A cold room does not have a temperature. It has a map.

The number on the control panel belongs to one sensor, usually at the evaporator return. The product lives in the rest of the volume, where three things set the local temperature: how far the air travels, what the load blocks, and how much heat the equipment releases into the space.

ASHRAE puts the requirement plainly in Chapter 21 of the Refrigeration volume: the air moved by the fans has to reach every part of the room uniformly. That is a design result, not a byproduct of hanging a unit cooler on a wall.

Why the jet dies before it reaches the far wall

A unit cooler does not distribute air. It throws a jet. The fan openings discharge parallel streams that merge, within a few opening diameters, into one jet aimed down the room, and that jet drags the surrounding air with it. That entrainment costs it velocity.

ASHRAE Fundamentals describes the decay of a free jet with a compact expression:

V_x = K * V_0 * sqrt(A_0) / X

V_0 is the discharge velocity, A_0 the effective discharge area, X the distance from the outlet and K a constant of that outlet. Velocity falls as 1/X: at twice the distance, roughly half the velocity. If throw was never checked against the real length of the room, the far end receives nothing. The Refrigeration volume states the consequence in its unit cooler selection guidance: ignoring throw and unit location produces “areas of stagnant air and hot spots in the refrigerated space.”

The second obstacle changes every day

The load is the other half of the problem, and it changes every shift. Air does not pass through product; it goes around it. Behind each stack, on the face away from the equipment, a pocket of air sits on the floor and stops renewing. Chapter 21 is blunt: an elaborate distribution system is worth nothing if the stacking pattern blocks the airflow.

That pocket has a price in product. Chapter 22 reports that the shelf life of fresh meat peaks at 29 degrees Fahrenheit (minus 1.7 Celsius) and is cut in half when the meat is held at 36 degrees Fahrenheit (2.2 Celsius). A spread of 7 degrees Fahrenheit inside one room costs half the shelf life of whatever sits in the warm end.

First family: the duct is the diffuser

The first family of textile ducts attaches to the discharge of the unit cooler or air handler through a collar and turns a concentrated jet into a path: the air enters the fabric and leaves distributed along the whole room.

The consequence that matters is velocity. Because the discharge area is the whole surface of the duct rather than a fan opening, the air arrives over the product slowly. Chapter 30, on meat products, warns that strong drafts and high-velocity airstreams striking the product cause local overdrying and non-conforming output.

One point is often misunderstood: inside a refrigerated room the textile duct is not insulated, and does not need to be. Supply air and room air sit at practically the same temperature, so there is no thermal difference to insulate against. The demanding part is the material, a technical textile rated for the minimum temperatures of industrial refrigeration.

In a food room the system also comes down in sections at its zippers, is washed in plant and returns with a technical report.

The heater that runs inside your freezer

Any evaporator working below freezing accumulates frost. Moisture from the product, from infiltration and from door operation freezes on the coil, bridges the fins and closes the air passages. Capacity falls. That is why the defrost cycle exists: the controller stops the fan, cuts cooling and energizes the heater rods inside the fin block.

The industry already treats that heat as a liability. Chapter 14, on forced-circulation air coolers, recommends delaying fan restart until the coil surface temperature approaches its normal level, so the room is not heated unnecessarily after defrost.

But a stopped fan seals nothing. The discharge opening stays open, the air warmed by the heaters is lighter, and it leaves through the top of that opening into the room. The same chapter, under frost control, puts a number on it: depending on the method, as much as 80 % of the unit defrost heat load can end up inside the enclosure, a load not normally carried into the room heat gain calculation. It shows up on the bill, not on the sheet.

The same heat is paid for twice

First as the electricity drawn by the heater rods, then as the compressor work to pull that heat back out of the room. The FAO reaches the same conclusion in Animal Production and Health Paper 92, on meat cold store operation: defrosting is an expensive operation that consumes energy already incorporated in the store, and the lower the storage temperature, the more the heat load is affected. Written as a balance, the real energy of the cycle is:

E = E_defrost + Q_leak / COP

E_defrost is the heater electricity, Q_leak the share that reached the room, COP the system coefficient of performance. In freezing COP is low, so removing each unit of leaked heat costs a meaningful fraction of what it cost to produce.

