Neometrix / Marine Compressed Air Systems / Shipboard Compressed Air Drying Unit / NMX-ADU-27

NMX-ADU-27 · ENGINEERED-TO-ORDER CLASS — TWIN-TOWER DESICCANT · CONTINUOUS DUTY · MARINE GRADE

Shipboard Compressed Air Drying Unit. One tower dries the air. The other tower dries itself.

Name: Shipboard Compressed Air Drying Unit
Brand: Neometrix
SKU: NMX-ADU-27

A single bed of desiccant can only hold so much moisture before it saturates. A unit built for continuous duty never asks one tower to do the whole job.

While one tower dries the incoming air, the other is being purged of the moisture it adsorbed minutes earlier - regenerated and ready, so the cycle can switch without ever interrupting the dry air downstream.

Discuss a drying unit Capability note (PDF)

A twin-tower compressed air drying unit: two identical vertical pressure vessels mounted side by side on a steel skid, connected by plain unmarked piping and valves, with a small control panel box between them, on a clean concrete yard, and no people

Fig · 01 — A twin-tower desiccant drying unit, skid-mounted for shipboard fitting — illustrative render.

The towers

twin desiccant bedsone dries, one regenerates

The dew point

well below freezingdesiccant, not refrigerant

The regeneration

purge-drivenno external heat needed

The duty

continuousnever fully offline

Status

engineered to orderno unit yet delivered

ISO 9001ISO 14001ISO 8573-1 referenceIACS Unified Requirements referenceTwin-tower desiccant drying

Overview

Why control air answers to a stricter rule than the rest of the ship's air.

Because the air that starts an engine can tolerate a trace of moisture. The air that runs its automation cannot.

Fig · 02 — The system in one picture: wet air in, one tower drying while the other regenerates, dry air out to the control-air range.

A dryer that never stops is not one tower working harder. It is two towers taking turns.

What the unit is actually for

It sits downstream of the ship's own compressor, taking the compressed air already generated and drying it to the standard that control air, instrument air and automation systems need - a stricter standard than starting air or general service air ever has to meet.

Why the dew point rule is so much stricter here

Moisture that would be harmless in a starting-air line becomes a real problem inside a pneumatic controller: it can freeze in exposed piping during cold-weather operation, corrode valve internals from the inside, and turn a clean signal into a slow or false one.

Why desiccant, not refrigeration

A refrigerated dryer's dew point ceiling stops just above freezing, because it cannot cool the air below the point where its own condensate would ice up the equipment. A desiccant bed has no such floor - it reaches a dew point well below freezing, which is the class of performance shipboard control air is actually held to.

Why two towers, not one

A single bed of desiccant adsorbs moisture until it saturates, then it has to stop and regenerate - which would mean wet air downstream every cycle. Pairing two towers so one always regenerates while the other dries means the delivered air is never interrupted.

Why the regeneration half of the cycle is the one people forget

It is easy to picture the tower that is drying air and forget the one doing nothing visible - venting down, being purged, repressurising. That offline half of the cycle is exactly what makes the online half possible next time the towers switch.

The cycle

Draw, adsorb, switch, depressurise, purge, and repressurise.

Six steps, running continuously, so that from outside the unit the only thing anyone ever sees is dry air arriving on schedule.

FIG · 02SHIPBOARD COMPRESSED AIR DRYING UNIT · DRAW / ADSORB / SWITCH / DEPRESSURISE / PURGE / REPRESSURISE — TWO TOWERS, NEVER BOTH OFFLINE

The step that decides whether the unit actually works is the one nobody downstream ever sees: purging the offline tower, on schedule, every time.

Fig · 03 — The same desiccant bed, three states: saturated, mid-cycle, and freshly regenerated — a drawing of the idea, with no values.

1 · Draw

Compressed air from the ship's own compressor enters whichever tower is currently online.

2 · Adsorb

The desiccant bed pulls moisture from the air as it passes through; dry air leaves for the control-air range.

3 · Switch

On a timed interval, the inlet valves swap - the online tower goes offline, and the regenerated one takes over.

4 · Depressurise

The tower coming offline vents down toward atmospheric pressure, ready to be purged.

