Heat guide
Heat Exchangers and Energy Balance
A heat exchanger transfers thermal energy between fluid streams while usually keeping the fluids physically separate.
Energy balance on each stream
For sensible heating or cooling with approximately constant specific heat, each stream transfers Q̇ = ṁcpΔT. In an ideal adiabatic exchanger, heat lost by the hot stream equals heat gained by the cold stream.
Real measurements may not balance perfectly
Heat loss to surroundings, property variation, measurement uncertainty and unsteady operation can create differences between the hot-side and cold-side energy estimates.
Flow arrangement changes performance
Parallel flow, counterflow and crossflow produce different temperature profiles and effectiveness. Counterflow can achieve a larger average temperature driving force for many applications.
LMTD and effectiveness-NTU methods
Detailed design often uses the logarithmic mean temperature difference with overall conductance UA, or the effectiveness-NTU method when outlet temperatures are not all known.
Phase change needs a different energy term
Condensers and evaporators can transfer large amounts of latent energy with little refrigerant temperature change. A simple sensible-heat calculator does not capture that behaviour.
Effectiveness links real performance to the theoretical maximum
Heat-exchanger effectiveness compares the actual heat-transfer rate with the maximum thermodynamically possible rate for the inlet conditions and heat-capacity rates of the two streams. The effectiveness-NTU method is especially useful when outlet temperatures are unknown but exchanger size and flow properties are available.
This complements the logarithmic mean temperature difference method, which is convenient when terminal temperatures are known. Both approaches describe the same energy-transfer device from different sets of known information.
Heat exchangers transfer energy without mixing the streams in many designs
A typical heat exchanger keeps hot and cold fluids physically separated by a wall while allowing energy to conduct through that wall and convect between each fluid and the surface. Shell-and-tube, plate and finned-tube designs arrange area and flow differently to achieve the required duty.
Some exchanger types allow direct contact, so the general concept is broader than one construction.
Temperature difference changes along the exchanger
The hot and cold streams usually change temperature as they flow, so one inlet temperature difference does not represent the entire device. The log-mean temperature difference method provides an effective driving difference for many steady exchanger calculations when terminal temperatures are known.
Counterflow arrangements can maintain a larger useful temperature difference over the exchanger than comparable parallel-flow arrangements.
Fouling and pressure drop are practical design constraints
Deposits on heat-transfer surfaces add thermal resistance and can reduce performance over time. Increasing fluid velocity can improve convection but also raises pressure drop and pumping power.
A practical exchanger design therefore balances heat-transfer area, flow arrangement, allowable pressure loss, fouling, cleaning access and material compatibility rather than maximising one coefficient in isolation.
Counterflow can preserve a larger temperature driving force
In a counterflow exchanger, the two fluids move in opposite directions, which can maintain a useful temperature difference over more of the exchanger length than an equivalent parallel-flow arrangement. This often allows a higher effectiveness for the same overall conductance and heat-capacity rates.
The outlet temperature of one stream can approach the inlet temperature of the other without violating the second law because the local hot stream remains hotter than the local cold stream at every position in an ideal counterflow exchanger.
Fouling adds resistance and changes pressure drop
Deposits on heat-transfer surfaces add thermal resistance and can reduce flow area. The exchanger may then transfer less heat while requiring more pumping power. Industrial design therefore includes fouling allowances, cleaning access and monitoring rather than assuming clean-surface performance indefinitely.
A declining outlet-temperature approach can indicate fouling, but changes in flow rate, inlet conditions or control settings can produce similar symptoms. Diagnosis should use a full operating data set.