Filter driers and sight glasses

Tuesday, 1 March, 2011

It's time to turn our attention to another pair of standard fittings in liquid lines: the filter drier and the sight glass. The importance of these two components is often (and undeservedly) underestimated. The filter drier and sight glass protect the refrigeration system and provide an important indication of any problem involving the refrigerant, moisture, or contamination in the system.

Chapter 1. Filter driers and sight glasses

It's time to turn our attention to another pair of standard fittings in liquid lines: the filter drier and the sight glass. The importance of these two components is often (and undeservedly) underestimated. The filter drier and sight glass protect the refrigeration system and provide an important indication of any problem involving the refrigerant, moisture, or contamination in the system. 

Chapter 2. Filter driers- general function

Filter driers are usually installed in the liquid line of a dry-expansion refrigeration system, where they have a dual function.First, they trap coarse particulate contamination and copper shavings, and second, they capture any moisture present in the system. For this purpose, modern filter driers are equipped with a drier core consisting of a 100% molecular sieve for optimal drying performance and filtering.The filter drier should be replaced each time the refrigeration system is opened.

Drying capacity 

If you look at a filter drier data sheet, you will always see the term "drying capacity". This specifies the amount of refrigerant that can be dried by the filter drier with a specific type of refrigerant at a specific temperature, based on a certain moisture concentration in the refrigerant before and after the drying process. 

For example, a DML 53 drier can reduce the moisture concentration in 8.5 kg of R134a refrigerant from 1050 ppm to 75 ppm at 24 °C. Here "ppm" stands for "parts per million" and refers to the number of water molecules (H2O) in the refrigerant relative to the number of molecules of refrigerant. 

If the refrigerant charge of the planned system is not yet specified, a suitable filter drier can be chosen based on the liquid capacity. The liquid capacity refers to the cooling capacity in kilowatts when the drier is installed in the liquid line.

Practical tip: 

Refrigeration system engineers can simply assume that the specified ppm values shown in the datasheets/technical brochures before and after drying are realistic, and for practical purposes they only have to concentrate on the type of refrigerant and the refrigerant charge of the system.

Chapter 3. Selection and size

This information is adequate for selecting a suitable filter drier based on the rated drying capacity or liquid capacity. 

The internal volume of the standard drier is generally significant in this regard. For this reason, it is common practice for manufacturers to indicate the internal volume of the filter drier in the type designation. 

For example, with the Danfoss DML 53 the "53" indicates an internal volume of 5 cubic inches and a connection size of 3 (10 mm). The connection size number in the type designation is divided by 8 to obtain the corresponding inch dimension (for example, with "DML 53" the connection size is 3, which means 3/8 inch and corresponds to 10 mm in metric units). 

The type designation may also include the suffix "s" (such as DML 82s), where "s" stands for "solder" and indicates a solder connection. If the "s" suffix is not present, the unit is designed for flare connection.

Practical tip:

If during a service call you are forced to use a drier model that you happen to have in your service van, you can use the last number of the type designation to select a substitute with the right connection size.

Chapter 4. Standard driers and Bi-flow driers

There are many different models of filter driers. Standard models (such as DML) are most commonly used in commercial plant construction. They have a solid core that is permanently bonded to a compact housing. 

These standard driers are designed for only one direction of flow, as indicated by an arrow on the drier housing.

This is usually fully adequate. However, if you encounter a situation with bi-flow operation, such as a heat pump system with a four-way reversing valve, you can either arrange two standard driers in parallel with opposing flow directions and fit each of them with a check valve in the appropriate flow direction, or instead select a bi-flow drier (such as the Danfoss DMB). 

Bi-flow driers can be used with liquid flow in either direction without the risk that previously filtered out foreign matter will subsequently be released into the system.

Burn-out driers and cartridge driers with replaceable cores

In a situation where you actually need a standard drier but the pipe connection size is in the range of 22 mm or larger, you can consider using a cartridge drier with a replaceable core, such as the Danfoss DCR.Cartridge driers are available for use with single or multiple solid filter cores.

The main advantages of this model are very easy replacement of the filter element without any soldering work or dismantling of the piping, as well as moderate service cost because the housing remains in the system and does not have to be purchased anew. 

Like other filter driers, cartridge driers are most often used in the liquid line. However, the suction line is the preferred installation location when a cartridge drier is used as a burn-out drier (with a 48 DA cartridge).

This may be necessary if too much acid has been formed due to water penetration into the refrigeration system or a motor burn-out. 

Most burn-out cartridges or burn-out filter driers contain also a significant amount of aluminium oxide because it is ideally suited to absorbing acids. 

