Control cabinet air conditioning

The cabinet air conditioning or the housing air conditioning to the in cabinet built in the housing or electronic components from harmful environmental influences such as dust , moisture and temperature protection. The term is mainly used for heat dissipation measures.

Passive and active air conditioning

Passive air conditioning

Passive air conditioning is the natural heat dissipation without the aid of additional devices. When the housing is closed, the heat is dissipated through the housing walls. In the case of open housings, air is also exchanged through the air inlet and outlet openings ( natural convection , i.e. without the use of fans). The prerequisite for passive air conditioning is that the ambient temperature is lower than the desired internal temperature.

Examples of measures to improve passive air conditioning are

Attachment of ventilation slots in the housing walls
thermal coupling of hot components directly to the housing wall

Active air conditioning

If the passive air conditioning is not sufficient, the heat must be specifically dissipated from the housing using additional devices. For this purpose are used:

Filter fan or roof fan
Air / air heat exchanger
Air / water heat exchanger
Refrigerators

Air conditioning components

For the choice of components it is crucial whether the devices are open, i. H. breathable, or closed, d. H. are impermeable to air. While the heat in open devices can be dissipated via an air stream, in closed devices it is only possible through the housing walls or with the help of a heat exchanger. Whether a device is designed to be open or closed depends primarily on the degree of protection required in accordance with DIN EN 60 529: 2009-9.

Control cabinet surface

The heat dissipation through the closed control cabinet surface (passive air conditioning) depends on the type of installation of the cabinet, the heat transfer coefficient of the walls and the difference between the inside and outside temperature. The following formula is used for the estimation:

{\ displaystyle Q = U \ cdot A \ cdot (T_ {i} -T_ {u})}

Q: heat emission of the cabinet surface (in W)

U: heat transfer coefficient (in W / (m ² K))

A: effective cabinet surface (in m ² )

{\ displaystyle T_ {i}}

: Internal housing temperature (in ° C)

{\ displaystyle T_ {u}}

: Outside temperature (in ° C)

This formula does not take into account the temperature and air flow distribution in the cabinet, it only uses an indoor and an outdoor temperature and a general value for the heat transfer coefficient. It is therefore only suitable for a general estimate. Methods of numerical flow simulation are used for more precise calculation of the temperature and air flow distribution in a control cabinet .

Heat transfer coefficients of common control cabinet walls:

Painted sheet steel: U = 5.5 W / (m ² K)
Stainless steel sheet: U = 4.5 W / (m ² K)
Aluminum: U = 12.0 W / (m ² K)
Double-walled aluminum: U = 4.5 W / (m ² K)
Polyester: U = 3.5 W / (m ² K)

Effective control cabinet surface

The type of installation of the cabinet is taken into account with the effective control cabinet surface. A housing that is free on all sides in a room can give off more heat than a device that is set up on a wall or in a niche. The formulas for calculating the effective cabinet surface area are specified in DIN VDE 0660 Part 500.

Housing installation type according to DIN VDE 0660 part 500	Calculation formula for the effective control cabinet surface
Single housing free-standing on all sides	${\ displaystyle A = 1 {,} 8 \ cdot H \ cdot (B + T) +1 {,} 4 \ cdot B \ cdot T}$
Single housing for wall mounting	${\ displaystyle A = 1 {,} 4 \ cdot B \ cdot (H + T) +1 {,} 8 \ cdot T \ cdot H}$
Start-end housing free-standing	${\ displaystyle A = 1 {,} 4 \ cdot T \ cdot (H + B) +1 {,} 8 \ cdot B \ cdot H}$
Start-end housing for wall mounting	${\ displaystyle A = 1 {,} 4 \ cdot H \ cdot (B + T) +1 {,} 4 \ cdot B \ cdot T}$
Middle housing free-standing	${\ displaystyle A = 1 {,} 8 \ times B \ times H + 1 {,} 4 \ times B \ times T + T \ times H}$
Middle housing for wall mounting	${\ displaystyle A = 1 {,} 4 \ cdot B \ cdot (H + T) + T \ cdot H}$
Middle housing for wall mounting with covered roof area	${\ displaystyle A = 1 {,} 4 \ times B \ times H + 0 {,} 7 \ times B \ times T + T \ times H}$
	B = switch cabinet width H = switch cabinet height T = switch cabinet depth

