The Steam Table That Defines the Limit
A chemical plant requiring 150 kW of heat at a process temperature of 160°C has access to steam at 12 bar, which provides a saturated temperature of 191°C. The metallic heat exchanger specified for this duty is a shell-and-tube unit with a 6 mm stainless steel tube wall. The PTFE alternative must be evaluated against the same duty.
At 12 bar and 191°C, the saturated steam pressure is well within the range of metallic heat exchangers. For PTFE, the first constraint is the maximum operating pressure. Standard PTFE heat exchangers are rated for 0.5–1.0 MPa (5–10 bar) at moderate temperature, but the pressure rating decreases as the temperature increases. At 190°C, the allowable working pressure for a typical PTFE tube may be 0.3–0.5 MPa, which is below the steam pressure required.
The second constraint is the temperature-dependent strength. PTFE retains approximately 50% of its room-temperature tensile strength at 190°C, and the creep rate increases significantly. The tube wall must be thicker to withstand the pressure at the elevated temperature, which increases the thermal resistance and reduces the heat transfer coefficient.
The third constraint is the internal stress that develops when the tube wall is heated by the steam. The steam side of the tube is at 191°C, and the process side is at 160°C, which creates a temperature difference of 31°C across the wall. In a low-conductivity material, this temperature difference produces a significant thermal stress that adds to the pressure stress.
The Pressure Containment Limit
Pressure containment is the most fundamental limitation. The hoop stress in a thin-walled tube under internal pressure is proportional to the pressure and the tube diameter and inversely proportional to the wall thickness. For a PTFE tube with a 6 mm outer diameter and a 1.0 mm wall, the hoop stress at 10 bar internal pressure is approximately 30 MPa. The tensile strength of PTFE at 190°C is approximately 10–12 MPa, which means the tube cannot contain the pressure with an adequate safety factor.
Increasing the wall thickness reduces the hoop stress but increases the thermal resistance. A 2.0 mm wall reduces the hoop stress to 15 MPa, which is within the strength limit, but the thermal resistance doubles, and the heat transfer coefficient decreases. The heat exchanger must be larger to compensate, which increases the capital cost and the space requirement.
The alternative is to reduce the tube diameter. A 3 mm outer diameter tube with a 1.0 mm wall has a hoop stress of 15 MPa at 10 bar, which is within the strength limit. The smaller diameter tube has a higher surface area per unit volume, which partially compensates for the lower heat transfer coefficient.
The Temperature-Dependent Strength Limit
The tensile strength of PTFE decreases with temperature. At 20°C, the tensile strength is approximately 25–30 MPa. At 100°C, it is approximately 15–18 MPa. At 190°C, it is approximately 10–12 MPa. At 250°C, it is approximately 5–6 MPa.
The decreasing strength limits the allowable stress and the allowable pressure. A heat exchanger that is rated for 10 bar at 100°C may be rated for only 4–5 bar at 190°C. The derating factor is typically 0.5–0.6 at 190°C compared to the room-temperature rating.
The derating is not a linear function of temperature. The strength decreases slowly up to 150°C and more rapidly above 200°C. The practical limit for high-pressure steam service is approximately 150–180°C, above which the pressure rating becomes too low for most industrial steam systems.
The Creep Limit
Creep is the slow deformation of a material under sustained load. PTFE creeps at room temperature and the creep rate increases with temperature. At 190°C, the creep rate can be 10–20 times higher than at 20°C.
In a heat exchanger, the creep manifests as a slow increase in tube diameter and a slow decrease in wall thickness. The tube expands under the internal pressure, and the wall thins. Over time, the tube may rupture or the tube-to-tube sheet joint may leak.
The creep limit is typically expressed as the maximum stress that produces less than 1% strain in 10,000 hours. For PTFE at 190°C, this stress is approximately 3–4 MPa, which is much lower than the tensile strength. The creep limit, not the tensile strength, is the controlling parameter for long-term service.
The Thermal Stress Limit
The thermal stress in the tube wall is proportional to the temperature difference across the wall, the coefficient of thermal expansion, and the modulus of elasticity. For PTFE, the coefficient of thermal expansion is approximately 12 × 10⁻⁵ /°C, and the modulus at 190°C is approximately 200 MPa.
For a temperature difference of 31°C across the wall, the thermal stress is approximately 7.4 MPa. This stress adds to the pressure stress and reduces the allowable pressure. In a high-pressure steam application, the thermal stress can be a significant fraction of the total stress, and it must be included in the design calculation.
The thermal stress can be reduced by reducing the temperature difference across the wall, which means reducing the heat flux. A lower heat flux requires a larger surface area, which increases the capital cost.
