The Thermal Gradient That Develops During Startup
A chemical processing facility starts up a PTFE heat exchanger by opening the steam valve and allowing the steam to flow into the shell. The steam enters at 120°C, while the heat exchanger and the process fluid are at 25°C. The temperature difference at the steam inlet is 95°C.
The steam heats the shell and the tube sheet at the inlet end first. The tube sheet expands as it heats, but the tubes and the tube sheet at the outlet end are still cold. The differential expansion between the hot inlet end and the cold outlet end creates a bending moment in the tube bundle and a shear stress at the tube-to-tube sheet joints.
The thermal gradient also develops through the tube wall thickness. The steam side of the tube wall heats faster than the process side, and the temperature difference across the wall creates a thermal stress. For a PTFE tube with a 0.8 mm wall and a thermal diffusivity of 0.1 mm²/s, the time required for the wall to reach thermal equilibrium is approximately 6 seconds. A rapid startup that changes the steam temperature faster than this equilibrium time creates a temperature gradient across the wall that adds to the thermal stress.
Why the Ramp Rate Matters
The ramp rate determines the magnitude of the thermal gradients and the resulting stress. A slow ramp allows the temperature to equalize across the tube bundle and the tube wall, which reduces the gradients and the stress. A fast ramp creates large gradients and high stress.
The stress from a fast ramp can exceed the yield strength of the PTFE, particularly at the tube-to-tube sheet joints where the stress concentration is highest. The stress can also exceed the fatigue limit of the PTFE, which means that repeated startups with a fast ramp can cause cumulative damage and eventual failure.
The ramp rate effect is particularly important for PTFE because the material has a low thermal conductivity and a high coefficient of thermal expansion. The low conductivity means that the temperature equalizes slowly, and the high expansion coefficient means that a given temperature difference produces a large expansion. The combination makes PTFE more sensitive to thermal shock than metal heat exchanger materials.
The Ramp Rate That Protects the Tubes
The safe ramp rate depends on the temperature range of the startup and the design of the heat exchanger. For a typical PTFE heat exchanger with a 0.8 mm wall and a fixed tube sheet, the safe ramp rate is approximately 1–2°C per minute for a startup from ambient to 120°C. For a heat exchanger with a floating tube sheet, the safe ramp rate can be higher, approximately 2–3°C per minute, because the floating tube sheet accommodates the differential expansion.
The ramp rate should be controlled by the rate of steam valve opening. A slow-opening valve or a bypass arrangement for warm-up allows the steam flow to increase gradually, which limits the rate of temperature rise. A fast-opening valve causes a rapid temperature rise and a high thermal gradient.
The ramp rate should also be controlled during the cool-down period. A rapid cool-down, such as when the steam valve is closed suddenly or when cold water is introduced, creates the same thermal gradients in reverse. The safe cool-down rate is typically the same as the safe heat-up rate.
The Effect of Tube Wall Thickness
The tube wall thickness affects the thermal gradient across the wall during startup. A thinner wall has a lower thermal resistance and a smaller temperature gradient for a given ramp rate. A thicker wall has a higher thermal resistance and a larger temperature gradient.
For a 0.5 mm wall, the thermal gradient at a ramp rate of 2°C per minute is approximately 15°C. For a 1.0 mm wall, the gradient is approximately 30°C. The thicker wall has a higher thermal stress and a lower safe ramp rate.
The effect of wall thickness on the safe ramp rate is significant. A heat exchanger with a 0.5 mm wall can tolerate a ramp rate that is approximately twice the ramp rate of a heat exchanger with a 1.0 mm wall.
The Effect of Tube Sheet Design
The tube sheet design affects the stress at the tube-to-tube sheet joints during startup. A fixed tube sheet constrains the axial expansion of the tubes, which creates a compressive stress in the tubes and a bending stress in the tube sheet. A floating tube sheet allows the tubes to expand freely, which reduces the stress.
For a fixed tube sheet, the safe ramp rate is approximately 1–2°C per minute. For a floating tube sheet, the safe ramp rate is approximately 2–3°C per minute. The floating tube sheet is the preferred design for service with frequent startups.