Examples

A free-standing, closed switch cabinet with steel walls, with the dimensions (height × width × depth) of 2.2 m × 0.6 m × 0.6 m, which is set up in a room with a temperature of 20 ° C and its middle Inside temperature is 40 ° C, it emits the following heat output through its walls:

{\ displaystyle Q = 5 {,} 5 \, \ mathrm {W / (m ^ {2} K)} \ cdot 5 {,} 256 \, \ mathrm {m} ^ {2} \ cdot (40 \, ^ {\ circ} \ mathrm {C} -20 \, ^ {\ circ} \ mathrm {C}) = 578 {,} 16 \, \ mathrm {W}}

If the same cabinet is placed in the middle of a row of cabinets, its heat output is lower:

{\ displaystyle Q = 5 {,} 5 \, \ mathrm {W / (m ^ {2} K)} \ cdot 4 {,} 200 \, \ mathrm {m} ^ {2} \ cdot (40 \, ^ {\ circ} \ mathrm {C} -20 \, ^ {\ circ} \ mathrm {C}) = 462 {,} 00 \, \ mathrm {W}}

Filter fan

Control cabinet with filter fan (at the bottom of the door)

The filter fans are a cost-effective option for active control cabinet air conditioning . The precondition for their use is that the control cabinet environment is relatively clean. The ambient temperature must be below the desired internal cabinet temperature.

Air / air heat exchangers

Air / air heat exchangers are used when the switch cabinet has to be closed due to the degree of protection. Here, too, the ambient temperature must be lower than the desired internal temperature. A temperature difference of at least 10 K is recommended so that the heat exchanger works effectively.

The heated indoor air is sucked in by a fan in the upper area of the cabinet and passed through the heat exchanger. In counterflow principle outside air flows drawn from a separate fan, through the heat exchanger. The cooled indoor air leaves the heat exchanger and is blown back into the cabinet. The heat transfer takes place without direct contact between the air flows. Dust, moisture or dirt cannot get into the cabinet.

The performance of the air / air heat exchanger is characterized by the specific heat output parameter . It indicates how much heat the device can dissipate per degree of temperature difference between inside and outside.

Air / water heat exchanger

Compared to air / air heat exchangers, air / water heat exchangers (see also water cooling ) achieve high cooling capacities with very little space requirements. They are used for switching cabinet cooling when the heat output to be dissipated is very high or when the internal temperature is to be reduced to below the ambient temperature.

Similar to the air / air heat exchanger, the heated indoor air is sucked in by a fan and fed into the heat exchanger. The heat is absorbed by the water as a cooling medium and transported to a remote location through a water return line. In this way, entire rows of cabinets can be cooled. A prerequisite for the use of air / water heat exchangers is that a closed cooling water circuit is available.

The performance of the air / water heat exchanger depends on the one hand on the difference between the temperature inside the housing and the water inlet temperature and on the other hand on the volume flow of the water flowing through it. The air / water heat exchangers are by a characteristic map characterized, from which one can read off the cooling performance for the specific application.

Refrigerators

Switch cabinet cooling devices are also used for large amounts of heat to be dissipated. You work with a compression refrigeration machine . The heated indoor air is sucked in by a fan and passed through an evaporator, where it transfers its heat to the refrigerant . Cooled in this way, it is then returned to the cupboard. The cooling units have a condensate drain.

Control cabinet heating

Control cabinets for use in unheated rooms or outdoors are often equipped with heating. It should prevent the formation of condensation . In the case of outdoor applications, an excessive drop in component temperature should also be prevented.

literature

Styppa, Heinrich: Air conditioning for housings, machines and systems . Verlag modern industry, 2005 (The Library of Technology, Volume 284), ISBN 3-937889-28-0
Klingberg, Gottfried: Control cabinet and housing air conditioning in practice, Part II . Mählin Werbung Düsseldorf, 1996, ISBN 3-923270-07-0
Project planning manual for control cabinet cooling. Süddeutscher Verlag onpact Munich 2008, ISBN 978-3-937889-74-0

Web links

Rittal manual: Enclosure & process cooling (PDF file; 8.4 MB)