Chilled Water Based PAU Systems

The cooling coil is one of the most important components of a chilled water PAU.

Its capacity, number of rows, fin arrangement, face velocity, water flow and pressure drop must be matched to the required air conditions.

Supply and return water connections connect the PAU coil to the chilled water piping network. Correct valve installation, insulation, venting and balancing are essential for reliable cooling performance.

Outdoor air enters through a properly designed intake section before passing through the required filtration stages.

Depending on the application, a PAU may include pre-filters, fine filters or multistage filtration arrangements.

Filter selection should consider indoor air-quality requirements while also accounting for the additional resistance created across the filters.

The supply fan must deliver the design airflow while overcoming resistance from filters, coils, dampers, ductwork and other components.

Fan and motor selection should therefore be based on both airflow and external static pressure.

Double-skin or insulated PAU construction may be selected where improved thermal insulation, condensation resistance, acoustic performance or mechanical strength is required.

Control valves regulate chilled water through the cooling coil according to cooling demand.

Temperature and humidity sensors provide feedback to the control system, while the condensate drain tray and correctly designed drain trap remove moisture collected during dehumidification.

Reliable drainage is especially important in high-humidity applications.

Large offices and IT parks require continuous ventilation for occupied areas while maintaining indoor comfort.

A centralized chilled water fresh-air system can treat outdoor air before supplying it across multiple floors or HVAC zones, reducing the fresh-air load handled by terminal cooling equipment.

Hotels, malls and institutional facilities experience changing occupancy and ventilation demands.

A properly controlled energy-efficient chilled water PAU can coordinate fresh-air delivery with building operating schedules and occupancy requirements while supporting temperature and humidity control.

Healthcare applications can require carefully controlled ventilation, filtration, temperature and humidity.

PAU selection for hospitals should therefore consider project-specific air-change requirements, filtration levels, pressure relationships and other applicable HVAC design criteria.

Industrial buildings with centralized chilled water infrastructure can use chilled water PAUs for ventilation air treatment, process-support spaces, control rooms, production support areas and other conditioned zones.

System selection should be based on the actual outdoor-air load and environmental requirements of each application.

The first step is determining how much outdoor air the building or zone requires.

Airflow may be specified in CFM or m³/h and should be calculated according to occupancy, ventilation requirements, exhaust requirements and building pressure strategy.

Oversizing unnecessarily increases equipment and operating costs, while undersizing can compromise ventilation performance.

The PAU must be selected for the temperature and moisture content of the outdoor air entering the system.

A proper chilled water PAU cooling load calculation considers both sensible heat and latent moisture load.

In humid climates, latent capacity can be particularly important because the unit must remove moisture as well as reduce air temperature.

Cooling capacity can be expressed in TR or kW.

The required capacity depends on airflow, entering-air conditions and the target leaving-air temperature and humidity.

Compress India evaluates these project parameters when recommending suitable PAU configurations rather than relying only on nominal airflow.

Coil performance depends directly on the entering and leaving chilled water temperatures available from the central plant.

The PAU should therefore be selected using the actual design conditions of the chilled water system.

Changes in chilled water supply temperature can significantly affect cooling and dehumidification performance.

The required chilled water flow depends on cooling capacity and the selected chilled water temperature difference.

The coil's water-side pressure drop must also be considered when checking pump performance and available differential pressure.

These parameters influence coil selection, pipe sizing and balancing requirements.

When humidity control is important, supply-air temperature alone is not sufficient.

The design should also consider the required supply-air dew point and the amount of moisture that must be removed from outdoor air.

This is particularly relevant for a chilled water PAU for dehumidification.

The fan must overcome resistance created by filters, cooling coils, dampers, silencers, ductwork and fittings.

Calculating the required PAU external static pressure allows the fan to be selected for the actual operating point rather than only nominal airflow.

Before connecting a new PAU to an existing plant, the available chiller capacity, pump capacity, chilled water flow and differential pressure should be evaluated.

