Donaldson DHP 0025 AX-2280 AX Bora High-Pressure Air Dryer

A refrigeration dryer for compressed air up to 50 bar

The Donaldson Bora DHP 0025 AX to DHP 2280 AX is a range of refrigeration dryers built for compressed air at high pressure, up to 50 bar(g) for the smaller models and 45 bar(g) for the largest. High-pressure air from PET bottle blowing, compressed air energy storage and process lines carries much less water vapour per cubic metre than air at 7 bar, so a dryer designed for that pressure range is needed instead of a standard fridge dryer.

Fifteen models cover 25 to 2,280 m3/h referred to 40 bar(g) under ISO 7183, with a pressure dew point of +3 C. The compressed air is pre-cooled in an air-to-air exchanger, chilled in the refrigerant exchanger, drained automatically and reheated by the incoming air, and a hot-gas bypass controls the cooling capacity at partial load.

Technical specifications

Model data

• DHP 0025 AX: Flow m3/h 25; Pressure drop bar 0.25; Power kW 0.16; Air connection BSP 3/8″; Weight kg 28
• DHP 0135 AX: Flow m3/h 135; Pressure drop bar 0.23; Power kW 0.46; Air connection BSP 3/4″; Weight kg 39
• DHP 0315 AX: Flow m3/h 315; Pressure drop bar 0.20; Power kW 0.70; Air connection BSP 1″; Weight kg 89
• DHP 0810 AX: Flow m3/h 810; Pressure drop bar 0.23; Power kW 1.45; Air connection BSP 1 1/2″; Weight kg 156
• DHP 1260 AX: Flow m3/h 1,260; Pressure drop bar 0.22; Power kW 2.55; Air connection BSP 2″; Weight kg 252
• DHP 2280 AX: Flow m3/h 2,280; Pressure drop bar 0.20; Power kW 3.50; Air connection BSP 2″; Weight kg 391

The sheet gives data for all fifteen models; the table shows six of them, including power consumption as printed (at 50 Hz).

Sizing for real conditions

The nominal flow is for 40 bar(g), 35 C inlet, 25 C cooling air and a +3 C dew point. The sheet therefore supplies correction factors: for inlet temperature from 1.20 at 25 C down to 0.39 at 65 C, for dew point from 1.00 at +3 C to 1.37 at +10 C, for working pressure from 0.57 at 15 bar(g) to 1.10 at 50 bar(g), and for the temperature of the cooling air. Multiply them to obtain the corrected flow and pick the model whose rating is above it.

In overload the dryer only trips when the dew point rises above about +20 C, so a short-term peak does not stop the plant. Fully enclosed metal cabinets protect against damage and dirt, and the units are described as light and compact for transport and installation. Larger Bora high-pressure models, DHP 2400 to DHP 6000 in air-cooled (AX) and water-cooled (WX) form for 45 bar, are covered by a separate Donaldson data sheet.

Key features of the Bora high-pressure range

• Pressure-rated air circuit for 45 or 50 bar(g) service, so the dryer can sit directly on the high-pressure side of a booster or HP compressor.
• Air-to-air pre-cooler and reheater that recovers cold from the outgoing air, which lowers the refrigeration load and prevents condensation on downstream pipework.
• Hot-gas bypass capacity control that keeps the dew point stable when air demand varies.
• Automatic condensate drain, removing the need for manual draining on each shift.
• Compact enclosed cabinet suitable for compressor rooms where floor space is limited.
• Alarm contact and serial communication on the larger sizes, useful where the dryer is monitored from a central control system.

Why pressure matters for drying

When air is compressed, the water vapour it contains is concentrated into a smaller volume. At 40 bar the compressed air reaches saturation much more readily than at 7 bar, and the condensed water must be removed before the air is used. A refrigeration dryer cools the air to just above freezing, so most of the vapour condenses and is drained away. The +3 °C pressure dew point means that liquid water will not form in the downstream network as long as the pipework stays warmer than that temperature, which is normally the case indoors.

Worked sizing approach

1. Find the actual free air delivery of the compressor and convert it to the reference conditions used in the sheet.
2. Identify the worst-case inlet air temperature after the compressor aftercooler; in summer this is often the controlling value.
3. Identify the maximum cooling air temperature at the dryer location, which in a compressor room may be several degrees above outdoor ambient.
4. Apply the correction factors for inlet temperature, ambient, working pressure and required dew point from the data sheet, multiply them, and divide the required flow by the result.
5. Choose the model whose nominal flow is equal to or above the corrected value.

Because the inlet temperature factor falls sharply at high temperatures, an effective aftercooler upstream of the dryer is often more economical than a larger dryer.

Installation and service practice

• Install a coalescing pre-filter upstream to protect the heat exchanger from oil and particles, and an appropriate filter downstream according to the required air class.
• Leave space around the cabinet for cooling air and keep condenser fins clean; blocked fins quickly raise the dew point in dusty environments.
• Pipe the condensate drain to an oil-water separator rather than directly to a drain.
• Use pressure-rated fittings and hoses that match the dryer working pressure.
• Record the dew point reading regularly; a gradual rise usually points to dirty condensers, refrigerant problems or a blocked drain.

Use in Oman and GCC plants

Typical applications in the region include PET bottle blowing in beverage and water bottling plants, high-pressure test benches, and process lines needing dry air at elevated pressure. The upper ambient limit of +50 °C must be respected: in summer, compressor rooms without good ventilation can exceed it, so extract fans or ducted cooling air are recommended. Refrigerant type differs between small and large models, which matters for service contractors and refrigerant availability.

Information to include in an RFQ

• Model code, or the flow, working pressure and dew point required.
• Inlet air temperature and maximum ambient temperature.
• Power supply voltage and frequency.
• Need for filters, alarm contact or serial communication.

Documents and quotation

DHP 0025 AX-2280 AX: request a quote from this page. The attached Donaldson Bora DHP 0025 AX to DHP 2280 AX high-pressure refrigeration dryers data sheet (11/2012) is the reference, and the working pressure, inlet temperature and the sheet date (11/2012) with the current model code should be checked against it. Also see Compressors.