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Dimensioning of Refrigeration Components - Part 3: Filter Driers

28 November 2011 | Author: Stephan Bachmann
Filter driers are mostly installed in liquid lines in dry expansion refrigeration plants, where they have a dual function. Firstly, they catch coarse particles of dirt and copper filings, and secondly they also bind moisture in the system. For this purpose, modern filter driers are equipped with a drying block made of 100% molecular sieve for optimum drying capacity and filtering. The filter drier should be replaced whenever work is done on the refrigeration system.
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Quick Selection
To choose the right drier for a specific system, you must first consider the dimensions of the liquid line pipe. As there are usually several filter drier sizes for a connection diameter, the drying capacity can be a good additional aid for making the right selection. For a quick selection – without further consideration of the drying capacity or liquid capacity – you should always select the somewhat bigger drier. The drier sizes "DML 83", "DML 163" and "303", for example, all have a 3/8” or 10 mm flare connection. As a rough selection, sizes "163" or "303" should generally be chosen. The choice of a somewhat bigger drier is never a real disadvantage (apart from the somewhat bigger construction length).



Filter drier DML copper plated solder connections



DML flare connections





DML – cut away drawing
Pressure Drop
Generally speaking with the component selection the resultant pressure drop should always be considered. With the filter drier this is not necessary in every case, as it only has a marginal pressure drop in use with the respective pipelines. If you find a filter drier in an existing system, which, despite the usual condensing temperatures (e.g. 45°C and without a subcooler being installed), forms condensation or even ice on the pipe surface downstream from the filter drier, then the drier is most probably clogged up with dirt particles. With measurement connections before and after the drier, the service manometer then indicates a correspondingly high pressure drop when checked. In such cases there is no alternative but to replace the drier.
Drying Capacity
Looking at the respective datasheets we repeatedly see the term "drying capacity". This indicates the amount of refrigerant that can be "dried" by the filter drier for a specified refrigerant and specified temperature. This is based on the specific moisture content of the refrigerant before and after the drying process. A "DML" drier, for example, can therefore bring 8.5 kg of R134a refrigerant at 24°C from 1050 ppm to 75 ppm. "ppm" stands for "parts per million", and in a specific case indicates the number of water molecules (H2O) in the refrigerant per million refrigerant molecules. If the refrigerant charge volume of the system to be planned has not yet been specified, the suitable filter drier can also be dimensioned according to the liquid capacity. "Liquid capacity" is the cooling capacity in "kW" for installing the drier in the liquid line. The ppm values before and after drying can be confidently accepted by the system builder as practical; they are based only on the refrigerant and the refrigerant charge of the system.
Selection and Size
A suitable filter drier can therefore already be dimensioned via the drying and liquid capacity. Generally the internal volume of the standard drier is always important here. It is therefore now the norm for the manufacturer to include the filter drier's volume in the type reference. In the previous example we were talking about a "Danfoss DML 53". The "53" has an internal volume of  5 inch3 and a 3 (3/8” or 10 mm) connection size. The connection size in this type reference is then divided by 8 to get the corresponding connection size in inches (e.g. "DML 53": connection 3, i.e. 3/8 and is equal to 10 mm as a metric unit). For a service function you can select the right replacement for the connection size using the last digit of the type reference. The additional "s" may have to be observed here (e.g. "DML 82s"). It stands for "solder" and therefore identifies a soldered connection. If the additional "s" is not there, it is a flange connection.


Standard and Bi-Flow Drier
Filter driers are available in the most diverse models. The standard drier (e.g. "DML") is most frequently found in commercial system construction. It has a solid material core and it forms an inseparable unit with its compact housing. These standard driers are designed for only one flow direction (arrow direction on the drier housing), which is usually entirely sufficient. Should you, however, actually have a truly bi-flow operation – as found, for example, in a heat pump with a 4-way reversing valve – then you can either arrange two standard driers in parallel with opposite flow directions, and for each of them mount a check valve in the respective flow direction, or choose a bi-flow drier ("Danfoss DMB"). Bi-flow driers can be used in both directions without the risk of recently filtered out foreign objects being returned to the system.




Housing filter drier DCR for exchangable core



DCR for 2 cores - cut away drawing




Core for DCR
Filter Drier with Replaceable Solid Core
If a standard drier is actually required, but the line connection sizes are already in the 7/8” or 22 mm and higher range, a housing filter drier can be used for swap-out drying blocks ("Danfoss DCR"). These housing filter driers are available for operation with one or more solid cores. The main advantage of this variant is the very easy exchange of the drying cores without any soldering or dismantling of the lines and the moderate service price, as the housing remains in the system and does not have to be bought again. The main application area for this housing drier is also the liquid line.

If, out of interest, you want to illustrate the enormous water absorption capacity and the high moisture volume that is bound together in the ambient air, then you can set-up the following easy example. You open a drier block core (e.g. "48 DM") and set the block on a scales. Now you note the weight and observe the colour of the drying core. A day later the colour of the drier will have darkened significantly and the weight will have increased considerably.
Conclusion
Filter driers are indispensable for a compression refrigeration system's smooth operation. If you shy away from the cost of configuring the filter drier via the drying capacity, you can choose the connection dimension and selection of the bigger model (bigger volume) for the corresponding pipe dimension. The pressure drop will then generally also be OK.
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