In the event of a burn-out, the cartridge should be replaced afterward at regular intervals, and at the end it should be replaced by a coarse filter cartridge (type 48F). 

Burn-out driers are also available with non-replaceable cores (type DAS) for small pipe connection sizes. 

If you are interested in actually seeing the enormous moisture absorption capacity of a drier along with the large amount of moisture present in the ambient air, you can perform the following simple experiment. 

Remove an unused drier core (such as a type 48 DM) from its sealed can and place it on a postal scale. Record the weight and note the colour of the drier core (you can also take a picture). 

One day later, the drier core will have become much darker and distinctly heavier.

Chapter 5. Collector: Face-seal driers and Pencil Driers

In relatively small systems where the cost of a receiver is difficult to justify, a combined filter drier/receiver (type DMC) can be used.It basically consists of a large housing that combines the functions of a receiver and a drier core.The world's most commonly used type of drier, thanks to kitchen appliances, is the pencil drier.

Most pencil driers are copper-coloured (and made from copper; other types of driers are painted) and filled with silica gel beads as the drying agent. A pencil drier is fitted immediately ahead of the capillary tube in almost every refrigerator. 

Another type of drier is the face-seal drier. This type is rarely encountered by refrigeration system engineers who work exclusively with stationary systems. Face-seal driers are normally used in transportation refrigeration systems, and they are often referred to as "O-ring driers".

A face-seal drier is actually a standard drier, but with a different type of threaded connection than the flare-fitting models. The seal between the drier and the pipe fitting is provided by an O-ring using a face seal, as the name indicates.

Practical tip: 

A filter drier with an O-ring and the suffix "FS" (such as DML305(FS)) is a face-seal drier.

Chapter 6. Selection

To select a suitable drier for a specific system, first consider the pipe diameter of the liquid line.

As there are usually several filter sizes available for a given connection diameter, the drying capacity is also a good selection criterion. 

For quick selection without further consideration of the drying capacity or liquid capacity, you should always choose a slightly larger drier. 

For example, the drier sizes DML 83, DML 163 and DML 303 are available with 10-mm flare connection. For a rough selection, you should always favour the 163 or 303 size. There is never any real disadvantage to choosing a slightly larger drier, aside from the somewhat larger installation length. 

Pressure drop 

Generally speaking, when you select a component you must always pay attention to the pressure drop across the component. This is normally not necessary with filter driers because they have only a small pressure drop when used with suitable pipe dimensions. 

Practical tip: 

If condensation or even ice forms on the pipe at the outlet side of the filter drier in an existing system, despite a normal condensing temperature (such as 45 °C) and without a subcooler installed, the most likely cause is that the filter is clogged with dirt particles. 

If measuring ports are available before and after the drier, a check with a service pressure gauge will show a correspondingly high pressure drop. 

The only remedy in this case is to replace the drier.

Chapter 7. Sight Glasses Function

Sight glasses are related to filter driers in a certain way. In a system with an expansion valve, a sight glass is usually fitted in the liquid line immediately after the filter drier.In theory, the sight glass can be fitted at any desired location on the liquid line. However, placing it close to the expansion valve is especially advisable.

Generally speaking, a standard sight glass with an indicator also has a dual function. It monitors whether the moisture content of the refrigerant is within an acceptable range, and it indicates whether liquid refrigerant is always present at the expansion component. If the moisture content is OK, the indicator colour is green and no further action is necessary.

If the indicator is yellow, there is a problem with the moisture content of the refrigerant. Excessive moisture content can cause icing of the expansion valve if the evaporating temperature is 0 °C or lower. In addition, excessive moisture in the system can react with the oil normally used in modern refrigeration systems (polyester oil) to form acids. 

Frothing in the sight glass indicates either a shortage of refrigerant or that the refrigerant in the liquid line is not entirely in the liquid state. 

Neither of these situations is good for the system, so they should at least be recognised. 


Standard sight glasses are available with solder fittings and flare fittings with male threads, as well as with male and female threaded fittings for direct connection to a filter drier. 

Sight glasses are also available without housings for screw-in installation, and as saddle-mount versions. Saddle-mount sight glasses are fitted to the sides of especially large-diameter pipes using suitable adapters.

Solder connection

Special care must be taken when installing a sight glass with solder fittings. This should normally be done using hard solder with a nitrogen blanket, as is generally known.This prevents the formation of scale inside the pipe or the sight glass.Using nitrogen is especially advisable with a sight glass, as otherwise a black coating can easily form on the inner surface. Internal ash formation can be avoided by the consistent use of dry nitrogen.