Heat exchanger basics

Heat exchangers are commonly used in a variety of plant processes to transfer energy from one fluid or gas to another without mixing the two substances.

04/01/2000


Heat exchangers are commonly used in a variety of plant processes to transfer energy from one fluid or gas to another without mixing the two substances. As integral parts of comfort and process heating and cooling applications, they do, in most cases, perform efficiently and effortlessly for years. However, because they are parts of larger systems, they are often installed and forgotten, leading to problems down the road or less-than-optimum performance. A little knowledge about heat exchangers and how they operate can help plant engineers make better choices and install and maintain these devices more appropriately and cost effectively.

Each of the major types-shell-and-tube, spiral, and plate-are described and discussed here. Included are illustrations, operating principles, and applications. For more detailed information on the numerous designs, styles, and configurations of heat exchangers that are available, visit some of the web sites described in the resource guide at the end of this article.

Plant Engineering magazine acknowledges with appreciation the special contributions made to this article by Alfa Laval Thermal Inc., Richmond, VA, and ITT Heat Transfer, Buffalo, NY. Headline art courtesy ITT Heat Transfer.

Shell-and-tube heat exchangers

The shell-and-tube heat exchanger is probably the most common type found in industry. It is widely used in the process industries as well as in many types of HVAC equipment. Uses for these workhorses of heat transfer include heat removal in chillers, condensers, and reboilers, process stream cooling, and as critical parts of evaporative cooling and refrigeration systems.

Shell-and tube units consist of round tubes mounted in cylindrical shells. Components include the head, tube bundle, and shell. They can be built to any diameter or length. Tube bundles are typically hydrostatically tested. Many are ASME-designated as pressure vessels. The shell is a cylinder of seamless or rolled and welded pipe with a bolting flange at each end. Carefully placed holes in the tubesheets accommodate the tubes. Baffles help create the turbulence needed for heat transfer.

Shell-and-tube heat transfer technology has remained essentially the same over the years. Most recent developments include enhanced surface tubes that let units achieve approach temperatures of as small as 2-3 deg F. True counterflow construction is now commonly used to avoid heat-transfer limiting pinch points. Finally, some units feature double-wall construction, essentially a second tube within each tube. Double-wall construction offers significant leak protection and safety, though at higher cost.

Equipment comes in many design variations. Straight tube and U-tube configurations are popular. Compact U-tube units save space, feature removable, replaceable bundles, and are more frequently found in HVAC applications (although today some are finding their way into process situations). Fixed, straight-tube arrangements are more often specified in process situations because they can handle higher pressures and are easier to clean mechanically. Among other configurations is the straight tube, floating tubesheet unit, which features a removable tube bundle, a stationary tubesheet at one end, and a floating head at the other end to accommodate differential expansion and relieve stresses.

Spiral heat exchangers

A spiral heat exchanger is constructed by rolling two relatively long metal strips around a mandrel to form two concentric spiral channels. The channels are alternately welded on opposite ends to form a hot and cold channel. Welding the channels eliminates the potential for any cross-contamination of fluids and is analogous to a welded tube to tube-sheet joint in a shell-and-tube heat exchanger.

On one side, the hot fluid enters the center nozzle of the hot cover and flows in a spiral outward to a nozzle on a peripheral header. The cold fluid simultaneously enters a peripheral header and flows countercurrently to the hot fluid to the center nozzle on the cold side cover. Removable cover heads with full-face gaskets are used to seal the open end of the channels and prevent bypassing of a respective fluid from the peripheral header to the center nozzles. The heads are easily removed to allow access to all heat transfer surfaces.

The countercurrent monochannels give exceptionally high convective heat transfer coefficients due to the high turbulence and secondary flow effects (eddy currents and vortices). The monochannel also minimizes the potential for fouling to occur because any buildup in the channel results in an increase in local velocity at that point, an action that tends to flush the deposit away. When a spiral heat exchanger does require cleaning, all heat transfer surfaces are readily accessible by simple removing the heads.

Spiral heat exchangers are particularly effective for handling sludges, liquids in suspension including slurries, and a wide range of viscous fluids. Their design and fabrication make them well suited for controlling viscosity, a vital parameter when abrasive or corrosive fluids must be handled. The spiral heat exchanger is also used as a condenser and evaporator.

Plate heat exchangers

The plate heat exchanger consists of a series of thin, corrugated alloy plates, which are gasketed and compressed together inside a carbon steel frame. Once compressed, the plate pack forms an arrangement of parallel flow channels. The two fluids (hot and cold) flow countercurrent to each other in alternate channels. Each plate is fitted with a gasket to direct the flow, seal the unit, and prevent fluid intermixing. Plate heat exchangers are frequently found in a wide range of heating and cooling applications in the chemical, petrochemical, petroleum, pulp and paper, and pharmaceutical industries, as well as in many wastewater treatment applications.

