Demystifying the Specification of Heat Exchangers for Special Plumbing Applications
By Jose R. Rodriguez, P.Eng.
Plumbing engineers design domestic hot water systems for use with plumbing fixtures and to supplement hydronic or central forced-air heating systems, and a main component of this process is establishing an accurate input of the potable water and domestic hot water for the project.
To help plumbing engineers accomplish this task, the following information can be used to help select and design heat exchangers and understand the thermodynamic principles behind their use. These principles can be applied to special plumbing applications such as swimming pools, water fountains, and solar thermal applications of domestic hot water (DHW). Most of the information comes from drawings and specifications that included materials and methods of construction for the heat exchangers on particular projects. Typically, each designer writes specifications using their own style or master specs from their company to describe what they think is most important for the project. Thus, specifications usually lack consistency, and hopefully this article will help close the gap between drawings and specs from HVAC and plumbing engineers.
Heat Transfer Theory
Heat can be transferred to water by three methods:
1. Radiation: Energy is transferred by electromagnetic radiation (for example, a solar thermal application for DHW use).
2. Conduction: Energy is transferred between solids or stationary fluids by the movement of atoms in the molecules (for example, DHW generation using a fluid-to-fluid heat exchanger as a hot water generator).
3. Convection: Energy is transferred by mixing part of a medium with another part (for example, generating hot water for a swimming pool using a bypass flow design in the heat exchanger).
Most selections and designs of heat exchangers are made by the manufacturer’s representative using their selection software, which is based on their proprietary manufacturing. The basic job of the heat exchanger is to transfer specific quantities of heat under a specific set of conditions. A key parameter in the selection process is the calculation of the heat transfer area required.
The thermodynamic principles are shown below:
Q = U x A x LMTD
Q = GPM x 500 x P x C x CF x ?T
A = Q
Q = Heat load in British thermal units per hour (Btuh)
U = Overall heat transfer coefficient in Btuh x square feet x ºF
A = Heat transfer area in square feet
LMTD = Long mean temperature difference in ºF
GPM = Flow rate in gallons per minute
P = Specific gravity
C = Specific heat in Btu/lb x ºF
CF = Fluid correction factor to take into account,
which changes specific gravity and specific heat
?T = Fluid temperature rise in ºF
The value for LMTD depends solely on how the flow media is directed. The most effective way is to conduct flow in the direction of the countercurrent flow, as shown in Figure 1.
The LMTD can be calculated using the difference between the incoming and outgoing temperatures of the two fluids (the hot water side and the cool water side) according to the following equation:
LMTD = ?T – ? t
?T = T1 – t2 (temperature on the hot side end)
?t = T2 – t1 (temperature on the cold side end)
The overall heat transfer (U) is calculated based on the film coefficiency of the plates or tubes, the geometry of construction, the thickness of the plates or tubes, the material’s thermal conductivity and the Reynolds’s number.
To proceed with the specification, the plumbing engineer must know five of the following parameters:
• Heater capacity
• Temperatures on the hot side (in or out)
• Temperatures on the cold side (in or out)
• Flow rate on the cold side and/or the hot side
Although the principles of heat transfer work the same regardless of the media, the applications are differentiated as water/water, water/glycol or steam/water.
Brazed Plate Heat Exchanger
The brazed plate heat exchanger is the most versatile type used in plumbing applications. Brazed plate heat exchangers are very compact and inexpensive for use with water/water or water/glycol applications. They are recommended for use in domestic hot water, swimming pool and solar thermal applications for both residential and light commercial construction. (The brazed plate heat exchanger is one variation of the plate gasket heat exchanger described in detail in the next section.)
The corrugated plates provide a very high heat transfer coefficiency, resulting in a very compact unit. The stainless steel plates are vacuum brazed together to form a durable and integral piece that can handle the highest pressures and temperatures. The brazing process eliminates the need for the gasket and roller joints commonly used in traditional heat exchangers.
Double-wall heat exchangers consist of two standard plates welded together at the four corner ports to form one assembly plate. An air space, or leak path, is created between the two plates for fluid passage. If one plate fails, the fluid is delivered to the atmosphere to avoid cross-contamination. Some plumbing codes in the U.S. and Canada mandate the use of double-wall heat exchangers in domestic, potable water and solar thermal applications.
Plate and Frame Heat Exchanger
During the last few decades, the plate and frame heat exchanger (see Figure 2) has emerged as a viable alternative to the traditional shell and tube heat exchanger. They typically are used for water/water and water/glycol plumbing applications for domestic hot water generation or occasionally in swimming pool and potable water applications.
With plate and frame heat exchangers, spacing is provided between the plates for fluid flow. The gasket directs and controls the liquid flow for distribution. The plate’s corrugation provides structural strength for the heat transfer area, allowing better heat transfer and high liquid turbulence. These units can be single-wall or double-wall plates.
Shell and Tube Heat Exchanger
Shell and tube (S&T) heat exchangers (see Figure 3) offer the greatest flexibility for temperature and pressure for water/water, water/glycol, and steam/water plumbing applications.
In water/water and water/glycol applications, the baffles around the tube bundle direct the flow of the fluid, with the space between optimized to achieve better heat transfer and pressure drop. In steam/water applications, the baffle is designed to allow the condensate that collects at the bottom of the shell to flow freely to drain. This explains why the baffles for the different applications are designed in different configurations.
In addition to the indirect storage-type heaters well known in the plumbing world, the steam convertor is another typical application that might be used for domestic hot water or potable water systems in some provinces of Canada and certain areas of the U.S. such as New York City. The selection of a shell and tube unit can also be used for swimming pool applications and to recover the energy from steam condensate to preheat domestic hot water.
