Design

ALDACS are autonomous Air Handling Units: — They require neither an extra cooling device nor a cooling tower.

The suply air (SUP¹) temperature and humidity content are handled in processes that are, prctically, independent of each other.

An ALDACS is essentially built of four fundamental components and a few auxiliary ones, particularly heat recovery heat exchangers, ventilators and circulator pumps, control devices and sensors.

While ventilators and circulator pumps, control devices and sensors are in principle available for purchase on the market, all other components have, currently, to be purpose designed and manufactured. In our own developments, we have set on two concepts:

— Modularity, and

— Stackability

to facilitate and simplify the manufacture of components for systems of very different capacities.

For components where these concepts are fully applicable, individual design and manufacture are required. Heat recovery heat exchangers are typical cases, although the stackability in the manufacturing phase can be advantageously applied.

Modularity on the other hand, applies to both component manufacture and Unit assenbly: — Components built as assemblies of individual functional elements, can themselves be integrated in series, parallel, and series + parallel assemblies, as capacity multipliers.

These concepts are illustrated in the three following figures.



Figure 1 – Assembling a heat + mass exchanger by stacking five-stream elements.



Figure 2 – Assembly of a single five-stream heat + mass exchanger element.



Figure 3 – Exploded view of a single five-stream heat + mass exchanger element with all its main parts.

Challenges

Aqueous solutions of alkali halides, as liquid desiccants, are a pretty corrosive lot! Very few metals and metall alloys have a satisfactory service life when used in contact with them. The costs of metalic components for this service would be prohibitive, anyway. Polymeric materials represent the best alternative for the manufacture of components that contact these solutions directly. And their manufacturing energy requirements are significantly lower than that of metals. On the other hand, polymeric materials present themselves considerable challenges when compared with metalic parts:

1. The mechanical properties of polymeric materials are connsiderably different from those of common metals and their alloys, and can vary substancially with temperature, e.g. their tensile strenght;

2. The coefficients of thermal expansion of polymeric materials are one order of magnitude larger than those of metals. This means that even small temperature differences may lead to warping of parts, and to difficulties by sealing of connections;

3. Bonding of polymeric parts is in general more expensive than for metalic ones, and if adhesive bonding is excluded, more complex. Additionally, only thermoplastic polymers can be easily welded in practice;

4. Polymers suffer from the equivalent of metal oxydation: changes of the polymer structure with ageing, which weakens their mechanical properties, and reduces service life;

5. Bonding large assemblies, and assemblies of large parts, e.g. heat recovery heat exchangers (HRHX) and indirect evaporative coolers, (IEC) adds complexity and the need for large bonding equipment. It is feasible, nevertheless!

Overcoming these challenges requires new approaches to design, and new solutions.

Solutions

A few solutions can be discussed, though they are not all at the same stage of development:

1.  Manufacture the single parts as those depicted in Figure 3 above, for example, and assemble them by adhesive bonding;

2.  Manufacture the single parts and assemble them by welding, although various welding techniques might be required;

3.  Use additive manufacturing AM techniques to create functional elements as single pieces.

Adhesive Bonding of thermoplastics works reasonably well in applications where the mechanical and thermal stresses are minimal. Wherever they become larger, variable, or bonding failure is absolutely to avoid, welding is the only practicable option.

Two main techniques are suitable for welding thermoplastics whenever an effective bonding is required:

— Laser Transmission Welding, LTW;

— Electromagnetic Bonding, EMB.

LTW uses infrared radiation (NIR²) to weld parts together, where one part is transparent to, and the other absorbent of radiation in the wavelength of the laser used.

LTW of two laser transparent parts requires an NIR absorbing insert to convert the laser radiation to heat at the interface between the parts to weld. Common LTW lasers operate in the 800 . . 1024 nm wavelength range.

EMB devices operate in the radio frequency range (wavelength ~ 13.6 MHz typically). Radiation at this frequency can penetrate deeply into thermoplastics. EMB requires as well, and allways, an insert (susceptor³) to convert the radio waves energy into heat at the interface between the parts to weld.

The criteria to select a welding technique, the design, construction and manufacturing method of the single parts, have to be considered as one process.

As manufacturing methods three main options are common:

1 — Extrusion;

2 — Injection Moulding;

3 — Deep (vacuum) drawing.

The three methods have been successfuly used in the manufacture of single parts depicted in Figure 3, above.

All three methods impose their characteristic advantages and limitations, and, in fact, determine which welding technique is suitable to bond which parts. As a general rule, long weld seams are better done using LTW, seams between non-laser transparent parts, or through thick layers, are better done using EMB. Designing to use any of these bonding techniques imposes specific constraints as well.

Naturally, both LTW and EMB are suitable to bond assemblies of parts manufactured by AM, subjected to the same constraints indicated above.

Recent developments based on AM, reduce the number of parts to be welded drastically: — from 27 (twenty seven) to 7 (seven), and the number of LTW operations to 4 (four), while eliminating the need for EMB completely.

Notes & References