What is behind the recooling of electric rooms?
VFDs generate certain heat load that is dissipated into the ambient. Despite the high efficiency of the VFD, there is some portion of losses that heats up the electric room where the equipment is installed. Hence, it is important to ensure proper recooling of electric rooms. And that is also the topic of this post.
Vast majority of VFDs is installed indoor, either in a conventional building or in a pre-fabricated building (‘container’). Temperature inside an electric room needs to be maintained within a defined range. Extreme temperatures would negatively impact the reliability and lifetime of the equipment and thus shall be avoided. Moreover, reasonable indoor temperature is required for a comfortable work of the personnel.
To maintain a stable temperature inside the room means that the losses dissipated into the room must be balanced with a cooling equipment of more or less the same capacity. For example, if the VFD dissipates 30 kW of heat load into the ambient then the cooling system shall be able to evacuate same amount of heat from the room.
Methods of recooling of electric rooms
Now how to evacuate the losses from the electric room? Most common methods are:
- Room ventilation
- Room coolers via water-to-air heat exchangers
- Air conditioning (HVAC)
- Combination of above methods
Each method obviously has specific advantages and drawbacks. Room ventilation is rather simple. The investment is low and reliability high (e.g. n+1 fans). However, air from outside usually can’t be directly used and needs some treatment.
Room coolers are a good option if a cooling water is available. Otherwise it may not be very efficient solution.
Air conditioning can cool down the room to a defined temperature. The drawback is the high investment cost, high energy consumption (operation cost) and non-negligible maintenance.
Thinking of the pros and cons of each method it sounds very logical to combine the methods. For example, ventilation can be used most of the time and HVAC is only turned on to cover the “extreme” conditions.
Considerations to reduce the heat load
The investment into the purchasing of a cooling system as well as the operation cost can be quite significant. In addition, high air flow is not very comfortable for the operators inside the room either. Thus, the primary goal shall be to minimize the heat load of the electric room. It can be achieved by several means. Let’s look at some of them…
Selection of VFD cooling
Type of VFD cooling is essential choice with respect to losses dissipated into electric room. Air cooled VFD removes 100% of its losses into the ambient. Liquid cooled VFD has much better balance: only some 4 to 10% of the total losses heat up the room while 90 to 96% of losses are removed through the forced cooling system.
Water is generally much better coolant than air. The thermal capacity is by factors higher. Thus, comparably small water flow rate is required to evacuate the given amount of losses. This is illustrated in Figure 2:
The volumetric amount of water to remove certain heat load is 4’000-times less than the volume of air needed for the same. Especially at high power the air cooling reaches its limits and water cooling provides clear advantages.
Air ducting
Air cooled VFDs dissipate all losses into the cooling air. In the conventional solution the VFD sucks the air from the room through inlet openings (typically integrated in the front of the VFD panel) and blows the heated air back into the room. In order to reduce the heat load of the room an air duct may be used. The warm air is then not discharged into the room but guided with the air duct outside of the room. In this way the requirements on recooling of the room can be significantly reduced. As a bonus, the air duct may also reduce noise level in the room. However, there are some basic rules to follow otherwise the air ducting may not work as intended. Upgrade to Premium Subscription to learn more.
Engineering recommendations for air ducting:
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Enclosure recooling
As we already learned, liquid cooled VFDs dissipate only small portion of their overall losses into the ambient. Nevertheless, in very high power range even that portion can be few dozens kW. A solution to further reduce this portion of losses is to recool the air inside the VFD panel with air-to-water heat exchanger. By doing that the losses into air are reduced. Moreover, it also allows to achieve higher ingress protection (IP class) of the enclosure.
- Roof mounted fans on liquid cooled VFD ⇒ approx. 8-10% of total losses dissipated into the electric room
- Air-to-water heat exchangers to recool VFD enclosure ⇒approx. 3-4% of total losses dissipated into the electric room
Liquid-cooled VFDs with higher IP class of enclosure may support additional air-to-water heat exchangers inside the VFD cabinet. Such heat exchangers couple most of the losses of air-cooled components (e.g. busbars, capacitors, control hardware) back to the cooling water circuit. Thus, the losses into the electric room are once again significantly reduced as visualized in Table 2.
Summary
Recooling of electric rooms may be a technical challenge in case of high heat load. Moreover, the required recooling capacity drives the investment cost as well as operating cost (energy consumption of recooling system, more intensive maintenance etc). Therefore, it is important to consider the recooling of electric rooms from early stage.
The measures to reduce the heat load of electric room include selection of VFD cooling, minimizing the portion of total losses dissipated into the room or eventually using an air duct.
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References
[1] How to choose a medium voltage VFD: Cooling type, https://mb-drive-services.com/choose-mv-vfd-cooling/
[2] Advantages of liquid-cooled drives, https://mb-drive-services.com/advantages-of-liquid-cooled-drives/
[3] Enclosure protection, https://mb-drive-services.com/enclosure-protection/
