ME421Heat Exchanger and

Steam Generator Design

Lecture Notes 8 Part 1Gasketed-Plate Heat Exchangers

Introduction• Initially developed for the food industry: easy to clean.• Now, alternative to shell-and-tube HEX for low- to

medium-pressure applications and liquid-liquid heat transfer.

• Design is specialized and proprietary.• Manufacturers have computerized design procedures for

their HEX.• Typical gasketed-plate HEX consists of plates with

gaskets and the frame, which includes a fixed plate, a compression (pressure) plate, an upper carrier bar, a guidance bar, a support column, and tightening bolts and nuts.

Typical Gasketed-Plate HEX

Flow Pattern: Single-Pass Counterflow

Mechanical FeaturesPlate Pack and Frame• Plates are pressed together – the holes at the corners form

continuous tunnels/manifolds, leading the fluid from the inlet of the plate pack to the narrow channels between plates

• Plate pack tightened by mechanically/hydraulically• Two fluids flow in alternating channels, mostly in counterflow• Heat transfer occurs through the thin plate wall• There can be several hundred plates in a frame, held together

by bolts that hold the stack in compression• Carrier and guidance bars are bolted to the fixed frame and

they support the plates• Plate pack corresponds to the tube bundle in a shell-and-tube

HEX, but the two sides of the plate have identical hydrodynamic characteristics unlike two sides of a tube

Gasketed-Plate HEX Assembly and Gasket

Mechanical FeaturesPlate Pack and Frame (continued)• Plate: A sheet of metal precision-pressed into a corrugated

pattern– Largest single plate: ~4.3 m high x 1.1 m wide– Heat transfer area for a single plate: 0.01 – 3.60 m2

– To avoid poor distribution of fluid across the plate width, minimum length/width ratio: ~1.8

– Plate thickness: 0.5 – 1.2 mm– Plate spacing: 2.5 – 5.0 mm– Hydraulic diameter: 4 – 10 mm

• Leakage prevented by gaskets• Common plate materials are stainless steel, titanium; see

Table 10.1 for others and respective thermal conductivities (wrong unit in table)

• Plate number and size determined by: flow rate, fluid properties, pressure drop, and temperature requirements

Mechanical FeaturesPlate Types• Various corrugation types are available• Chevron type is most common, but also washboard pattern• Prediction methods rely on experimental data• Washboard: promotes turbulence by continuously changing

flow direction and velocity• Chevron: flow channel provides swirling motion to the fluids

– Chevron angle, , 25o – 65o range, is reversed on adjacent plates so that corrugations provide numerous contact points when plates are pressed together

– Chevron angle determines pressure drop and heat transfer characteristics of the plate

– Plates can be very thin, ~0.6 mm

Plate Types and Chevron Angle

Typical Chevron Plates

Chevron Plate

Operational CharacteristicsGasketed-plate HEX can be opened for inspection, cleaning, maintenance, or rebuilding within the length of the frame.

Operational CharacteristicsMain Advantages• Gasket design minimizes internal leakage risk. External

leakage is easily detected.• Flexible design through a variety of plate sizes and pass

arrangements• HT area easily accessible → change configuration to suit

different process requirements by changing number of plates• Efficient HT: High HT coefficients for both fluids due to

turbulence and small hydraulic diameter• Compact (large HT area/volume ratio), low weight; ~1500 m2

surface area in one unit• Heat losses are negligible, no insulation required• If gaskets fail, intermixing of fluids cannot occur• Low fouling due to high turbulence and low residence time

Operational CharacteristicsMain Advantages (continued)• More than two fluids in one unit possible, by using

connecting plates• Transition to turbulence occurs at low Re, 10 – 400• Due to thin walls, wall resistance minimized• About 82% of the theoretical LMTD is utilized• Similar cost with tubular HEX, if tubes are made of costly

material like stainless steel• Lower costs for handling, transportation, and foundations, as

well as cleaning.

Operational CharacteristicsPerformance Limits• Gaskets impose restrictions

– Operating temperatures (160oC – 250oC)– Pressures (minimum 25 – 30 bar)– Nature of fluids

• Friction factors are high but channel lengths are short and flow velocities are low so pressure drops can be kept within limits for single-phase flow applications

• Upper limit on the plate size due to available presses, thus largest units ~1500 m2

• Maximum design pressure ~1 MPa• Not suitable for air-to-air or gas-to-gas applications• Not suitable for high viscosity fluids• Velocities lower than 0.1 m/s not used• Only specially-designed units for evaporation/condensation

Passes and Flow Arrangements• Pass: group of channels in which the flow is in the same

directionU: All four ports are on the fixed-head plate, permits disassembly without disturbing external piping. Flow distribution less uniform than Z-arrangement.

(single pass)

(single pass)

Passes and Flow Arrangements

2 x 3 / 2 x 3 configuration (two-pass configuration with 3 channels), counterflow except in the central plate (parallel flow)

2 x 4 / 2 x 4 configuration (two-pass configuration with 4 channels), counterflow except in the central plate (parallel flow)

Passes and Flow Arrangements

2 x 4 / 1 x 8 configuration (two-pass / one-pass flow system), one half of HEX in counterflow, the other half is in parallel flow (asymmetrical system). Used when one fluid has a much higher mass flow rate or smaller allowable pressure drop.

Passes and Flow Arrangements• Multipass arrangements require ports on both fixed and

movable head plates• Usually symmetric conditions prevail (same number of

passes and number of channels per pass for both fluids• Maldistribution may be a problem, must be considered in

design

Applications• Widely used in chemical, pharmaceutical, food, dairy

industries, hygiene products, biochemical processing; due to ease of cleaning

• Also used as process heaters/coolers• Mostly liquid-to-liquid turbulent flow• Corrosion and fouling problems in large systems can be

transferred to plate HEX

ApplicationsCorrosion and Fouling• Highly preferred to resist corrosion, but expensive materials• Corrosion allowance is much smaller than that for tubular units• High turbulence may result in erosion problems• Less fouling than tubular units

– High turbulence maintains solids in suspension– Uniform velocity profiles and no low-velocity zones– Plate surfaces are smooth– Corrosion product deposits, to which fouling can adhere, are absent– High heat transfer coefficients keep wall temperatures low, thus

crystallization is minimized– Frequent cleaning can be easily scheduled

• Assuming a pressure drop of about 30 kPa per NTU, Table 10.4 lists recommended fouling factors

• In general, 5% excess NTU for low fouling duties, 10% for moderate fouling, 15-20% for high fouling