Second family: a damper that gravity closes

The second family closes that leak with no electronics, no actuators and no maintenance: a short textile sleeve installed on the fan discharge. While the fan runs, air pressure keeps it inflated and open.

When the controller stops the fan for defrost, the pressure disappears and the sleeve falls under its own weight, covering the opening and cutting the path the heat escaped through. The heater energy stays inside the housing, where the ice is, so the cycle concentrates and finishes sooner. That protects product as well: Chapter 14 warns that an excessively long defrost cycle can drive an unacceptable rise in product core temperature.

Two families, one design

Distribution and defrost are not two separate products. One spreads the air while the equipment cools; the other seals the discharge while it defrosts. Both are sized for the geometry and load of one particular room.

That sizing is not a catalogue exercise. ASHRAE says so explicitly: guesses and rules of thumb are not an acceptable basis for selecting equipment with the right airflow. Before any fabric is cut, the room is simulated with the real load and stacking pattern of the project, so velocities, temperatures and air path are known in advance.

Frequently asked questions

Does a textile duct inside a cold room need insulation?

No. The supply air and the room air are at practically the same temperature, so there is no thermal gradient to insulate against, and an insulated duct would add weight and cleaning difficulty with no benefit. The demanding requirement is the fabric itself, which has to remain serviceable at the minimum operating temperatures of the room.

Does the 80 % figure for defrost heat apply to every installation?

No. ASHRAE states it as an upper bound that depends on the defrost method, so electric, hot gas and air defrost do not behave alike. The value of the figure is that it sets the order of magnitude of a load that is usually absent from the room heat gain calculation entirely.

How is the hygiene of the air system documented in a food plant?

The textile sections are separated at their circumferential zippers, taken down, washed industrially and reinstalled, and each unit returns with a technical report of the cycle. That report is what turns an unverifiable claim about duct cleanliness into an auditable record.

The room already has a unit cooler. Does it have to be replaced?

Normally not. Both families connect to the existing equipment: the distribution duct to its discharge through a collar, and the defrost sleeve to the fan outlet. What changes is how the air is delivered and what happens to the discharge opening during defrost, not the refrigeration plant.

The decision criterion is not the duct. It is where the temperature spread in the room comes from. If the far end of the room is warm and the equipment is sized correctly, the problem is throw and distribution, and the first family applies. If the room is at low temperature and the defrost cycles are frequent and long, part of the load is heat the equipment is putting back in, and the second family applies. Most industrial rooms are in both situations at once, which is why the two are designed together, against the real geometry and the real stacking pattern, and not chosen from a catalogue.

Request a technical sizing study for your cold room from the DUCTECOL engineering team at fabricduct.co.

The post Textile Air Ducts in Cold Rooms: Throw, Load and Defrost first appeared on DUCTecoL.

A building that resists being conditioned

Few commercial buildings are harder to condition than a terminal. Ceilings run from roughly 33 to 130 ft (10 to 40 m). Glazed curtain walls pick up solar gain all day long. Occupancy arrives in pulses: a thousand people at a gate that was empty twenty minutes earlier, then empty again.

ASHRAE states the consequence directly in its guidance for transportation centers. In an airport, the single largest problem is often thermal drafts created by large entranceways, high ceilings and long passageways with openings to the outdoors. Not installed capacity. Not the chiller. Air movement.

That diagnosis moves the argument from tonnage to distribution.

Why cold air does not fall from a 100 ft ceiling

Supply air leaves an outlet as a jet, and a jet does two things on the way down.

First, it slows. In the fully developed zone of the jet, the ASHRAE Handbook — Fundamentals chapter on space air diffusion gives the centerline velocity as Vx = Kc3 · Vo · √Ao / X: velocity falls inversely with throw distance. The constant is not a generic figure. It comes from outlet testing to ASHRAE Standard 70, and the tabulated values are published as examples only.