5 · Purge

A fraction of the newly dried air is throttled through the depressurised tower, carrying its adsorbed moisture back out.

6 · Repressurise

The purged tower repressurises and stands ready to come back online at the next switch.

Work content

What the unit contains, element by element.

Read it as a checklist: a unit missing a row will buy that row back later, usually the first time the dew point drifts and nobody notices for a week.

Close view of the control panel of a twin-tower air drying unit, isolated against a clean workshop floor: one dew-point gauge, two indicator lights and a single toggle switch on a plain stainless panel, and no people

Fig · 04 — The cycle controller and dew point monitor, isolated against a clean shop floor — illustrative render.

Element	What it does	What matters
Desiccant towers (twin)	hold the desiccant bed	one online while the other regenerates, always
Desiccant material	adsorbs water vapour from the air	selected for the dew point and duty required
Switching valves	route air to whichever tower is online	timed to swap before a bed saturates
Purge orifice / valve	throttles dry air into the offline tower	regenerates the desiccant with no added heat
Pre-filter	removes bulk liquid and oil upstream	protects the desiccant bed from fouling
After-filter	removes desiccant dust downstream	keeps the delivered air particulate-free
Dew point monitor	reads the delivered air's moisture content	confirms the unit is doing its job, not just running
Differential pressure indicators	show when a filter element needs changing	protect the desiccant bed and the system downstream
Cycle controller	drives the switching and purge sequence	keeps the two towers correctly in step
Moisture indicator	a visual check on desiccant condition	a quick confirmation between monitoring intervals
Safety relief provision	protects the towers from overpressure	standard pressure-vessel practice
Corrosion-resistant enclosure	survives the marine atmosphere	protects the controls and valves, not just the towers
Shock & vibration qualification	survives shipboard mounting	proven for the motion a hull actually transmits
Mounting & piping connections	fits the ship's existing compressed-air range	matched to the vessel's own pipework
Testing & documentation	proves dew point and cycle performance	before handover, with records

The row that decides whether the delivered air actually stays dry, not just whether the unit is running, is never the desiccant alone. It is the cycle controller. A full tower of fresh desiccant on a controller that switches late still delivers wet air.

Fig · 05 — Two towers, one controller: while one dries the incoming air, the other is mid-regeneration, ready to swap in.

Full specification — expand

System	Twin desiccant towers, switching valves, a purge orifice or valve, pre- and after-filtration, a dew point monitor, differential-pressure indicators, a cycle controller, a corrosion-resistant enclosure, and testing and documentation before handover
The One Idea	One tower dries the air. The other tower dries itself - continuous dry air is only possible because the offline tower is always being regenerated, ready to take over the instant the cycle switches
Why Desiccant	A refrigerated dryer's dew point ceiling stops just above freezing. A desiccant bed reaches a dew point well below freezing, which is the class of performance shipboard control and instrument air is held to
Why Two Towers	A single bed saturates and must stop to regenerate. Pairing two towers so one always regenerates while the other dries means the delivered air is never interrupted
Regeneration	The offline tower is purged using a fraction of the unit's own dried air, expanded to near-atmospheric pressure, which carries the adsorbed moisture back out of the desiccant - no external heat required in the heatless configuration
Standards	ISO 8573-1 is the reference for the delivered air's quality class. IACS Unified Requirements are the reference framework for compressed air systems aboard classed vessels. Acceptance of the finished unit rests with the customer and their inspection authority
Configurations	Heatless (pressure-swing, no external heat), heated (blower-purge, lower purge-air loss), and heat-of-compression (regenerated from the compressor's own discharge heat)
Scope Boundary	This is the drying unit itself - desiccant towers, switching valves, purge control - not the compressor that generates the ship's compressed air, and not the same product as the fixed, delivered, land-based system on the dry & oil-free compressed air system page
Status	Neometrix engineers shipboard compressed air drying units to order, and no delivered air drying unit is claimed.

Configurations

One twin-tower discipline, three ways to regenerate.

The towers, the valves and the dew point target are shared. What changes is where the energy to dry the offline tower comes from.

Heatless

Pressure-swing regeneration

No external heat source at all - the simplest, most common choice where reliability and few moving parts matter more than purge-air economy.

