Supply air handling unit
The casing guides supply air through its sections. A water, glycol or DX coil heats or cools the air; access panels and removable elements allow servicing.
By equipment
Heat exchange coils for air handling units
An overview of heat exchange coils in air handling units. Identify the air path and component first, then open the supply-air or extract-air guide for that coil and its duty.
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.
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.
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
A combined supply/extract AHU may contain separate heating, cooling and recovery coils. Run-around recovery connects coils through a liquid loop. Plate and rotary air-to-air recovery exchangers are different constructions, not automatically CuAl coils.
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
Refrigerant distributor
A device feeding the parallel circuits of a DX evaporator.
Divides refrigerant between the selected coil circuits.
DX-specific where used; the expansion valve has a separate function.
What to inspect
Damaged small tubes or leakage signs require diagnosis.
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.
Which coil, in which air path
Mark the specific heat exchange component on a diagram or photograph and identify its air path. This overview helps establish the coil and its place in the unit. Use the supply-air guide for supply-air treatment, or the extract-air guide for a run-around heat recovery coil. Check the working fluid, function and construction separately; the AHU designation does not establish them automatically.
Define function and working medium separately
A water heating coil, a liquid cooling coil and a DX evaporator require different inputs. The unit name alone does not select the coil type. Record the airflow and air condition, thermal duty and working medium for the relevant section. If several heat exchange elements are present, mark the specific one on a drawing or photograph.
Fitting an existing casing
An installation check relates the coil dimensions to its frame, airflow direction and connection positions. Distinguish coil dimensions from the available opening and the overall unit dimensions. For replacement, also show access to the coil and surrounding components. Meeting the thermal duty still requires these integration conditions to be checked.
IN PRACTICE
Three interfaces that must fit
For an existing AHU, check the coil against the casing, the air path and the service connections together.
Casing
Opening, fixing points and room for removal.
Air path
Finned face, airflow direction and sealing.
Connections
Medium, connection side and access for service.
Service access depends on the casing, connection side and available clearance around the unit.
REVIEW THE RESULT
How to check a coil within an AHU
Position in the air path
Mark the required coil on the unit layout and show the airflow direction. The enquiry should identify its function, working fluid and the air conditions immediately upstream of this coil.
Fit and removal
Keep AHU casing, removable coil and access-opening dimensions separate. Check the removal path as well as fixings and connections; matching envelope dimensions do not establish replacement and service access.
Interfaces and included components
Record fixings, the interface with the air path and connection positions on the agreed drawing. List the drain pan, seals and other components separately in the specification when they are part of the task.
SELECTION INPUTS
Inputs to prepare
- Unit type and model; supply air, another arrangement or unknown.
- The required coil function and its position along the airflow path.
- Airflow, entering air condition and required leaving condition.
- Working medium and its documented operating conditions.
- Coil and opening dimensions, connections and allowable pressure drop.
- Overview, nameplate and the relevant assembly marked on a drawing.
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
Are AHU and supply air unit exact synonyms?
Supply air specifies the purpose of the airstream. The broader term AHU does not describe the full air treatment arrangement. Retain the specific unit type in the enquiry rather than replacing it with a broader label.
Is every ventilation heat recovery unit a Cu-Al coil?
No. Heat transfer alone does not identify the construction. Establish the technology and function of the specific component first; the general term heat exchanger is insufficient.
Is the AHU model enough to identify a replacement coil?
The model helps start identification, but does not establish the installed coil construction. Available information on the coil, connections, dimensions and operating duty is also needed.