Adding cooling coils without checking plant capacity can reduce performance elsewhere in the chilled water network.

Properly sized chilled water supply and return piping helps maintain the required flow while controlling pressure loss.

Insulation and an effective vapour barrier are necessary to prevent surface condensation, energy losses and moisture damage.

A modulating chilled water control valve allows coil water flow to respond to changes in cooling demand.

Depending on the hydronic design, separate balancing valves may be used, or a pressure independent control valve—PICV—may combine flow limitation, balancing and control functions.

Valve selection should be based on design flow, available differential pressure and control requirements.

A professional PAU chilled water piping installation normally considers the service components required for reliable operation and maintenance.

These can include strainers, isolation valves, air vents, drain points, thermometers, pressure measurement provisions and appropriate connection accessories.

When several PAUs, AHUs or FCUs share the same chilled water network, each branch should receive its design water flow.

Hydronic balancing helps prevent some units from receiving excessive water while remote or higher-resistance circuits remain starved of flow.

Dehumidification occurs when humid air is cooled below its dew point.

Water vapour condenses on the chilled cooling coil and flows into the drain tray.

The resulting air leaves the coil with a lower moisture content before being supplied to the building.

Cooling-coil surface temperature depends partly on the temperature and flow rate of the chilled water.

Insufficient water flow or chilled water supplied at a temperature higher than the design requirement may reduce the coil's ability to reach the required leaving-air condition.

Stable humidity control requires coordination between chilled water flow, coil capacity, airflow and control sensors.

Monitoring supply-air temperature together with humidity or dew point can provide better control than relying on dry-bulb temperature alone in moisture-sensitive applications.

In some applications, air may need to be cooled sufficiently to remove moisture but would then be too cold for direct supply.

A properly engineered reheat arrangement can raise the dry-bulb temperature after dehumidification while maintaining the reduced moisture content.

Whether reheat is required depends on the project and control strategy.

High-humidity periods can be challenging because latent load may remain significant even when sensible cooling demand is relatively low.

Correct coil control, fan operation and plant coordination help prevent indoor humidity from rising during monsoon or part-load conditions.

Treated primary air can be supplied to FCU-served spaces, connected to AHU systems or delivered directly into occupied zones depending on the design.

Chilled water PAU integration with FCU systems can allow FCUs to address room loads while the PAU handles the ventilation-air load.

The duct network must distribute the required air quantity to each area without excessive pressure loss or noise.

After installation, airflow balancing helps confirm that the design air volume reaches individual floors, zones and outlets.

Fresh-air quantities should be coordinated with toilet exhaust, kitchen exhaust, process exhaust and other extraction systems.

Maintaining an appropriate balance between supply and exhaust helps achieve the intended building pressure relationship and reduces uncontrolled outdoor-air infiltration.

Installation planning starts with reviewing the available plant space, equipment dimensions, access routes and service requirements.

Adequate clearance should be provided for filter replacement, coil cleaning, fan servicing, valve maintenance and other routine activities.

Supply and return piping should be installed according to the approved hydronic arrangement, while duct connections should be properly sealed and supported.

Condensate drainage requires appropriate slope and trap configuration to ensure reliable removal of water from the PAU.

Before regular operation, chilled water pipework may require hydrostatic pressure testing in accordance with the project's approved procedure.

Flushing helps remove construction debris and contaminants, while air removal helps prevent circulation problems caused by trapped air.

After the system is operational, chilled water flow through the PAU coil should be checked against the design requirement.

Balancing valves, PICVs or other flow-control devices should be set appropriately, and modulating valve operation should be verified through its control range.

Commissioning should verify more than fan rotation and equipment start-up.

Airflow, temperatures, chilled water conditions, pressure values and—where relevant—humidity performance should be checked to confirm that the installed system is delivering the intended results.

Where the PAU is connected to a Building Management System, control commands, sensor readings, alarms, status signals and interlocks should be tested.