Heated

Blower-purge regeneration

An electric heater warms the purge air, using less of the unit's own dried output to regenerate the offline tower.

Heat-of-compression

Waste-heat regeneration

Regenerates using heat already carried in the compressor's own discharge air - the most efficient option, paired directly with a compatible compressor.

Fig · 06 — Three ways to regenerate, one twin-tower discipline: heatless, heated, or heat-of-compression, all built around the same switching cycle.

And the part that is not a tower or a valve at all, yet decides all three: the switching schedule — the timing that keeps one tower always regenerating before the other one saturates.

Where it is used

Wherever shipboard air has to run automation, not just spin an engine over.

The common thread is a pneumatic system that cannot tolerate the moisture a starting- air line shrugs off.

Engine room control air

Feeding the pneumatic controls and automation that keep machinery running, where a false signal from wet air is not a minor fault.

Deck machinery & instrumentation air

Supplying instrument-grade air to sensors and actuators exposed to the weather deck.

Cold-weather & high-latitude operations

Where a dew point above freezing means ice forming inside exposed control-air piping.

Automation & safety-system air supply

Wherever a pneumatic signal has to be trusted the first time, not checked and re-checked.

FAQ

Common questions.

Longer answers, for readers who want the reasoning.

Q · 01 Why does control air need to be so much drier than starting air?

Because the two kinds of air are asked to do very different jobs, and moisture only becomes a problem in one of them. Starting air exists to deliver one large, brief burst of force to turn an engine over; a trace of moisture in that burst is essentially irrelevant to the outcome. Control and instrument air, by contrast, runs continuously through small-bore piping, precision valves and pneumatic instruments that are calibrated to respond to fine pressure signals. Moisture in that air can condense inside exposed piping and freeze during cold-weather operation, blocking the line entirely; it can corrode valve seats and instrument internals from the inside over time; and water droplets passing through a sensitive pneumatic controller can turn a clean, fast signal into a sluggish or outright false one. None of those failure modes shows up the day the system is commissioned - they show up months or years later, which is exactly why the drying standard for this class of air is written so much stricter than for starting or general service air.

Q · 02 What actually goes wrong if the dew point isn't low enough?

Three things, roughly in order of how quickly they show up. First, and fastest, is icing: in cold ambient conditions, moisture that has not been removed can condense and freeze inside exposed control-air lines, partially or fully blocking them exactly when the system is needed. Second is corrosion, which is slower but cumulative - water sitting inside valve bodies, actuators and instrument casings attacks metal surfaces that were never designed to be wet, and the damage is often invisible until the component fails. Third is signal quality: pneumatic instruments and controllers are calibrated assuming clean, dry air, and free water or even fine mist passing through them can delay, dampen or corrupt the pressure signal they are supposed to transmit faithfully. A drying unit that is sized and controlled correctly is what stands between the compressor and all three of these failure modes.

Q · 03 Why two towers instead of one bigger one?

Because a single bed of desiccant, however large, will eventually adsorb all the moisture it can hold and stop working - and a system that has to be taken offline every time that happens cannot supply continuous dry air. Making the one tower bigger only delays the saturation point; it does not remove it. Pairing two towers instead means one is always available to dry air while the other is being purged of the moisture it collected during its own turn online, and a timed valve sequence swaps their roles before either one saturates. The system as a whole never has to stop, because there is always a regenerated tower ready to take over - the capacity problem is solved by alternating duty, not by building a single tower big enough that nobody has to think about it.

Q · 04 Heatless, heated, or heat-of-compression - which is right for a ship?

It depends on what the installation can afford to trade against what. Heatless, pressure-swing regeneration needs no external heat source at all, which makes it the simplest and most reliable choice - fewer components that can fail, nothing to wire into a heater circuit - at the cost of using a larger fraction of the unit's own dried air just to regenerate the offline tower. Heated regeneration uses an electric heater to warm the purge air, which lets the unit regenerate using less of its own output, at the cost of one more component - the heater itself - to maintain and one more potential failure point in a machinery space. Heat-of-compression regeneration is the most efficient of the three, capturing heat the compressor is already producing as waste, but it only works when the drying unit is paired directly with a compatible compressor rather than fitted independently. For most shipboard retrofits, where reliability and simplicity outweigh the purge-air saving, heatless is the default starting point; the other two earn their complexity on larger, purpose-built installations.

