Steam Consumption in Heat Exchangers: Calculation and Design

Steam-heated heat exchangers transfer thermal energy from steam to water or another process fluid through a separating metal surface. As saturated steam condenses, it releases latent heat, which is transferred to the secondary fluid. The resulting condensate must be removed from the exchanger.

Steam consumption in heat exchangers depends on actual heat duty, not simply on the exchanger's published thermal rating. Secondary-fluid flowrate, inlet and outlet temperatures, steam condensing pressure and operating conditions must all be considered.

How heat transfer takes place

In an indirect steam heat exchanger, steam and the secondary fluid circulate in separate passages. Heat travels through the wall between them without the two fluids mixing.

When saturated steam condenses, it releases latent heat. Because saturation temperature varies with steam pressure, pressure in the primary space determines the available heating temperature.

Heat-transfer performance also depends on the condition of the surfaces, secondary-fluid circulation and effective condensate drainage.

Rated capacity versus actual steam consumption

Heat exchangers are often selected from standard equipment ranges, which can result in greater installed heat-transfer area than the process requires.

An exchanger may also be oversized to include design allowances or reduce pressure drop on the secondary side.

For existing systems, actual heat load can be assessed from measured secondary-fluid flow and flow/return temperatures. The pump nameplate flowrate should not automatically be used because it refers to a particular operating head and may not represent the installed duty.

Where necessary, suitable flowmeters can establish actual operating flow.

Calculating heat duty in continuous-flow exchangers

For a continuous-flow exchanger heating a secondary fluid without a phase change:

Q̇ = ṁ × cₚ × ΔT

• Q̇: heat duty, kW.
• ṁ: secondary-fluid mass flowrate, kg/s.
• cₚ: specific heat capacity, kJ/(kg·°C).
• ΔT: secondary-fluid temperature rise, °C.

Steam consumption can then be calculated from its evaporation enthalpy:

ṁₛ = 3,600 × Q̇ / h_fg

This gives steam flowrate in kg/h when heat duty is in kW and evaporation enthalpy is in kJ/kg. It is based on latent heat transfer without separately accounting for significant condensate subcooling.

Worked example: hot-water heating

Consider a continuous-flow steam heater with the following operating conditions:

The temperature rise is 11°C.

Q̇ = 7.2 × 4.19 × 11 ≈ 332 kW

The corresponding steam flow is:

ṁₛ = 3,600 × 332 / 2,139 ≈ 558 kg/h

This value represents the stated full-load condition. Actual steam demand will change if the secondary flowrate or temperature rise changes.

Storage and non-storage water heaters

A non-storage calorifier heats water as it flows through the exchanger, so heat duty is determined mainly by flowrate and temperature rise.

A storage calorifier contains a defined quantity of water and a heating element. The full water inventory must be raised to the specified storage temperature within the required recovery time.

Mean steam consumption is calculated using:

ṁₛ = m × cₚ × ΔT / (h_fg × t)

Here m is the heated water mass in kg and t is the recovery time in hours.

Worked example: heating 2,272 litres

A storage calorifier contains 2,272 kg of water and must heat it from 10°C to 60°C in 30 minutes.

Steam is supplied at 2 bar(g), for which the evaporation enthalpy is 2,163 kJ/kg.

ṁₛ = (2,272 × 4.19 × 50) / (2,163 × 0.5)

ṁₛ ≈ 440 kg/h

This is the average steam flowrate during heat-up. When the water is still cold, the instantaneous condensation rate can be higher.

Start-up conditions and condensate drainage

In systems that shut down overnight or at weekends, secondary-fluid temperature may be significantly lower than design return temperature when operation resumes.

The increased temperature difference creates a higher instantaneous condensation rate. If the control valve cannot admit enough steam, primary-space pressure can fall.

A reduction in steam pressure also lowers the differential pressure across the steam trap. If drainage capacity is insufficient, condensate may accumulate inside the heat exchanger.

Consequences include reduced active heat-transfer area, water hammer, vibration, mechanical stress and internal corrosion.

Steam-trap selection therefore needs to consider actual condensate flow, steam-space pressure, return-line backpressure and start-up behaviour.

A pump-trap or other suitable condensate pumping arrangement may be required where the available differential pressure is insufficient.

Plate heat exchangers

Plate exchangers consist of thin metal plates forming alternating channels for primary and secondary fluids. Heat passes between adjacent channels through the plates.

Corrugations improve plate rigidity against differential pressure while encouraging turbulent flow. This increases heat transfer and reduces stagnant areas that may promote fouling.