Comparing PTFE and Metallic Units in High-Pressure Steam Service
The following table summarizes the performance and cost characteristics of PTFE and stainless steel heat exchangers in high-pressure steam service at 12 bar and 191°C.
| Characteristic | Stainless Steel Heat Exchanger | PTFE Heat Exchanger | Advantage |
|---|---|---|---|
| Maximum operating pressure at 191°C | 25 bar | 4–5 bar | Stainless steel |
| Maximum operating temperature | 400°C | 260°C | Stainless steel |
| Tensile strength at 191°C | 140 MPa | 10–12 MPa | Stainless steel |
| Creep limit at 191°C | Not applicable | 3–4 MPa | Stainless steel |
| Thermal conductivity | 16 W/m·K | 0.25 W/m·K | Stainless steel |
| Overall heat transfer coefficient | 800 W/m²·K | 200 W/m²·K | Stainless steel |
| Surface area required for 150 kW | 5 m² | 20 m² | Stainless steel |
| Thermal stress at 31°C ΔT | 15 MPa | 7.4 MPa | PTFE |
| Chemical resistance to steam condensate | Good | Excellent | PTFE |
| Service life in steam service | 20–30 years | 8–12 years | Stainless steel |
| Capital cost for 150 kW duty | $45,000 | $38,000 | PTFE |
| Installation cost | $6,000 | $5,000 | PTFE |
| Annual maintenance cost | $1,500 | $3,000 | Stainless steel |
| Five-year total cost | $82,500 | $90,000 | Stainless steel |
The five-year total cost values assume a production value of $500 per hour and include the capital cost, the installation cost, and the maintenance cost over five years.
When PTFE Is Still the Better Choice
Despite the mechanical limitations, PTFE is still the better choice in several high-pressure steam applications.
Corrosive condensate service. If the steam condensate contains corrosive species such as chlorides or fluorides, the stainless steel unit may corrode, while the PTFE unit is resistant.
Low-pressure steam service. At steam pressures below 5 bar and temperatures below 150°C, the PTFE mechanical limitations are less severe, and the PTFE unit may be competitive.
High-purity service. If the process fluid is sensitive to contamination from metal ions, the PTFE unit is the better choice because it does not leach metal ions.
Small heat load service. For small heat loads, the larger surface area of the PTFE unit is less of a constraint, and the lower capital cost may favor PTFE.
Retrofit service. If the existing heat exchanger is a PTFE unit and the steam pressure is increased, the PTFE unit may still be adequate if the pressure is within the derated limit.
Design Modifications That Extend the PTFE Pressure Limit
Several design modifications extend the pressure limit of PTFE heat exchangers in high-pressure steam service.
Smaller tube diameter. A smaller diameter tube has a lower hoop stress for the same pressure and wall thickness. A 3 mm tube can withstand higher pressure than a 6 mm tube with the same wall thickness.
Thicker tube wall. A thicker wall reduces the hoop stress but increases the thermal resistance. The wall thickness must be optimized for the specific pressure and heat transfer requirements.
Reinforced tube construction. A PTFE tube with a metallic or composite reinforcement can withstand higher pressure than an unreinforced tube. The reinforcement may be a wire braid, a spiral wrap, or a composite layer.
Reduced operating temperature. Lowering the steam temperature reduces the thermal stress and increases the material strength. A lower steam pressure or a desuperheater can reduce the temperature.
Reduced heat flux. A lower heat flux reduces the temperature difference across the wall and the thermal stress. A larger surface area is required, which increases the capital cost.
Floating tube sheet. A floating tube sheet reduces the axial stress on the tubes and the tube sheet, which improves the pressure containment.
Specification Parameters for High-Pressure Steam Service
Equipment engineers evaluating PTFE heat exchangers for high-pressure steam service should confirm the following parameters with the supplier.
Steam pressure and temperature. Specify the maximum steam pressure and the saturated temperature.
Process temperature. Specify the process fluid temperature and the approach temperature.
Tube diameter and wall thickness. Specify the tube diameter and wall thickness based on the pressure and heat transfer requirements.
Pressure rating. Specify the pressure rating at the operating temperature, not at room temperature.
Creep limit. Specify the creep limit and the maximum allowable stress for long-term service.
Thermal stress. Specify the thermal stress calculation and the allowable stress.
Reinforcement. Specify whether the tube is reinforced and the reinforcement type.
Service life requirement. Specify the expected service life and the replacement schedule.
Matching PTFE Design to Steam Conditions
The limitations of PTFE heat exchangers in high-pressure steam service are pressure containment, temperature-dependent strength, creep, and thermal stress. These limitations can be managed by design modifications such as smaller tube diameter, thicker wall, reinforcement, and reduced heat flux. The practical limit for PTFE in high-pressure steam service is approximately 5 bar at 190°C, above which the design becomes impractical or the capital cost becomes prohibitive.
The selection decision should be based on a facility-specific analysis that incorporates the steam conditions, the process requirements, the capital cost, and the maintenance cost. A simple spreadsheet model with these parameters provides the information needed to make the decision.
For high-pressure steam service facilities evaluating heat exchanger options, the material selection should be based on the total cost of ownership over the expected service life, not just the capital cost or the thermal performance. A metallic unit with a higher capital cost may have a lower lifetime cost if it can handle the steam conditions without derating.
Equipment engineers and procurement specialists evaluating PTFE heat exchanger specifications for high-pressure steam or other demanding service can request a design review covering steam pressure, steam temperature, tube diameter, wall thickness, pressure rating, creep limit, and thermal stress to confirm the unit is matched to the actual steam conditions and process requirements of the facility.
