Heat Exchangers Economizers

Recovering Heat Beyond the Boiler

Once steam is generated, every unused degree in condensate, flue gas, or blowdown is money leaving the stack or the drain. Heat exchangers and economizers capture that energy and return it to feedwater, domestic hot water, process loads, or combustion air. Minnesota plants — especially institutional campuses with year-round steam — lean on these devices for efficiency, and DLI-style exams expect operators to know what each device does, how it fails, and what trips or symptoms appear when flow or fouling goes wrong.

Shell-and-Tube and Plate Heat Exchangers

A shell-and-tube exchanger puts steam (or hot condensate) on one side and the heated fluid on the other. Instantaneous water heaters, converter packages for hydronic loops, and process heaters are common. Steam usually enters the shell; condensate leaves through a trap set. If the trap fails closed, the shell floods and capacity drops. If the trap fails open, steam enters the condensate system and temperature control becomes erratic.

Plate exchangers pack large surface area into a small footprint and are popular for domestic hot water and low-temperature loads. Gaskets limit temperature and pressure; follow the nameplate. Fouling on either side raises approach temperature — the hot inlet runs hotter or the cold outlet never reaches setpoint.

Control valves on the steam or hot-water inlet modulate to maintain outlet temperature. Always confirm the exchanger has a working relief valve on the closed fluid side: if the cold side is blocked while steam still heats the bundle, trapped liquid expands and overpressurizes the heater. That relief is a safety device, not optional trim.

Feedwater Heaters and Deaerator Relationship

Direct-contact heaters and deaerators are specialized heat exchangers that mix steam with feedwater to heat and strip gases. Indirect feedwater heaters use tube bundles. Hotter feedwater means less fuel for the same steaming rate and less thermal shock to the boiler drum or shell. Keep heater levels, pegging steam, and vents in the ranges the manufacturer specifies; a deaerator that runs cold passes oxygen downstream to the economizer and boiler.

Economizers

An economizer is a gas-to-water heat exchanger in the boiler flue gas path. Feedwater flows through tubes or coils while leaving flue gas heats it before the water enters the drum or boiler inlet. Economizers are one of the highest-payback heat-recovery devices on steam boilers when flue gas is hot enough and feedwater is cool enough for a useful temperature difference.

Types and limits

• Noncondensing economizers keep gas-side surfaces above the acid dew point so sulfur-bearing flue gas does not condense and corrode tubes.
• Condensing economizers are designed for cool return water and clean fuels (often natural gas); they reclaim latent heat from water vapor in the flue gas and need corrosion-resistant materials and condensate drains.

If feedwater is already very hot (high condensate return), economizer duty shrinks — there is less temperature difference to exploit. If gas-side soot or fireside deposits build up, draft rises and heat transfer falls; clean per the boiler manual.

Operation and protection

Economizers need reliable feedwater flow whenever flue gas is hot. Firing into an economizer with no water flow can overheat tubes. Many packaged boilers interlock burner enable with feed pump status or flow switches. Watch inlet and outlet water temperatures and flue gas temperatures before and after the economizer. A sudden rise in gas outlet temperature at a given load often means fouling or bypassing.

Cold feedwater entering a hot economizer can sweat on the gas side if metal temperatures drop below dew point — a corrosion risk on sulfur fuels. Some plants recirculate to keep inlet water warm enough. Follow the boiler and economizer OEM limits; do not invent a colder setpoint to “squeeze more efficiency” if it violates dew-point margins.

Blowdown Heat Recovery

Continuous blowdown leaves the boiler near saturation temperature. A blowdown heat recovery exchanger or flash tank can preheat makeup water before the deaerator. Flash steam from high-pressure blowdown can feed the deaerator pegging line. Operators should keep recovery exchangers free of scale — concentrated boiler water fouls quickly if flow stops or if the unit is undersized.

Air Preheaters and Other Recovery

Larger plants may use air heaters to warm combustion air with flue gas. For exam purposes, know the parallel idea: hot flue gas still has usable energy after the boiler; an economizer, air heater, or condensing stack recovery can capture it if corrosion and draft limits are respected.

Fouling, Leaks, and Operator Response

Heat exchangers fail by fouling, corrosion, erosion, and gasket or tube leaks. Steam-to-water leaks contaminate the potable or process side — treat as a cross-connection emergency when domestic water is involved. Economizer tube leaks can put water in the breeching or threaten draft fans; trip the burner and follow the plant emergency procedure when inventory or breeching is at risk.

Approach temperature is your diagnostic: when hot and cold outlets draw closer than design at a given load, the surface is dirty or flow is short-circuiting. Log temperatures on major exchangers so drift is obvious.

Efficiency Mindset for the Exam

Questions often ask which device recovers heat from flue gas into feedwater (economizer), which device heats building water with steam (converter / shell-and-tube), or why an economizer needs flow when firing (tube protection). Without flow or with a flooded steam side, recovery stops and equipment is damaged. Pair heat recovery with good condensate return — the hottest free heat in the plant is condensate you already made.