Codes and Standards
Regardless of the type of heat exchanger, their construction and fabrication are governed by ASME’s Boiler and Pressure Vessel Code, Section VIII, Division 1. The ASME code prescribes mandatory guidelines for the fabrication of pressure vessels, including material selection, design, testing, and inspection. It covers all aspects of the construction of heat exchangers, except the types of service loads (other than pressure) and the thermal design.
International Association of Plumbing and Mechanical Officials (IAPMO) PS 092: Heat Exchangers and Indirect Water Heaters establishes acceptable standards for heat exchangers and indirect water heaters, including materials and marking and identification requirements.
The Uniform Building Code (UBC), International Building Code (IBC), and ASCE/SEI 7: Minimum Design Loads of Buildings and Other Structures describe seismic protection guidelines for heat exchangers.
As mentioned, in some U.S. and Canadian municipalities it is mandatory to use double-wall heat exchangers for domestic and potable water applications. However, some state and local plumbing codes recommend using single-wall heat exchangers except when a toxic fluid is being used, such as stipulated in New York City Plumbing Code Section 608.16.3 and Illinois Plumbing Code Section 890.1220.a.4. Is propylene glycol considered a toxic fluid? Although it and ethylene glycol might be toxic, some plumbing applications require them to generate domestic hot water on the solar loop system in cold climates. As plumbing engineers, we need to use our previous experience, knowledge and good judgment to design systems that do not conflict with the local code.
All materials used in the fabrication of heat exchangers are regulated by the ASME code and are subject to compliance. Most manufacturers do not recommended the materials of construction because these materials are subject to research and development by an expert in metallurgy. From my experience on past projects, I can suggest the following materials based on the plumbing application.
• Domestic hot water: All materials used in contact with DHW must be copper, brass, copper nickel or stainless steel for shell and tube heat exchangers. The plate-type heat exchangers use stainless steel with stainless steel liner connections.
• Potable water: To prevent cross-contamination with toxic fluids, double-wall tube of copper, brass or copper nickel is used in the construction of S&T exchangers. Plate-type heat exchangers use double-wall stainless steel plates with stainless steel liner connections.
• Swimming pool (chlorinated water) applications: Use copper nickel or brass tube bundles in S&T heaters and titanium plates for plate and frame heat exchangers.
• High-temperature hot water: For potable water use double-wall copper nickel tubes or stainless steel.
Stainless steel (a corrosive material in chlorinated water) typically is found in the specification of the heat exchanger for swimming pool applications. However, when I worked at the factory I learned that stainless steel bundle construction could be destroyed in three months of operation. Some engineers now are using plate and frame heat exchangers with stainless plates and specifying an increase in the mil size (thickness) of the plates than the manufacturer programs are defaulting to because titanium plates have a longer lead time and higher cost impact on the project.
Case Study: Thermal Design of a Heat Exchanger for a Swimming Pool Application
A heat exchanger is needed for a large outdoor swimming pool requiring a heat load of 9 million Btuh. A shell and tube heat exchanger will be required to satisfy the heating capacity, and it will be interconnected with the central heating plant using low-pressure steam convertors to preheat the pool water to 70°F. The pool’s heat exchanger will need to heat chlorinated water from 70°F to 80°F using hot water from 210°F to 140°F.
Using software from one of the major S&T heat exchanger manufacturers, the original specification can be found in Table 1.
The major constraints and limitations found after the original specification follow:
1. Multiple units should be selected for redundancy and space constraints. Remember that you need to allow room to remove the bundle for service.
2. The percent of chlorine in the water needs to be specified for noncorrosive materials that come in contact with the pool water.
Options for the original specification are:
1. For the head that is in contact with the pool water, the material can be steel with an epoxy coating to avoid future corrosion, copper nickel tubes and tube sheet, but space limitations may need to be addressed.
2. Bypassing a portion of the flow (half of the flow in this case, see Figure 5 for reference) and increasing the temperature rise from 10°F to 20°F (70°F to 90°F) on the heat exchanger can achieve the capacity (Btuh) required with smallest unit.
Using the option 2 approach, the results are as found in Table 2.
With the specification in Table 2, you can use noncorrosive materials for the components in contact with the pool water, such as a cast bronze head, copper nickel tubes, and copper nickel tube sheet, saving approximately 30–40 percent in costs compared to the original specification shown in Table 1, and space limitations are achieved as well.
With the growing number of green buildings and net-zero projects, plumbing engineers will become more involved in specifying heat exchangers for solar thermal applications, instantaneous and/or semi-instantaneous water heaters, and indirect water heaters and hot water generators that require tube bundles using water or steam as the heating media. Swimming pool heat exchangers are very well known in HVAC engineering and now are becoming more popular with plumbing engineers.
As you can see from the case study, by using a bypassing partial flow design on the heat exchanger and increasing the temperature rise, you can achieve the same heating capacity with a smaller unit. (Bypassing flow design uses the same principles as mixing valves.)
In the end, use your engineering knowledge and judgment to provide your clients with safe and cost-effective options for their projects.
Jose R. Rodriguez, P.Eng., is a Technical Services Engineer with Wallace Eannace Associates Inc., a leading manufacturer representative in the New York City area specializing in plumbing, heating/cooling, and industrial processes for fluid- and steam-handling equipment. He is a licensed Professional Engineer in Ontario, Canada and is an active member of ASPE and ASHRAE. He can be reached at firstname.lastname@example.org.