Second, it entrains room air. That is the working principle of mixing ventilation, and it has a cost: as the jet advances, both the supply velocity and the temperature difference between supply air and room air get smaller and smaller.

From 65 to 100 ft (20 to 30 m) that leaves two outcomes. Throw hard enough to reach the floor and the air arrives in the occupied zone as a draft, which is the visible failure. Throw less and it never arrives: the jet stalls in the upper volume, mixes into the warm stratified air already sitting there and loses the capacity to cool anything. The cooling was produced and paid for. It simply never reached a passenger.

The leakage that never enters the comparison

Then there is the metal itself: tons of galvanized sheet suspended over the heads of the public, assembled from transverse joints sealed by hand in the field.

Field work at Lawrence Berkeley National Laboratory measured ten large commercial duct systems at operating conditions. Three leaked less than 5 percent of duct inlet flow. The other seven leaked between 9 and 26 percent. That spread is not a product property. It is what field assembly does to a drawing.

The consequence is not only wasted fan power. ASHRAE recommends that a terminal building be designed to maintain a substantial positive pressure, precisely because air balance is hard to control across many outdoor openings, high ceilings and long, low passageways that are frequently not conditioned. ASHRAE does not attach a figure to the word substantial, and neither should a specification that quotes it. And air balance is what leakage disturbs first: air released into a ceiling plenum instead of the space it was sized for is still inside the building, but it has left the balance the designer calculated.

The duct is the diffuser

Fabric duct changes the geometry rather than the material list. There is no grille at the end of the run, because the fabric surface is itself the diffusion element: air is released gently along the entire length instead of being concentrated at a handful of outlets. Where it has to come down from height, the design uses high-throw or low-throw nozzles, sized and aimed to deliver into the occupied zone, which in a terminal means gates, check-in lines and seating.

That is consistent with the design intent ASHRAE describes for high-ceiling transportation spaces: concentrate conditioning where it is wanted and avoid disturbing the stratified air above. ASHRAE describes sidewall distribution as the means; what transfers to a textile system is the intent, not the hardware. Less air mass treated for the same conditions in the occupied zone means fewer kilowatts.

Sound follows the same logic. Diffusers and grilles typically generate their highest noise levels in the octave bands centered at 1000 Hz or above, which is the band that carries speech intelligibility. That is why a noisy concourse buries its own announcements. A textile surface diffusing at low face velocity does not whistle, drum or vibrate.

Weight is the last piece of it. A fabric run weighs a fraction of the equivalent metal and hangs from a single steel cable, so suspended load on the structure drops and installation can proceed at night, section by section, without taking a concourse out of service for a single day.

Nothing is guessed

Before a foot of duct is cut, the terminal is modeled in computational fluid dynamics: the air in the building divided into millions of cells, with the physics solved in each one for direction, temperature and velocity. It is the same class of numerical method used in aerospace and motorsport, and it takes high-performance computing to run.

It earns its keep because large volumes do not mix uniformly. Microclimates form. Flow descends in some regions while convective plumes rise in others, and a design that ignores the plume fights it. The plume needs room to develop, and extraction belongs where it rises. A terminal does not behave like a supermarket, a hospital or an office.

The output is numeric. Air speeds in the occupied zone are checked against ANSI/ASHRAE Standard 55. It is worth being exact about what that standard actually sets. What it limits is the mean speed the occupant is exposed to within the occupied zone, not the velocity in a duct or at an outlet face, and it publishes no single blanket maximum: the allowance moves with operative temperature, moves again where people can adjust the air themselves, and is judged separately at ankle height. Those are inputs to a comfort model for spaces designed for human occupancy, not duct design limits, and the standard does not specify setpoints.

From that velocity field comes the perforation pattern: diameters, rows and orientation, laser-cut and sewn at the plant.

