Today, hygienic air handling unit design, production and operation has become a specialized field. The reason for this is that hygienic air handling units are frequently needed not only in operating rooms and intensive care units, but also in pharmaceutical production facilities, food industry production and storage facilities, electronic processes, etc. In recent years, with the understanding of the importance of indoor air quality in human health, hygienic central applications have started to gain importance in comfort applications. Now, care is taken to use power plants that meet strict hygienic conditions, even in comfort devices. Intensive studies on the subject have been started recently and various standards have been prepared. Studies are carried out in companies operating in this field, and new products with improved quality are introduced to the market every day.
There are enough documents in the literature on the concept of hygiene, the concept of clean room, and indoor air quality. There are even articles in daily newspapers on the importance of the subject and the factors contrary to the concept of hygiene. For this reason, in this article, rather than dwelling on the concept of hygiene, the features that the ventilation system must have in order to create the hygiene conditions in air handling units, which are considered central, are emphasized. Presented article; This is a summary of the practice and research done by the authors. Studies have shown that the following standards regarding hygienic air handling units are widely used:
-prEN 13053 
Title of the standard:
Ventilation for Building-Air handling unit-Ratings and Performance for Components and Sections
This standard has been prepared by the European Committee for Standardization (CEN). The content of the standard is the capacity and performance measurements of the elements used in air handling units, and hygienic air handling units are also included in a part.
According to this standard, there are two types of hygienic air handling unit applications. The first explains the features that should be for standard applications, while the other talks about hygienic air handling units used in the air conditioning of places such as operating rooms and intensive care units that require high hygiene conditions.
-DIN 1946 / 4 
Title of the standard:
Heating, ventilation and air conditioning
HVAC systems in hospital
This standard has been prepared by VDI (Germany’s Chamber of Mechanical Engineers). This standard provides detailed information about the elements in air handling units to be used in hospitals or places requiring high indoor air quality.
-VDI (VEREIN DEUTSCHER INGENIEURE) [3,4]
Two standards regarding hygienic air handling units have been published by Germany’s Chamber of Mechanical Engineers:
1- Hygienic standards for ventilation and air conditioning Systems- Offices and assembling
2- Hygienic standards for ventilation and air conditioning Systems- Standards for Hygiene training
These two standards are more valid in Germany, and EN standards corresponding to these standards are being studied.
Title of the standard:
Recommendation Concerning Hygienic Aspects in Air Handling Unit
Eurovent; is an independent organization that has issued certificates on the capacities of heating, cooling and ventilation devices in recent years. He has prepared a cover letter about hygienic power plants. However, since it follows European standards, the conditions required in this article are very similar to the prEN 13053 requirements.
As mentioned above, studies on hygienic installations and hygienic air handling units have intensified in recent years. However, the rapid development of technology and the rapid elimination of deficiencies in this regard have created differences and incompatibilities between standards. This creates problems for those who are new to this subject. Apart from the inconsistencies between the standards, the practical application of the items described in the standard without going into details is another challenge. The conditions required or the items to be complied with in the standards are quite global and details are avoided. Whether the plant complies with the requirements of the standard is up to the plant designer and the implementer. For example, it is stated in the standards that the materials used in air handling units should be resistant to corrosion, but test methods are not explained. This means that the plant material can be galvanized, painted or stainless. However, it should not be forgotten that galvanized or dyed sheets are of different qualities, and any galvanized sheet or any dyed sheet cannot be used in hygienic power plants. For this reason, the application of the articles written in the standard in accordance with its purpose and purpose requires a separate knowledge and experience.
In this article, the conditions declared by the standards for each main element in air handling units are explained separately, comments are made on the desired conditions based on the experience of the authors, and information is given on how these conditions can be applied in practice.
The point to be noted in the following comparisons is that the prEN 13053 standard is a fairly new standard according to DIN V 1946/4, and it has been made mandatory for normal applications in the prEN 13053 standard based on some conditions required for hygienic plant in DIN V 1946/4. For this reason, some of the issues mentioned in DIN V 1946/4 but not covered in prEN 13053 sections for hygienic applications here are due to the fact that they should be applied in normal power plants.
PLANT CASSETTE STRUCTURE
DIN 1946 Part 4
Central cassette inner surfaces must be flat and cleanable. In particular, the doors of the fan, filter, humidifier and dehumidifier cells should have a sight glass and lighting. The sealing class in the switchboard cassette must meet the class -2 conditions described in DIN V 24 194/2 . Accordingly, the air leakage in the central cassette should not exceed the values given in Table-1 at different pressures.
prEN 13053 ve Eurovent
For standard applications, the inner surfaces of the central cassette should be easy to clean. Switchboards that require high hygiene conditions should be designed in such a way that they do not allow particles to accumulate in the joints between the parts used in the open profile or the switchboard cassette. This is even more important for the central core. All porous parts used should be covered with smooth and washable materials. There should not be any fasteners such as screws or nuts on the inner surface of the central cassette.
Rain and snow should be prevented from entering the cassette openings that open to the outside, such as the central suction, exhaust and fan mouth. Also, the air velocity should be kept low at these openings.
Thermal bridges in the switchboard cassette may cause condensation on the cassette surface. The cassette of the switchboard to be used should be selected considering the thermal conditions of the environment where the switchboard will be located and the thermal bridging coefficient measured according to En 1886 in the cassette.
The doors used should be such that they can observe the interior elements and lighting should be placed. This is especially necessary for fans, filters, humidifiers and cooling coil cells.
Central air leakage must be within the classes specified in EN 1886 [7,8]. In other words, it should be classified as in the tables below;
Apps and Comments
As understood in the standards above, the inner surface of the cassette is very important for hygienic applications. In general, a smooth flat interior surface that is easy to clean is desired. No indentation or protrusion is required. The central cassette inner surface should not allow dirt and dust to accumulate. In Figure-1, it shows an internal surface of the central unit, which is flat on all four sides.
DIN 1946 Part 4
All elements used in air handling units must be accessible or removable from both sides (right and left) for cleaning and maintenance purposes.
PrEN 13053 & Eurovent
The same issues mentioned above are mentioned in prEN 13053 and there is no additional issue.
Application and Comments
The most important feature of hygienic air handling units is that the inner surface of the unit cassette and all the elements used in the unit can be accessed from all sides and therefore cleaned. Therefore, there must be at least one service door or service panel for each element of the switchboard. It is more convenient to reliably remove elements that are difficult to reach inside the cassette. Figure 3 shows an example of a hygienic air handling unit. As can be seen, a service door has been placed for all the elements used in the switchboard. Doors placed on both sides of the elements ensured access to both sides of the elements.