DX Based Primary Air Handling Units

In a split arrangement, the air handling section with filters, DX coil and supply fan is installed in a plant room or indoor location, while the compressors and condenser are housed in a remote air-cooled condensing unit on the terrace, podium or another ventilated outdoor area.

The two sections are connected by insulated refrigerant piping. This arrangement keeps compressor noise and heat rejection away from occupied areas and is commonly used in commercial buildings.

A packaged DX PAU combines the air handling and refrigeration sections in a single factory-assembled unit, usually installed on a roof or an outdoor platform.

Factory piping and charging reduce site refrigerant work, but the location must still allow condenser airflow, service access and a fresh-air intake positioned away from exhaust outlets.

A DX PAU can also be connected to a VRF outdoor unit through an AHU connection kit, also known as a DX kit or EEV kit. The kit typically consists of an electronic expansion valve assembly and a control box that regulates refrigerant flow to the PAU coil based on sensor feedback.

This allows fresh-air treatment and room cooling to share the same VRF platform. Manufacturers set limits on connection ratios, combinations with indoor units, pipe lengths and operating ranges, so every selection must be checked against the VRF manufacturer's guidelines.

Where a suitable exhaust air stream is available, a heat recovery wheel, plate heat exchanger or heat pipe can be combined with the DX PAU to pre-cool the incoming fresh air and, depending on the device, remove part of its moisture.

Recovering energy from exhaust air reduces the load on the compressors and can allow a smaller condensing unit. The recovery device should be chosen considering exhaust air quality, cross-contamination risk, space and maintenance access. Explore our heat recovery units (HRU / ERV).

With hot gas reheat, part of the hot refrigerant gas leaving the compressor is diverted through a reheat coil placed after the DX cooling coil. The air is first cooled to remove moisture and then reheated to a suitable supply temperature using heat that would otherwise be rejected at the condenser.

Modulating hot gas reheat is particularly useful in humid weather, when outdoor air needs dehumidification but the space needs little sensible cooling.

In regions with cold winters, a reverse-cycle, or heat pump, DX PAU can cool and dehumidify fresh air in summer and monsoon and warm it in winter using the same refrigerant circuit.

Heat pump heating capacity falls as outdoor temperature drops, so the design should consider the lowest outdoor conditions at the site and whether supplementary heating is required.

The DX coil is a finned-tube heat exchanger, usually with copper tubes and aluminium fins, in which the refrigerant evaporates. Coil rows, fin spacing, circuiting and face area must suit the required airflow, cooling capacity and leaving-air condition.

A refrigerant distributor feeds the coil circuits evenly, while a thermostatic expansion valve (TXV) or electronic expansion valve (EEV) controls refrigerant flow to maintain correct superheat at the coil outlet. Coils exposed to salty or corrosive air may need protective fin coatings.

The condensing unit contains the compressors, condenser coil, condenser fans and refrigeration safety devices. Scroll compressors are widely used in DX PAU applications as fixed-speed units, tandem sets or inverter-driven variable-speed compressors.

Multiple compressors, multiple refrigerant circuits or inverter compressors allow cooling output to follow the changing fresh-air load, improving temperature and humidity stability at part load.

Outdoor air enters through weather louvres with bird screens and a manual or motorised damper. Filtration typically includes pre-filters and fine filters, with higher-grade stages added where indoor air quality or healthcare requirements demand it.

Every filter stage adds resistance, so filtration must be selected together with the supply fan. Read more about our filtration and air cleaning systems.

The supply fan may be a belt-driven centrifugal fan, a direct-driven plug fan or an EC fan, selected to deliver the design airflow against the total static pressure of the unit and duct system.

Double-skin insulated panels reduce heat gain, prevent condensation on the casing and limit noise breakout, which matters when the PAU carries cold, dehumidified air through a warm and humid plant room.

A microprocessor or DDC controller manages compressor staging or speed, fan operation and alarms based on supply-air temperature, humidity or dew point and outdoor conditions.

Typical protection includes high- and low-pressure switches, compressor overload protection, phase failure and reversal protection, anti-short-cycle timers, airflow proving and coil frost protection.

A DX PAU removes large quantities of moisture from outdoor air, particularly during the monsoon, so a sloped, corrosion-resistant drain pan and a correctly sized trap are essential.

In draw-through units, where the coil section operates under negative pressure, the trap must be designed for the fan static pressure so that condensate drains freely and air is not drawn in through the drain line.