Aesthetics and maintenance are specified, not inherited

Color is a decision, not a default. A duct can contrast with the architecture and read as part of it, or be specified in a low-visibility tone that lets it recede into the ceiling structure. Internal hoops are equally optional: with them the duct holds its shape when the system cycles off, without them it breathes with the equipment.

Maintenance never shuts the terminal down. Sections come down run by run, go out for industrial laundering and come back with a service report for each one.

Frequently asked questions

How do you get air down 100 ft without creating a draft?

Not by increasing throw. The jet decays inversely with distance and entrains room air the whole way, so more supply velocity buys arrival at the cost of a draft. The workable route is directed delivery from nozzles sized for the occupied zone, with the resulting air speeds verified against ASHRAE Standard 55 rather than against a catalog throw figure.

Does conditioning only the occupied zone violate any ventilation requirement?

No. It separates two questions that get confused. Outdoor air rates and air changes are set by the applicable ventilation standard and are unaffected by the diffusion element. What changes is where the conditioned air is placed, and ASHRAE explicitly describes concentrating conditioning where it is wanted in high-ceiling transportation spaces.

How does fabric duct compare to metal on leakage?

Metal leakage in large commercial buildings is a field variable, not a catalog value: LBNL measured 9 to 26 percent of inlet flow in seven of ten systems, and less than 5 percent in the other three. A textile system has no hand-sealed transverse joints at all, so the field variable that produces that spread does not exist in it.

Is CFD necessary, or is it a sales exercise?

For a terminal it is necessary, because the geometry has no precedent in smaller buildings. Ceiling height, glazing load and pulsed occupancy produce microclimates and convective plumes that no rule of thumb predicts, and the perforation pattern is derived from the resulting velocity field rather than chosen from a standard product.

The decision in a terminal is not made by comparing the installed price of a linear foot of duct. It is made by asking three questions the traditional comparison usually leaves out: what fraction of the design airflow actually reaches the occupied zone after distribution losses, what air speed the passenger standing at a gate is exposed to at that flow, and what sound the diffusion element contributes in the octave bands where announcements have to be understood. A system that answers all three with measured or computed numbers is specifiable. One that answers them with a catalog throw figure quoted to a standard terminal velocity is a hypothesis about a building that has no equivalent at small scale.

Request a CFD-based design and sizing study for your terminal at fabricduct.co.

The post Air distribution in airport terminals: the fabric duct case first appeared on DUCTecoL.

A space that manufactures its own corrosive agent

An indoor pool is one of the most demanding spaces in air conditioning. The water surface evaporates continuously, and ASHRAE models that evaporation with an equation where air velocity over the water is a direct variable: going from 30 to 125 fpm across the surface raises evaporation by roughly 30 percent. The designer walks a tightrope. Move too much air and the latent load spikes. Move too little and the vapours stagnate exactly where people breathe.

The reference criteria are narrow: air 1 to 2 degrees Celsius above water temperature without exceeding 30 degrees Celsius, design relative humidity between 50 and 60 percent, and the space held at negative pressure so moisture does not migrate into adjacent areas. Outside that range humidity finds the cold surfaces: glazing, frames and, above all, the hidden roof structure. The ASHRAE manual itself warns that in the worst case the roof could collapse from corrosion caused by water condensing on the structure.

Chloramines: the chemical price of disinfection

The chlorine that keeps the water safe reacts with what bathers bring in, sweat, urea and organic residue, and produces chloramines. The most aggressive is trichloramine, the familiar pool smell that irritates eyes and airways. It is not clean chlorine. It is new chemistry, denser than air, accumulating right above the surface.

A study published in Frontiers in Built Environment in 2022, using CFD modelling on a Montreal pool, measured 1,910 ppb of trichloramine in the swimmer breathing zone at water level, against markedly lower values at standing height. Its conclusion is uncomfortable for common practice: the ventilation minimums set by standard are insufficient to clear that accumulation.

That vapour does not stay low. It rises, condenses on the cold metal of the roof, dries, and the cycle repeats, concentrating chlorides exactly where nobody cleans. The British Health and Safety Executive states it plainly: chloramines are the most important factor in stainless steel corrosion in the pool environment.