The proper choice of gasket materials is important for the reliable operation of plate heat exchangers. For nearly 60 yr, these units have primarily used elastomer gaskets to seal the unit, direct the flow, and prevent fluid intermixing. Commonly used elastomers today are variations of three basic materials: nitrile, ethylene propylene diene terpolymer (EPDM), and Viton. Nitrile is the most common and is suitable for fluids such as water, oils, and foodstuffs. EPDM is used for fluids such as water, steam, dilute acids, amines, and strong alkalines. Viton is the most expensive material and is typically used for aggressive fluids such as concentrated acids and some petroleum oils.

Basic heat transfer theory

The design of any heat exchanger is governed by the following equations.

Q =U 3 A 3 (f 3 LMTD)

Where:

Q=Rate of heat transfer, Btu/hr (duty)

A=Net heat transfer area, sq ft (surface area)

U=Overall heat transfer coefficient

(Btu/hr 3 hr 3 sq ft 3 deg F)

f=LMTD correction factor for

nonideal flow (dimensionless)

LMTD = Log mean temperature difference, deg F

The goal is to minimize the surface area requirement, and thus the cost, of a given heat exchanger.

Restating the equation to solve for A yields:

A = Q/U 3 (f 3 LMTD)

The heat transfer area is minimized by maximizing the U value and LMTD for a given heat transfer duty. Examining the various parts of this equation, the heat transfer requirement or duty is usually user defined. It is expressed as a desire to heat or cool a certain flow-rate of fluid by a given amount. Duty is calculated in this way:

Q = M 3 C

p

3 DT

Where:

M = Flow rate of fluid, lb/hr

C

p

= Specific heat of fluid, Btu/lb/deg F

DT = Temperature change of fluid, deg F

The LMTD is calculated as:

DT

1

- DT

2

divided by L

n

3 DT

1

/DT

2

The arithmetic average can be used, but does not account for the diminishing returns effect caused by close temperature approaches (see illustration above). In general, countercurrent flow gives the greatest LMTD values with cocurrent giving the smallest. In most shell-and-tube equipment, the flow is actually a combination of both, and a correction of up to 30% may be required from the ideal countercurrent values calculated.

The overall heat transfer coefficient, or U value, is calculated as the sum of various resistances to heat transfer that might be encountered. Its basic form is:

1/U = 1/h

hot

+ Rf

hot

+ t/k + Rf

cold

+1/h

cold

Where:

h = Individual film coefficient (how effective the fluid is in transferring its heat to the wall)

Rf = Fouling resistance, measured as thickness of fouling layer divided by the thermal conductivity of the fouling material

t/k = Resistance of the heat transfer surface, measured as the wall thickness divided by the thermal conductivity of the wall material, all units are sq ft/ hr/deg F/Btu

The way to maximize the overall heat transfer coefficient, U, is by maximizing the individual heat transfer coefficients, h, and by minimizing the resistance due to fouling, Rf. The film coefficients are affected by the physical properties of the fluids (viscosity, thermal conductivity, and specific heat), and the degree of turbulence of the fluid. Both plate and spiral heat exchangers enhance the effectiveness of heat transfer by inducing turbulence into the fluid (see illustration above, right).

The fouling resistance, Rf, is minimized by limiting the buildup of fouling on the heat transfer surface. This condition is primarily controlled by wall shear stress. Again, both plate and spiral heat exchangers have inherently higher wall shear stresses than conventional tubular equipment. In general, for equal pressure drops, plate and spiral heat exchangers have 16 and 4 times greater wall shear stress, respectively. Additionally, spirals have the added benefit of being a single channel device, which also minimizes the fouling tendency of fluids.

Because of their high shear stresses, induced turbulence, and countercurrent flow paths, both plate and spiral heat exchangers do well in optimizing the heat transfer capabilities of the equipment.

Information for the heat transfer theory section was provided by Alfa Laval.

A guide to heat exchanger resources

For more information about heat exchangers,


visit the web sites listed here.


www.alfalaval.com


Alfa Laval Thermal Inc.


www.baltimoreaircoil.com


Baltimore Aircoil


www.bellgossett.com


Bell&Gossett


www.brownfintube.com


Brown Fintube Co.


www.exothermics.com


Exothermics, Inc.


www.flatplate.com


FlatPlate, Inc.


www.heatcraft.com


Heatcraft, Inc.


www.ittstandard.com


ITT Standard


www.kemco.net


Krueger Engineering and Mfg. Co., Inc.


www.modine.com


Modine Mfg. Co.


www.tranter.comTranter, inc.





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