Comparison of plate constructions

In suitable plate heat exchangers, steam condensation and condensate subcooling can take place within a single unit.

If condensate discharges to an atmospheric receiver, reducing its temperature also reduces flash-steam generation and associated vent losses.

Shell-and-tube heat exchangers

Shell-and-tube exchangers contain a bundle of tubes inside a cylindrical shell. Tube sheets separate primary and secondary fluids.

In common horizontal steam-to-water heating arrangements, steam condenses inside the tubes while secondary water flows through the shell.

A typical configuration has one shell pass and two tube passes, using U-shaped tubes.

This design requires only one tube sheet, but U-tubes can be more difficult to clean or replace. Floating-head designs provide easier tube-bundle removal and are relevant where larger temperature differences occur.

The number of tube passes is selected to achieve the intended flow velocity. Depending on design priorities, shell-side arrangements may also be modified to limit pressure drop.

Overall heat-transfer coefficient U

The overall heat-transfer coefficient depends on the secondary fluid, flow conditions, steam condensation and fouling.

The following indicative values apply to steam-heated shell-and-tube exchangers and include an allowance for fouling.

Dropwise condensation can increase heat transfer, but it is difficult to maintain reliably. Design calculations therefore generally assume filmwise condensation.

Alternative exchanger arrangements

Corrugated-tube exchangers use profiled tubes to encourage turbulence and reduce fouling in low-viscosity fluids. In this arrangement, steam flows on the shell side.

Spiral heat exchangers have two separate spiral channels. Their single-pass construction can accommodate fluids with significant fouling or suspended solids while maintaining relatively low connection-to-connection pressure loss.

Reboilers use steam to vaporise part of a secondary process fluid, such as in distillation service. Circulation may be pump-driven, generated naturally by density differences, or take place around tubes immersed in a liquid pool.

For vaporising duties, the heat balance must include the energy required for the secondary fluid's phase change.

Estimating actual demand in heating installations

As a preliminary building-heating estimate near an outdoor design temperature of −1°C, indicative allowances are 30–40 W/m³ for building volumes up to 3,000 m³ and 15–30 W/m³ for larger volumes.

These figures are approximate. Actual heating demand depends on air changes, insulation, windows, roofs and other building heat losses.

An installed system's demand can also be estimated from measured steam consumption under known temperature conditions.

Suppose an installation uses 227 kg/h of steam when indoor temperature is 19°C and outdoor temperature is 7°C. The temperature difference is 12°C.

At an outdoor design temperature of −1°C, maintaining 19°C indoors creates a 20°C difference.

Assuming an approximately proportional relationship:

Design steam flow ≈ 227 × 20 / 12 ≈ 378 kg/h

For more direct confirmation, actual steam consumption should be measured with an appropriate steam flowmeter.

Engineering selection criteria

Selecting a heat exchanger requires consideration of actual heat duty, flowrates, temperatures, allowable pressure drop, fouling, cleaning access, operating pressure and load variation.

The exchanger, control valve and condensate drainage system must be assessed together. Even sufficient installed heat-transfer area cannot provide the intended duty consistently when condensate accumulates in the steam space.

Conclusion

Steam consumption in a heat exchanger follows from process heat duty and the latent heat released when steam condenses.

In continuous-flow applications, flowrate and secondary-fluid temperature rise determine the load. For storage heating, the main parameters are water inventory and required recovery time.

Exchanger construction, steam pressure, fouling, pressure drop and condensate drainage must be considered as parts of the same engineering system.

Frequently Asked Questions

How is steam consumption calculated for a continuous-flow heat exchanger?

From heat duty Q̇ = ṁ·cₚ·ΔT and the evaporation enthalpy of steam at its actual condensing pressure.

Why can rated exchanger capacity differ from actual duty?

The exchanger may be oversized, while actual process flowrates and temperatures may differ from design conditions.

How is steam flow calculated for a storage calorifier?

Using water mass, temperature rise, recovery time and the steam evaporation enthalpy.

Why can steam condensation increase during start-up?

Colder secondary fluid creates a larger temperature difference and a higher instantaneous condensation rate.

When is a pump-trap needed?

When steam-space pressure is insufficient to discharge condensate against return-line backpressure.

What is the main advantage of gasketed plate exchangers?

They can be dismantled for cleaning and, where the design permits, have their heating area modified.

Why do brazed exchangers require care during thermal cycling?

Their rigid construction is more susceptible to thermal fatigue from frequent or rapid temperature changes.

How can installed exchanger duty be checked?

By measuring secondary-fluid flow and inlet/outlet temperatures or using an appropriate steam flowmeter.