What happens to metal hanging over the water

Uster was not an isolated case. In 2001, in Steenwijk, the Netherlands, the suspended ceiling of another pool came down with the air ducts included, through the same mechanism in 304 stainless fixings. In 2003 it happened again in Finland. The pattern is always the same: highly stressed parts, in a chlorinated atmosphere, that nobody washes.

The technical detail that misled a generation of designers is temperature. Stress corrosion cracking in austenitic steels was considered a problem above roughly 55 degrees Celsius. In pools it occurred at thirty. That is why the HSE concludes that grades 304 and 316, perfectly valid underwater or at the pool edge, must not be used in safety-critical load-bearing components inside the hall atmosphere.

A duct lives precisely in that zone: against the roof, in the layer where chlorides concentrate most, with no cleaning regime at all. So it is worth being direct about the materials specified out of habit:

• Galvanised sheet. It corrodes, and the rust ends up dripping onto the pool basin.
• Pre-insulated aluminium. Sandwich panels with aluminium faces suffer chloride pitting in pool atmospheres.
• Fiberglass duct board. Rigid board with foil facing absorbs moisture through the cut edges and encourages microbial growth in an environment that is humid by definition.
• Stainless steel. The grade that genuinely resists is not 316: it is alloys with around 6 percent molybdenum, the same specified for desalination, at three to four times the cost of conventional stainless. Even then, guidance requires inspection of critical components at least twice a year.

Why technical fabric changes the problem

Fabric duct does not compete with metal on corrosion resistance. It steps out of the equation. Synthetic materials take no part in the electrochemistry that cracks steel. There are no chlorides pitting a surface and no stresses propagating a crack.

The selection, however, is not generic. At DUCTECOL we work with a wide range of technical textiles, polyamides, aramids and other polymers, and depending on the process in each zone we run a chemical compatibility study before defining the material. And not only the fabric: threads, hooks and every component of the system are chosen to withstand that specific atmosphere.

On top of that, the air pattern the manual asks for is exactly what fabric produces by design: sweeping dry air across the glazing so it does not condense, barely touching the water surface, and diffusing uniformly with no draughts over bathers. Laser micro-perforations and calibrated nozzles allow that pattern to be tuned metre by metre, something that grilles on rigid ductwork achieve only with far more costly engineering.

Weight, cost and maintenance

A fabric duct weighs around a tenth of the equivalent metal run. On a structure already working in an aggressive environment, that difference in suspended load is not minor. Industry documentation reports savings between 30 and 70 percent in total installed cost against metal ductwork, counting transport, installation and maintenance. Against the high-grade stainless a pool would actually require, the gap is wider.

Maintenance closes the argument. The duct comes down by its zippers and is washed at 40 degrees Celsius. No repainting, no passivation, no crack inspection twice a year. And hygiene is audited by washing, something a rigid system cannot offer.

Frequently asked questions

Does fabric duct resist chlorine without coatings?

Synthetic materials take no part in the electrochemical corrosion that attacks steel. Even so, resistance depends on the specific textile and on the chemistry of each hall, which is why selection follows a chemical compatibility study and extends to threads, hooks and accessories.

What is the design relative humidity for an indoor pool?

Between 50 and 60 percent as a design value, with air 1 to 2 degrees Celsius above water temperature and not exceeding 30 degrees Celsius. The hall must be kept at negative pressure relative to adjacent spaces.

Why is specifying 316 stainless not enough?

Because the British HSE advises against grades 304 and 316 in safety-critical load-bearing components inside the pool atmosphere, following the stress corrosion collapses documented since 1985. The grades that do resist cost three to four times more than conventional stainless and still require periodic inspection.

How is a fabric duct cleaned in a pool?

It comes down by its zippers and is washed at 40 degrees Celsius. That cycle replaces the repainting, passivation and inspection that metal ductwork demands in a chlorinated environment.