Office buildings and IT/ITES floors cooled by VRF or ductable split systems usually need a separate unit to treat ventilation air. A DX PAU supplies cooled, dehumidified fresh air to each floor or zone, improving indoor air quality while reducing the latent load on terminal units.

VRF indoor units and ductable splits are generally selected to cool recirculated room air and are not intended to handle large quantities of untreated outdoor air. A DX PAU pre-treats the fresh air, allowing terminal units to focus on room loads and reducing the risk of condensation at diffusers during humid weather.

See our DX air conditioning systems for related VRF and split system services.

Retail and hospitality spaces combine high occupancy with significant exhaust, especially from kitchens. A DX PAU can supply conditioned make-up air that balances exhaust and keeps spaces comfortable during peak hours.

Healthcare buildings without a central plant, or areas that must operate independently, can use DX PAUs to provide filtered and dehumidified fresh air.

Selection should follow project-specific requirements for air changes, filtration, pressure relationships and redundancy. Critical areas may need multiple compressors, standby capacity or additional filtration.

Classrooms, auditoriums and libraries experience large swings in occupancy and need adequate ventilation to control CO2 levels. DX PAUs with demand-based controls can adjust fresh-air treatment to timetables and occupancy while maintaining minimum ventilation.

Corridors, lobbies and guest rooms need a continuous supply of fresh air with controlled humidity to prevent odours, condensation and mould. DX PAUs can serve individual wings or floors independently, supporting phased opening and flexible operation.

In factories without central chilled water, DX PAUs can treat fresh air for control rooms, quality laboratories, canteens and administrative blocks. Unit sizing should reflect the fresh-air quantity, heat and moisture conditions and operating hours of each area.

Airflow is set by occupancy, floor area, exhaust quantities and pressurisation needs, using the ventilation standard specified for the project, such as the National Building Code of India (NBC 2016) or ASHRAE Standard 62.1.

Overestimating fresh air increases compressor and fan energy, while underestimating it compromises indoor air quality and building pressure.

A DX PAU must be selected for the dry-bulb and wet-bulb or dew point conditions at the project location. In coastal and humid cities, the peak latent load often occurs during the monsoon, when outdoor temperature is moderate but moisture content is very high.

The DX PAU cooling load calculation should therefore check both peak dry-bulb and peak dew point design conditions, not only the hottest summer day.

Required cooling capacity, expressed in TR or kW, depends on airflow, entering-air enthalpy and the target leaving-air condition.

Fresh-air load varies widely across summer, monsoon and winter, so the number of compressors, refrigerant circuits and inverter capacity control matter as much as peak capacity.

For humidity control, the coil must deliver air at a dew point low enough to absorb the latent load of the conditioned spaces. This sets the coil leaving-air temperature and the required apparatus dew point (ADP).

If the dehumidified air is colder than needed for supply, reheat may be required.

Higher face velocity reduces coil size but increases the risk of condensate being carried off the coil into the ductwork. Cooling coils are commonly selected at or below about 2.5 m/s (approximately 500 fpm) unless moisture eliminators are provided.

Coil rows and fin spacing affect dehumidification capacity, air-side pressure drop and ease of cleaning.

Air-cooled condensing unit capacity falls as outdoor temperature rises. In many parts of India summer temperatures can exceed 45 °C, so condensing units should be selected for the project's design ambient with adequate margin and confirmed for high-ambient operation.

Units installed in coastal locations should have corrosion-protected condenser coils.

The supply fan must overcome resistance from louvres, filters, the DX coil, heat recovery devices, reheat coils, dampers, silencers and the duct network. Calculating external static pressure for the actual duct layout lets the fan be selected for its true operating point.

Suction and liquid lines must be sized to keep pressure drop within limits while maintaining sufficient refrigerant velocity. Each manufacturer specifies maximum equivalent pipe length, vertical rise and height difference between the indoor and outdoor sections.

Exceeding these limits reduces capacity and can damage compressors, so the condensing unit location should be fixed with these constraints in mind.

Compressor oil circulates with the refrigerant and must return to the compressor. Suction risers, oil traps and pipe slopes should follow the manufacturer's guidelines, especially where the condensing unit sits above or below the PAU.

Poor oil return is a common cause of premature compressor failure in DX systems.

Copper refrigerant pipes should be brazed while dry nitrogen flows through them. Nitrogen purging prevents oxide scale from forming inside the pipes, where it could otherwise block strainers and expansion valves or damage compressors.