Comparing Ramp Rates for Different Configurations
The following table summarizes the safe ramp rate and the expected cycle life for different PTFE heat exchanger configurations in service with a startup from 25°C to 120°C.
| Configuration | Tube Wall Thickness | Tube Sheet Design | Safe Ramp Rate | Thermal Gradient at Safe Rate | Expected Cycle Life |
|---|---|---|---|---|---|
| Standard fixed tube sheet | 0.8 mm | Fixed | 1.0°C/min | 20°C | 500–800 cycles |
| Standard fixed tube sheet | 0.5 mm | Fixed | 1.5°C/min | 15°C | 800–1,200 cycles |
| Floating tube sheet | 0.8 mm | Floating | 2.0°C/min | 20°C | 1,500–2,500 cycles |
| Floating tube sheet | 0.5 mm | Floating | 3.0°C/min | 15°C | 2,500–4,000 cycles |
| Floating tube sheet with expansion joint | 0.8 mm | Floating | 3.0°C/min | 20°C | 2,000–3,500 cycles |
| Floating tube sheet with slow-opening valve | 0.8 mm | Floating | 2.0°C/min | 20°C | 2,000–3,000 cycles |
The expected cycle life values assume a startup from ambient to operating temperature once per day and a cool-down to ambient once per day. The cycle life is longer for slower ramp rates and for floating tube sheet designs.
Design Features That Increase the Safe Ramp Rate
Several design features increase the safe ramp rate and reduce the thermal shock risk.
Floating tube sheet. A floating tube sheet allows the tubes to expand freely, which reduces the stress at the tube-to-tube sheet joints.
Thinner tube wall. A thinner wall reduces the thermal gradient and the thermal stress.
Expansion joints in the shell. A bellows expansion joint in the shell allows the shell to expand independently of the tubes, which reduces the differential expansion stress.
Slow-opening steam valve. A slow-opening valve limits the rate of steam flow increase and the rate of temperature rise.
Bypass for warm-up. A bypass arrangement allows a small steam flow to warm the heat exchanger gradually before the main steam valve is opened.
Temperature monitoring. Temperature sensors at the heat exchanger inlet and outlet allow the ramp rate to be monitored and controlled.
Operating Practices That Protect the Tubes
Beyond design features, operating practices can protect the tubes from thermal shock.
Controlled startup procedure. A written startup procedure that specifies the ramp rate and the valve opening sequence ensures that the operators follow the safe startup practice.
Gradual steam valve opening. Opening the steam valve gradually over 15–30 minutes allows the temperature to rise at a controlled rate.
Warm-up bypass. Using a warm-up bypass to introduce a small steam flow before the main valve is opened reduces the initial temperature difference.
Cool-down control. Allowing the heat exchanger to cool gradually after shutdown, rather than closing the steam valve suddenly or introducing cold water, reduces the thermal gradient.
Thermal cycle counting. Tracking the number of startups and the ramp rate provides a basis for predicting the service life and scheduling maintenance.
Specification Parameters for Thermal Shock Protection
Equipment engineers specifying a PTFE heat exchanger for service with frequent startups should confirm the following parameters with the supplier.
Startup frequency. Specify the expected number of startups per day or per year.
Temperature range. Specify the temperature range of the startup and the cool-down.
Ramp rate. Specify the required ramp rate based on the tube wall thickness and the tube sheet design.
Tube wall thickness. Specify the tube wall thickness. A thinner wall allows a higher ramp rate.
Tube sheet design. Specify a floating tube sheet for service with frequent startups.
Expansion joints. Specify whether expansion joints are provided in the shell.
Steam valve type. Specify a slow-opening valve or a bypass arrangement for warm-up.
Temperature monitoring. Specify the temperature sensors and the monitoring method for tracking the ramp rate.
Matching Ramp Rate to Heat Exchanger Design
The startup ramp rate determines the thermal gradients and the stress in a PTFE heat exchanger during startup. A slow ramp allows the temperature to equalize and reduces the stress. A fast ramp creates large gradients and high stress that can exceed the strength of the PTFE and cause premature failure.
The safe ramp rate depends on the tube wall thickness and the tube sheet design. For a standard fixed tube sheet with a 0.8 mm wall, the safe ramp rate is approximately 1–2°C per minute. For a floating tube sheet with a 0.5 mm wall, the safe ramp rate can be 3°C per minute or higher.
For facilities evaluating PTFE heat exchanger options for service with frequent startups, the ramp rate should be specified in the procurement documents and verified at commissioning. The default configuration provided by the manufacturer may not include the design features that allow a higher ramp rate, and the resulting thermal shock may cause premature failure.
Equipment engineers and procurement specialists evaluating PTFE heat exchanger specifications for service with frequent startups or high thermal cycling can request a design review covering startup frequency, temperature range, ramp rate, tube wall thickness, tube sheet design, and expansion joints to confirm the unit is matched to the actual operating cycle and thermal shock requirements of the facility.