The materials discussion in a natatorium is not settled by comparing the price per metre of the first installation. It is settled by putting three variables on the table that traditional ductwork usually leaves outside the budget: the cost of the steel grade that genuinely resists that atmosphere, the inspection regime that material demands across the life of the building, and the load the structure has to carry in an environment that is attacking it. Once those three enter the comparison, technical fabric stops being the alternative and becomes the starting point.

Request the chemical compatibility study and sizing for your natatorium at fabricduct.co.

The post Why metal ductwork does not belong in an indoor pool first appeared on DUCTecoL.

Engineering tools built by a fabric duct manufacturer

Most HVAC sizing tools available online are designed for conventional metal ductwork. When engineers need to specify fabric ducts — a technology with fundamentally different airflow dynamics, material properties, and installation methods — those generic calculators fall short.

DUCTECOL Internacional S.A.S., a fabric duct manufacturer based in Colombia with exports across Latin America, North America, and beyond, developed Apps DUCTecoL to bridge that gap. Available at apps.ductecol.com, the platform provides three specialized web applications that support the complete design cycle of textile duct systems for HVAC projects.

Platform overview: three applications, one integrated workflow

AirFlow Sizer — Duct sizing and airflow velocity

AirFlow Sizer calculates the optimal fabric duct dimensions for a given HVAC system. Users input parameters such as required airflow (in m³/h or CFM), design temperature, available static pressure, and space geometry. The tool returns duct diameter or section, recommended length, perforation pattern, and estimated air distribution across the occupied zone.

Unlike generic duct calculators, AirFlow Sizer uses DUCTecoL’s actual product specifications — material weights, perforation configurations, and tested pressure-drop data — ensuring results that align with the manufactured product’s real-world performance.

AirFlow Quiet PRO — Acoustic performance analysis

Noise control is a critical requirement in commercial and institutional fabric duct installations. AirFlow Quiet PRO estimates the sound pressure level produced by the textile duct system under specific operating conditions, accounting for airflow velocity, fabric material type, perforation configuration, and suspension geometry.

This tool is particularly valuable for projects in noise-sensitive environments: educational facilities, healthcare spaces, retail stores, corporate offices, and food processing areas where acoustic comfort standards must be met.

AirFlow Studio PRO — Computational Fluid Dynamics (CFD) simulation

For complex projects requiring advanced analysis, AirFlow Studio PRO provides CFD simulation of airflow behavior inside and around the fabric duct. The tool generates visual representations of velocity, temperature, and pressure distribution across the installation space.

This level of analysis is especially relevant for large industrial facilities, data centers with high thermal densities, clean rooms, and any project where uniform air distribution is a critical operational factor.

Who can access the platform

Apps DUCTecoL is not a public tool. Access is restricted to three user categories: active DUCTecoL clients, authorized distributors, and DUCTecoL’s internal engineering team. This ensures the platform is used by qualified professionals who can fully leverage its technical capabilities.

The registration process collects professional information including full name, company, job title, country, email, WhatsApp number, and phone number. LinkedIn profile linkage is optional. After submission, the DUCTecoL team reviews and approves each account. Active clients and authorized distributors receive priority validation.

The manufacturer advantage

The core differentiator of Apps DUCTecoL is data integrity. Every calculation performed on the platform uses parameters derived from DUCTecoL’s actual manufacturing data: tested materials, certified perforation patterns, and validated performance metrics from completed installations across multiple industries and geographies.

For engineers, this means design specifications that match the product’s field performance. For distributors, it means technically backed proposals that build client confidence. For the industry as a whole, it represents a manufacturer investing in transparency and technical accessibility.

How to get started

Visit apps.ductecol.com and click “Solicitar Acceso” (Request Access) to complete the registration form. If you are an existing DUCTecoL client or distributor, your approval will be expedited.

For questions about the platform or DUCTecoL’s engineering services, contact the team via WhatsApp at (+57) 301 4529090 or through the contact form at fabricduct.co.

The post Apps DUCTecoL: Professional Engineering Tools for Fabric Duct Design and Analysis first appeared on DUCTecoL.