The completed piping is pressure tested with dry nitrogen at the pressure and duration specified by the manufacturer and checked for leaks.

The circuit is then evacuated with a vacuum pump to remove air and moisture, and a vacuum hold test confirms that the system is dry and leak-tight before refrigerant is charged.

Refrigerant is charged by weight according to manufacturer data, including additional charge for the installed pipe length. Suction superheat and liquid subcooling are then checked under stable operation to confirm correct charge and expansion valve performance.

DX PAUs use refrigerants such as R410A, R32 or R407C depending on the equipment. R32 is classified as mildly flammable (A2L), so installation, ventilation and handling must follow the manufacturer's safety instructions and applicable regulations.

Suction lines, and in VRF and heat pump systems both refrigerant lines, need closed-cell insulation of suitable thickness to prevent condensation and capacity loss.

Exposed pipes on terraces should be protected from UV and mechanical damage, and supports should allow for thermal movement without crushing the insulation. Learn about our thermal insulation and protection services.

Outdoor air during coastal summers and the monsoon carries far more moisture than indoor air at comfort conditions. If the PAU does not remove this moisture, it enters the building and raises indoor humidity even when room temperature feels acceptable.

When outdoor conditions fall below design, a DX PAU must reduce its capacity. If a single compressor simply cycles on and off, the coil warms during off periods and moisture held on the coil can evaporate back into the supply air.

Inverter compressors, tandem compressors, multiple refrigerant circuits and EEV control allow capacity to track the load more closely, keeping the coil cold enough to dehumidify while avoiding frequent cycling.

If a DX PAU is controlled only on supply dry-bulb temperature, compressors may unload once the temperature setpoint is met, allowing supply humidity to rise. For moisture-sensitive applications, the coil is controlled to a leaving-air temperature or dew point that achieves the required moisture removal.

Hot gas reheat reuses heat from the refrigeration cycle, making it an energy-efficient way to raise the temperature of dehumidified air. Electric heaters, hot water coils or wrap-around heat pipes are alternatives, depending on the project.

Whether reheat is needed depends on the supply strategy. Some designs deliver cool, dry fresh air directly to the space, while others supply air closer to room-neutral temperature to terminal units.

At very low load, such as cool nights or winter in northern India, coil surface temperature can fall below freezing and ice can form on the fins. Frost protection sensors, minimum capacity control and correct airflow help prevent coil icing, airflow loss and compressor damage.

Treated fresh air can be ducted directly to diffusers in each zone, to the ceiling return plenum or to the inlet of terminal units. Supplying it directly to the space keeps ventilation independent of terminal-unit fan operation.

The supply-air condition should be coordinated with terminal-unit selection so that sensible and latent loads are shared as intended.

When a PAU coil is connected to a VRF outdoor unit through an AHU kit, the coil capacity and internal volume must match the kit and outdoor unit ratings, and sensors must be installed at the locations specified by the manufacturer.

The selected control mode, such as supply-air, return-air or external signal control, should be coordinated with the BMS for stable operation.

Fresh-air ducts must deliver the design quantity to every floor and zone with acceptable pressure loss and noise. After installation, airflow at branches and outlets is measured and balanced. Explore our ductwork services.

Fresh-air supply should also be balanced against toilet, kitchen and process exhaust to maintain the intended building pressure and limit infiltration of untreated air.

Installation planning starts with a survey of plant rooms, shafts, terraces and access routes. The PAU needs clearance for filter removal, coil cleaning, fan and motor service and drain inspection.

The condensing unit needs unobstructed airflow, protection from hot-air recirculation between adjacent units, adequate structural support and safe service access, while remaining within the permitted pipe length and height difference.

Refrigerant piping is installed and supported according to the approved layout, and duct connections use flexible connectors to limit vibration transfer.

The condensate drain is installed with the correct slope and trap, and power and control cabling between the PAU, condensing unit and control panel is terminated and tested.

Before start-up, the refrigerant circuit is pressure tested, leak tested and evacuated to the vacuum level specified by the manufacturer. The correct refrigerant quantity is then charged and recorded.

During start-up, suction and discharge pressures, superheat, subcooling, compressor current, supply voltage and condenser fan operation are measured and compared with manufacturer data.

Safety switches and interlocks are tested to confirm that compressors stop safely under abnormal conditions.

Supply airflow, external static pressure, entering and leaving air dry-bulb and wet-bulb temperatures and supply-air humidity are measured to confirm that the DX PAU achieves the expected cooling and dehumidification under prevailing conditions.

