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
Coils for supply air handling units
Establish the air condition entering this particular coil and the required condition leaving it. Then match the duty and mounting arrangement to the supply air unit.
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
Supply AHUs can heat or cool incoming air using water, glycol or refrigerant coils. Heating and cooling may use separate sections; a wet cooler includes condensate management appropriate to the airflow.
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.
Outdoor air and the coil's position
A supply air unit may treat outdoor air, mixed air or air leaving heat recovery. The coil's entering temperature therefore need not equal the outdoor temperature. Show the order of any filter, heat-recovery device, heater and cooler, and mark the section concerned.
IN PRACTICE
Coil access in a supply-air unit
Two archive service illustrations show the relation between the casing, coil and removable components.
Access panel
The opened side exposes the coil and its connection area.
Extraction route
The partially withdrawn view makes access relevant; actual clearance and temporary supports require checking.
Section drawing
Confirm the frame, seals and connections for the real unit.
Service access depends on the casing, connection side and available clearance around the unit.
Winter heating and summer cooling
Heating requires the entering air condition and available fluid temperatures. Cooling additionally requires humidity and any dehumidification target. A water coil and a DX coil need different working-fluid data. Several seasonal duties should not be reduced to one capacity value without their operating conditions.
Measuring a replacement coil
Distinguish fin-pack dimensions, the outer frame and the casing opening. On one defined view, mark airflow, connection side, fixings and the coil-removal route. For a cooler, include the drain pan and drainage. These details make the supply-coil enquiry more specific than the general AHU designation.
Piping and protection within the project
The coil check includes allowable air and fluid pressure drops and coordination with piping and controls. Outdoor-air treatment also needs an explicit winter duty and freeze-protection response. A pump, valve or sensor belongs to the supply only when included in the agreed arrangement and component list.
REVIEW THE RESULT
What to check for a supply-air coil
Air immediately before the coil
Tie temperature, humidity and airflow to the coil location after any mixing or recovery. Document winter and summer duties separately; outdoor temperature is not always the coil entering temperature.
Fin pack, frame and opening
Compare all three sets of dimensions and connection positions on one agreed view. Define the connection side relative to the shown airflow direction and separately check the space needed to remove the coil.
Fluid circuit, condensate and winter protection
Agree the drain pan and drainage for a cooler, and winter duty and protection response for a water heater. Check pressure drops and the piping scope for the selected function without inferring supplied components from an image.
SELECTION INPUTS
Inputs to prepare
- Unit model and air-path diagram identifying the coil.
- Airflow and air condition immediately before the coil; outdoor-air fraction where applicable.
- Required leaving conditions for winter and summer duties separately.
- Working fluid, temperatures, flow rate and allowable pressure drops.
- Fin-pack, frame and opening dimensions; connections, fixings and airflow direction.
- Drain pan, drainage, service access and the known freeze-protection arrangement.
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
Does entering heating-coil temperature equal outdoor temperature?
Only if the air has not been conditioned upstream. Air mixing or heat recovery changes the entering condition, so the actual unit arrangement is needed.
Why measure the frame and fin pack separately?
The frame defines the mounting envelope; the fin pack is the active heat-transfer section. Matching one set of dimensions does not establish both installation fit and thermal performance.
Is a mixing assembly included with the coil?
It is a separate scope item. State whether the enquiry covers the coil alone, a piping assembly or compatibility with existing equipment.