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What Is a Temperature Approach? | Process Engineering Glossary
What Is a Temperature Approach?
In piping engineering and process engineering, temperature approach is the temperature difference between the hot stream and the cold stream at either end of a heat exchanger, or at the point of closest approach within the exchanger. It is the driving force for heat transfer at that location. A large temperature approach produces rapid heat transfer at that end and requires less heat transfer surface area. A small temperature approach produces slow heat transfer and demands more surface area for the same duty, but enables greater heat recovery and reduces the external utility consumption. The temperature approach is therefore the central design variable that governs the trade-off between heat exchanger capital cost and process energy cost.
Applications of Temperature Approach
Crude Oil Preheat Train
Crude oil atmospheric distillation preheat trains in refineries consist of a series of heat exchangers that heat the cold crude oil against successive hot product streams. The temperature approach in each exchanger is set by the inlet and outlet temperatures of both streams, which are governed by the overall preheat train design. Engineers balance the approach temperatures across the train to recover the maximum heat with the minimum total surface area, using pinch analysis to identify the temperature approach at which the overall capital and energy costs are minimised.
Feed-Effluent Exchangers
Feed-effluent heat exchangers around reformers, dehydrogenation reactors, and high-temperature catalytic reactors heat the cold feed against the hot reactor effluent. The temperature approach at the cold end of the exchanger determines how close the feed temperature gets to the effluent outlet temperature. A small cold end approach recovers more heat from the effluent and reduces the fired heater duty, but requires more exchanger area and creates a tight temperature constraint that the control system must manage when the feed or effluent flow rates change.
Refrigeration System Design
In refrigeration systems, the approach temperature between the refrigerant condensing temperature and the cooling medium temperature, and between the refrigerant evaporating temperature and the process stream being cooled, directly determines the coefficient of performance of the refrigeration cycle. Smaller approach temperatures produce more efficient refrigeration cycles but require larger heat exchangers in both the condenser and the evaporator. Optimising these approach temperatures is a key step in minimising the total energy cost of refrigeration systems.
Benefits of Understanding Temperature Approach
Correct Exchanger Sizing
Using the correct minimum approach temperature when calculating the LMTD ensures the heat exchanger is sized with the correct area. An excessively small approach temperature assumption produces an undersized exchanger with insufficient area to achieve the design heat recovery at the available temperature driving force. An excessively large approach temperature assumption produces an oversized exchanger with more area than needed and an unnecessarily large capital cost.
Energy Efficiency Optimisation
Systematically minimising the approach temperatures across a heat exchanger network, subject to the constraint that all approaches remain above the design ΔTmin, extracts the maximum heat recovery from the process streams and minimises external utility consumption. This optimisation, guided by pinch analysis, is the primary tool for energy efficiency improvement in new plant designs and retrofit projects.
Early Detection of Fouling
Monitoring the approach temperatures of operating heat exchangers detects fouling as a change in the approach temperature profile before it appears as a product quality problem or a utility overconsumption alarm. The approach temperature responds to fouling more sensitively than the outlet temperature alone because it combines information about both streams simultaneously.
Limitations to Consider
Approach Temperature Below the LMTD Correction Threshold
When the minimum approach temperature becomes very small relative to the overall temperature range of the exchanger, the LMTD correction factor F for multi-pass shell and tube exchangers falls below the acceptable minimum. This indicates that the exchanger configuration is thermodynamically inefficient. Splitting the duty between two exchangers or changing to a countercurrent configuration restores the F-factor and makes the design practical.
Variable Physical Properties
The LMTD method assumes constant specific heat for both streams, which produces a linear temperature profile along the exchanger and justifies the logarithmic mean as the correct average driving force. For streams with strongly varying specific heat across the temperature range, such as near-critical fluids or wide-boiling multicomponent mixtures, the actual temperature profile is non-linear and the LMTD method introduces error. Zone-by-zone integration of the heat duty against the local temperature difference gives a more accurate result.
Approach Temperature and Controllability
Very small approach temperatures in heat exchangers, especially those in closed heat integration loops, can create control difficulties because small changes in one stream temperature produce large changes in the other stream temperature. This strong thermal coupling between streams can cause oscillations in the integrated heat exchanger network that are difficult to damp with standard feedback control tuning. Allowing a somewhat larger approach temperature than the thermodynamic optimum in tightly integrated sections of the network is sometimes necessary to achieve acceptable control performance.
Temperature Approach FAQ
What is temperature approach in process engineering and how does it relate to heat exchanger sizing? Temperature approach is the temperature difference between the hot and cold streams at either end of a heat exchanger, representing the local driving force for heat transfer at that point. Process engineering uses the log mean temperature difference, calculated from the two terminal approach temperatures, in the equation Q = U × A × LMTD to determine the required heat transfer area. A smaller minimum approach temperature increases the LMTD correction factor requirement and increases the needed surface area for the same duty. The overall heat transfer coefficient (U) and the LMTD together determine the area needed to meet the design heat duty.
How does temperature approach interact with fouling and heat exchanger performance monitoring? As fouling factor deposits accumulate on heat transfer surfaces, the overall heat transfer coefficient (U) falls and the approach temperatures shift away from their clean-condition design values. Monitoring the approach temperatures against design values provides an early warning of fouling before the performance degradation appears as a utility overconsumption problem. The cold end approach temperature of a cooler typically increases as the exchanger fouls, because the hot stream cannot be cooled as deeply with reduced heat transfer. In contrast, the cold end approach of a film coefficient-limited exchanger where the controlling resistance is on the cold side may decrease as the overall performance drops, depending on the specific temperature configuration.
How does minimum temperature approach govern heat integration and pinch analysis? In heat integration and pinch analysis, the minimum temperature approach ΔTmin is the closest temperature difference allowed between any hot and cold stream in the composite curve diagram. It defines the pinch point and sets the minimum external utility requirements for the entire plant. A smaller ΔTmin allows more heat recovery between process streams at the cost of more heat transfer area in the network. The optimum ΔTmin minimises the total annual cost of the heat exchanger network by balancing the energy cost reduction against the capital cost increase. Every individual exchanger in the network, including the overhead condenser of a distillation column used for heat integration, must maintain its approach temperature above ΔTmin to avoid inadvertently transferring heat across the pinch and increasing both the hot and cold utility requirements above the minimum targets.
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