Q · 05 Is this the same as the dry & oil-free compressed air system on your site?

No, and the difference is worth being precise about. The dry & oil-free compressed air system page describes a complete, delivered, land-based package - an oil-free compressor, a refrigerated dryer, a receiver and a filter train, built and supplied for a research laboratory that needed certified air quality for clean optics and instrumentation. This page describes a different class of product entirely: a standalone drying unit, with no compressor and no receiver of its own, built to fit onto a ship's existing compressed-air range using desiccant rather than refrigeration - which reaches a far lower dew point than the refrigerated system on that other page ever needs to, because shipboard control air is held to a stricter standard. One is a delivered generation system for a lab; this one is a capability for a drying component aboard a vessel. They are related by subject and deliberately kept separate as pages.

Q · 06 Has Neometrix built one of these?

We would rather answer this plainly than let a page imply otherwise. Neometrix engineers shipboard compressed air drying units to order, and no delivered air drying unit is claimed. What stands behind the offer is adjacent and real: Neometrix already engineers compressed-air generation systems, one of which has, in fact, already been delivered and certified - see the dry & oil-free compressed air system page - built to a demanding air-quality class and proven by impurity test certificates rather than brochure claims. Pressure vessels, pneumatic controls and marine-grade fabrication are disciplines already run daily. So the honest position is this: the drying-unit class is engineered to order, the adjacent compressed-air engineering is in the building, and the first unit of this exact type will be built around a customer's compressed-air range and duty rather than lifted off a shelf.

Q · 07 Which standards apply, and who decides how dry is dry enough?

ISO 8573-1 is the public standard this page points to for classifying compressed air quality, including the particle, water and oil content bands a delivered air stream can be measured against. IACS Unified Requirements - the minimum class requirements shared across classification societies - are the reference framework for compressed air systems aboard classed vessels generally. Neither standard, on its own, sets the exact dew point a specific installation needs; that is decided case by case, against the customer's own control-air specification and the classification society under which the vessel is built. Neometrix does not claim a dew point figure on this page for exactly that reason - it belongs on a customer's own specification, not printed generically here. Acceptance of the finished unit rests with the customer and their inspection authority.

Q · 08 What do you need from us to quote?

Five things, and most of them describe the compressed-air range the unit has to fit into, not the unit itself. First, the inlet air condition: the pressure, flow and temperature of the compressed air the unit will receive from the existing compressor. Second, the dew point target, taken from the classification society's requirement or the customer's own control-air specification. Third, the duty profile: continuous or intermittent demand, and how much margin the installation needs. Fourth, the space and mounting envelope available for a twin-tower skid. Fifth, the regeneration preference, if one exists - heatless, heated, or heat-of-compression - or whether that choice should be left to the design review. From that we come back with a system definition you can check, a tower and valve layout, and a budgetary price.

The other compressed-air and adsorption systems, and how they differ from this one.

Three neighbours in the same air-quality and gas-conditioning family.

↗ The delivered, land-based sibling

Dry & Oil-Free Compressed Air System

A complete generation system - compressor, refrigerated dryer, receiver, filters - delivered for a research laboratory.

↗ The same air-quality discipline, aviation

Ground Air Supply Station (GASS)

Quality-controlled compressed air for engine testing, on the ground rather than at sea.

↗ The same adsorption principle, a different gas

PSA Nitrogen Generation Plant

Pressure-swing adsorption again, this time separating nitrogen from air rather than drying it.

Browse all Neometrix product lines.

Get a quotation

Tell us the inlet air, the dew point,
and where it has to fit.

The projects desk replies within two working days with a system definition you can check, a tower and valve layout, and a budgetary quotation. Write to [email protected] or use the form.

Enquire — drying unit Capability sheet (PDF) +91 7777 876 876

ISO 9001 / 14001 ENGINEERED-TO-ORDER CLASS — SHIPBOARD COMPRESSED AIR DRYING UNIT DRAW · ADSORB · SWITCH · DEPRESSURISE · PURGE · REPRESSURISE — TWO TOWERS, NEVER BOTH OFFLINE ENGINEERED IN NOIDA · INDIA