Project documentation can include relevant commissioning readings, control checks and equipment information as required by the project scope.

Temperature and humidity sensors can be used to modulate chilled water flow through the coil.

This allows cooling output to respond to changing outdoor-air and building conditions rather than operating continuously at maximum capacity.

A VFD can adjust fan speed where variable airflow operation is appropriate.

Depending on the application, ventilation demand may also be coordinated with occupancy schedules, air-quality sensors or other control inputs.

Any demand-based strategy should continue to satisfy the minimum ventilation requirements of the building.

Coordinating PAU operation with pumps and chilled water generation equipment can help prevent unnecessary operation and avoid running the PAU when cooling water is unavailable.

Suitable interlocks depend on the plant and BMS architecture.

Differential pressure monitoring can help identify increasing filter resistance.

BMS alarms can also be configured for temperature deviations, fan faults, sensor failures and other important operating conditions.

Performance trends provide useful information for maintenance and troubleshooting.

Monitoring chilled water supply and return temperatures can help identify changes in coil and system performance.

Unexpectedly low temperature difference may be associated with excessive flow, control issues, reduced heat transfer or other hydraulic conditions that require investigation.

Dirty filters increase resistance and can reduce airflow.

Contaminated cooling coils reduce heat transfer, while blocked drain trays or condensate lines can result in water accumulation and leakage.

Strainers within the chilled water circuit should also be inspected as required to maintain water flow.

Low chilled water flow can result from incorrect balancing, blocked strainers, closed valves, air accumulation, actuator problems, inadequate differential pressure or other system conditions.

Systematic diagnosis helps identify the actual cause rather than treating only the resulting temperature problem.

If a chilled water PAU is not cooling effectively, technicians should evaluate both the air and water sides of the system.

Possible areas for inspection include filters, airflow, coil cleanliness, chilled water temperature, water flow, valves, sensors and operating setpoints.

Persistent high humidity may additionally require assessment of latent load and coil dew-point performance.

Water around a PAU does not always indicate a leaking cooling coil.

Condensate drain blockage, incorrect traps, damaged insulation, vapour-barrier failure or condensation on cold pipes can also create visible moisture.

Inspection should identify the source before repairs are carried out.

Temperature and humidity sensors can drift over time and should be checked according to the maintenance strategy.

A structured chilled water PAU preventive maintenance or AMC programme can cover periodic inspections, cleaning, operational checks and identification of developing faults before they lead to major disruption.

Contact Compress India to discuss maintenance requirements for existing chilled water PAU installations.

Higher airflow generally requires larger fans, filters, coils and casing dimensions.

Cooling capacity, filtration level, fan configuration, casing specification and control requirements can therefore substantially influence the final equipment price.

The complete chilled water PAU installation cost can include more than the PAU itself.

Chilled water piping, insulation, valves, strainers, supports, duct connections, drainage, electrical work, BMS controls and commissioning requirements may all form part of the project.

Connecting a PAU to an existing chilled water system can be economical when sufficient spare cooling and pumping capacity is available.

If the plant has insufficient capacity, additional pumps, modifications or chiller upgrades may be required.

This is why existing plant conditions should be evaluated before finalizing a quotation.

Operating cost depends on the efficiency and operating hours of the fan, chilled water pumps and chiller plant.

Pressure losses, chilled water temperature, flow control, fan speed and maintenance condition can all influence energy consumption.

A chilled water PAU vs DX PAU comparison should consider the complete project rather than equipment price alone.

A chilled water PAU can integrate effectively into buildings with an existing central chilled water plant, while a DX PAU uses a refrigerant-based direct-expansion cooling arrangement.

Capital cost, plant infrastructure, capacity, redundancy, maintenance and operating strategy should all be considered when selecting between the two.

For an accurate recommendation, Compress India can review your airflow, cooling requirement and existing HVAC infrastructure.