “trade off” between life cycle performance of the entire system, and installed cost. In some instances, a midpoint for heat transfer performance is 2.5°. This allows for a smaller heat exchanger, with a slightly smaller available surface area, what the difference in approach temperature of 0.5° does not put an undue strain on other aspects of the system, although this should be reviewed by the stakeholders.

Two key aspects of HVAC operation affect heat exchangers: fouling and scaling.

Fouling is a complex process that occurs over time in heat exchangers. Higher flow velocities create more turbulence within the heat exchanger and prevent build-up of particulate matter. This is part of the reason that oversized heat exchangers present a risk. With excess sizing, channel velocity decreases, and particulates aggregate more easily. Building operators should be careful to maintain flow levels, and to consider accessories such as a back flush valve and or strainers to capture particulate matter.

Pressure drop within a system is a key indicator for fouling in a heat exchanger. If the pressure drops become elevated, it is often a sign that the heat exchanger is clogged or fouling. It’s not uncommon upon startup for rocks, building debris, dust and dirt from the construction site to be picked up in the cooling tower and become lodged in the heat exchanger. Maximum particle size in the media should not exceed 75% of the channel plate gap. Design Envelope pumps serve as highly accurate flow meters, and provide easily accessible data on a wide range of pump performance indicators. This capability can be very helpful for diagnosing and resolving any issues in conjunction with pressure drop.

Scaling is common due to heat and hard water. The best way to address and prevent scaling is through chemical cleaning, using citric acid, either in place or at a service center. High-pressure washing can address loose particles and may also help with scaling. Higher velocities create more turbulence, which helps avoid and prevent scaling and build-up of particles.

As with many components in an HVAC system, a program of regular inspection and maintenance is key to ensuring long term performance, efficiency and low cost operation.

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