For PAUs connected to a building management system, start/stop commands, sensor values, run status, trip signals and alarms are tested end to end. Learn more about our HVAC automation and BMS integration services.

Commissioning records of refrigerant charge, pressures, electrical readings, airflow and air-side measurements are provided according to the project scope.

The controller modulates compressor capacity to hold a leaving-coil temperature or supply dew point setpoint, with reheat managing the final supply temperature where provided. Outdoor temperature and humidity sensors allow setpoints and staging to adapt to seasonal conditions.

Staging logic should include minimum on and off times, lead-lag rotation to balance compressor run hours, and soft-start or inverter control to limit starting current.

A VFD or EC fan allows airflow to be set during commissioning, compensated for filter loading and adjusted for demand-based operation. CO2 or occupancy sensors can reduce fresh-air quantity in variable-occupancy spaces, provided the building's minimum ventilation requirement is always met.

When airflow is reduced, refrigeration capacity must reduce with it to avoid coil frosting.

Compressors should run only when supply airflow is proven. Alarms for high or low refrigerant pressure, compressor trips, coil frost, fan failure, dirty filters and sensor faults can be sent to the BMS or a remote monitoring dashboard for faster response.

DX PAU energy use depends on part-load compressor efficiency, condensing temperature, fan power and the share of load removed by heat recovery. Clean condenser coils, no hot-air recirculation, inverter compressors and efficient fans all help lower operating cost.

Clogged filters reduce airflow and can lead to coil icing. Dirty evaporator coils lower heat transfer and dehumidification, while dirty condenser coils raise head pressure, compressor power and the risk of high-pressure trips.

Drain pans and condensate lines should be cleaned regularly to prevent overflow and microbial growth.

Refrigerant leaks reduce capacity and can damage compressors over time. The leak should be located and repaired before recharging, followed by pressure testing, evacuation and charging to the correct quantity.

Low suction pressure and coil icing can result from low airflow, dirty filters, a slipping fan belt, low refrigerant charge, a faulty expansion valve or operation at very low load. The root cause should be identified before the coil is defrosted and the unit restarted.

High discharge pressure is often caused by dirty condenser coils, failed condenser fans, hot-air recirculation, refrigerant overcharge or non-condensable gases in the circuit. Repeated compressor trips may also point to voltage imbalance, faulty contactors or overload problems.

When a DX PAU is not cooling, both the air and refrigerant sides should be checked: airflow, filters, coil condition, refrigerant pressures, superheat and controls. Persistent high humidity can indicate short cycling, dry-bulb-only control, insufficient latent capacity or excessive airflow across the coil.

Water appearing downstream of the coil may come from high face velocity, a blocked drain, an incorrect trap or condensation on uninsulated surfaces.

A DX PAU preventive maintenance or AMC programme can include filter service, coil and condenser cleaning, drain cleaning, refrigerant pressure and leak checks, electrical inspections, sensor calibration and control verification.

Visit frequency should reflect operating hours, outdoor air quality and how critical the served areas are. Explore our HVAC AMC services.

Larger airflow requires larger fans, coils and casings, while higher latent load requires more coil rows and refrigeration capacity. Inverter compressors, multiple circuits, double-skin construction and higher filtration levels raise equipment cost but can improve performance and efficiency.

The installed DX PAU cost can include the condensing unit or VRF outdoor unit, refrigerant piping and insulation, structural supports, ductwork, condensate drainage, electrical cabling, controls, refrigerant charging and commissioning. Long pipe runs, difficult access or crane lifting can also influence the total.

Options such as heat recovery wheels, hot gas reheat, BMS integration and CO2-based demand control add capital cost but can lower operating cost or improve humidity control. Their value should be assessed against operating hours and local climate.

Running cost is driven mainly by compressor power, which depends on outdoor conditions, part-load efficiency and condensing temperature, together with supply fan and condenser fan power. Clean coils, correct refrigerant charge and sensible setpoints keep energy use under control.

A DX PAU vs chilled water PAU decision should consider the whole building, not just equipment price. A DX PAU avoids the investment in chillers, pumps and chilled water piping and can be added in phases, whereas a chilled water based PAU system is often the better fit where a central chilled water plant already exists or the total cooling load is large.

DX based PAU vs chilled water based PAU

Compress India can review your fresh-air requirement, installation space and existing HVAC infrastructure to recommend the right option for your project.