Extract air handling unit
An extract air path may contain a coil for heat recovery or a separate air-treatment duty. Its function is defined by the air and fluid circuits.
By equipment
Coils for extract air handling units
Identify the heat-recovery technology. An extract-side finned coil in a liquid loop and an aluminium plate heat exchanger are different components.
From the unit to its heat exchanger
Compare the complete unit, access to the coil and an exploded view. These examples show the relationship between the equipment casing and the heat-transfer section.
Combined supply and extract arrangement
The two-level arrangement illustrates separate air paths. Heating, cooling and recovery coils have distinct duties. A run-around recovery system uses coils connected by a liquid loop; plate and rotary air-to-air exchangers use a different construction.
Inside the heat exchanger
An extract coil can recover heat into a liquid loop serving a supply coil. Extract air composition and possible condensation matter; a simple extract fan does not imply that a coil is present.
Read the following parts as a typical CuAl coil anatomy. Actual circuiting and accessories vary with the equipment and duty.
Heat-transfer core
Copper tubes
Channels carrying the working fluid through the finned core.
Separate the fluid from air and conduct heat to or from the fins.
Core element of a CuAl finned-tube coil.
What to inspect
Leak traces, corrosion or visible damage.
Heat-transfer core
Aluminium fins
Thin metal sheets in thermal contact with the tubes.
Enlarge the surface exchanging heat with air.
Core element; spacing and profile vary.
What to inspect
Dirt deposits, damaged passages or corrosion.
Depends on circuit and construction
Return bends and hairpins
Curved tube sections connecting successive passes.
Turn the flow and form the coil circuit.
Layout-dependent; an integral hairpin and a separate return bend are different constructions.
What to inspect
Damaged bends or leakage at joints.
Depends on circuit and construction
Headers and connections
Manifolds linking multiple tube circuits to external connections.
Distribute or collect the working fluid.
Arrangement depends on circuiting; headers are not identical to a DX distributor.
What to inspect
Corrosion, joint damage or leak traces.
Depends on circuit and construction
Frame, casing and tube supports
End plates and supporting casing around the core.
Support the core and provide mounting interfaces.
Common; casing styles include end-plates-only constructions.
What to inspect
Deformation, corrosion or damaged mountings.
Where required by the duty
Condensate tray and drain
A collection surface beneath a coil that produces condensate.
Collect and remove condensed water.
Needed for wet cooling; often part of the equipment section.
What to inspect
Standing water, deposits, corrosion or leakage.
Where required by the duty
Droplet eliminator
A downstream separator for droplets carried by the airflow.
Intercepts droplets and lets collected water drain.
Conditional on carryover risk and equipment design; not every cooling coil has one.
What to inspect
Contamination, damage or water carryover.
External piping
External hydronic assembly
A separate piping package with valves and, where specified, a pump.
Controls fluid delivery and connects the coil to the system.
External to the finned core; component list follows the circuit design.
What to inspect
Leaks or control faults are system issues, not proof of coil damage.
Condition signs help identify what needs inspection; they do not identify the fault by themselves. Service methods and intervals follow the equipment manual.
The coil in a run-around loop
In a liquid-coupled recovery system, extract and supply coils are linked by a fluid loop and can occupy separate air paths. This differs from plate or rotary recovery; heat transfer in the extract system does not by itself establish a Cu-Al construction.
IN PRACTICE
The unit and an accessible coil section
A closed-unit example and a separate disassembly view explain the enclosure and the component level.
Unit envelope
The exterior view shows panels and connection areas, not the internal section sequence.
Coil section
The separate open view makes the finned block and removed components visible.
Actual duty
Confirm the coil function and operating conditions from the unit documentation.
Service access depends on the casing, connection side and available clearance around the unit.
Replacing one coil affects the paired system
Checking an extract coil requires information about the supply side and the shared fluid loop. Compare both airflows and air states, fluid temperature and flow, coil pressure drops and circulation conditions. Recovery effectiveness cannot be assigned as a fixed percentage from one photograph or coil size.
Moist or contaminated extract air
State humidity, potential contaminants and cleaning access. If surface condensation is expected, check the drain pan and drainage; in cold operation, assess frost risk and its control response. Do not transfer materials, fin spacing or protection arrangements from an ordinary supply coil without checking the extract environment.
Identification and measurement
Identify the extract coil on the unit diagram and show its links to the circulation assembly and supply coil. Record the frame, opening, connection side and removal access. If the component is a plate exchanger, the enquiry must follow that construction; a Cu-Al coil is not a direct substitute.
REVIEW THE RESULT
What to check in an extract recovery coil
Thermal balance of the paired coils
Compare the extract and supply coil duties together with intermediate-fluid flow. Record both entering air states and the calculation boundary; do not assign constant effectiveness without this paired assessment.
The common hydraulic loop
Check fluid type and concentration, both coil and pipework resistances, and pump operating conditions. Coordinate the replacement with the remaining loop, not only the mounting envelope.
Condensate, frost and cleaning
Assess condensation and frost from surface temperature and extract-air state, then agree drainage and the protection response. Check contaminants and fin access separately; glycol content does not prove that the air-side surface remains frost-free.
SELECTION INPUTS
Inputs to prepare
- Recovery type and a diagram linking the extract and supply sections.
- Airflows, temperatures and humidity for both airstreams.
- Loop-fluid type and concentration, temperatures and flow rate.
- Paired-coil data, pump information and available head where known.
- Extract contaminants, cleaning requirements, condensate handling and frost protection.
- Dimensions, connections and photographs of the replacement coil and surrounding components.
What do you need to do?
Select a new coil
Define operating conditions first. The input checklist above describes the data needed for this application. The enquiry form adapts to the selected duty; it does not calculate a coil rating.
Replace an existing coil
Add the equipment nameplate, photographs, overall and finned dimensions, airflow direction, connections and access constraints. Matching the envelope alone does not confirm performance or compatibility.
Prepare an OEM specification
Separate fixed interfaces from dimensions open to change. Attach the current drawing revision, duty points, material requirements, quantity and acceptance criteria. Manufacturing feasibility is confirmed for the specific specification.
Final performance, materials and compatibility require a project-specific selection and an approved drawing.
Questions about this application
Is every aluminium heat-recovery exchanger a Cu-Al coil?
No. Material alone does not identify the technology. A plate block and a finned-tube coil have different flow paths and selection requirements.
Can the extract coil be replaced without supply-coil data?
Measurement can start independently, but checking run-around recovery performance needs the paired coil and common loop. Keep unknown parameters explicitly identified in the enquiry.
Does glycol eliminate air-side frosting?
No. Protecting the loop fluid from freezing and preventing frost on a surface exposed to moist air are separate issues. Both airstream conditions and an agreed protection response are needed.