Choosing a cooling unit
for electrical enclosures & server racks
Updated:
Filter fan, heat exchanger or compressor cooling unit? The path from power loss to the right type of cooling.
🌡️ Go straight to the cooling capacity calculation →Choosing the right cooling unit is not a matter of gut feeling, but a short calculation plus two or three clear decisions. A unit that is too small lets the enclosure overheat; one that is too large wastes energy and money. This guide applies equally to electrical enclosures with frequency converters and control technology and to 19″ server racks with an active IT load.
Short answer: first add up the power loss of all installed components. Then the decisive question: if the surroundings are cooler than the desired internal temperature, a filter fan is often enough. If the surroundings are just as warm or warmer, you need an active compressor cooling unit — and a sufficiently sealed enclosure for it (at least IP 54).
Step 1: determine the power loss
The basis of every sizing is the power loss — the heat given off by all installed components together. Add up the power losses from the data sheets: frequency converters, power supplies, transformers, contactors and controllers in the electrical enclosure; servers, switches, UPS and PDU in the server rack. Estimating is not enough — frequency converters in particular contribute considerably.
This heat load, the enclosure dimensions and the ratio of internal to ambient temperature determine the required cooling capacity (guide value according to IEC 60890). You do not have to calculate this by hand:
→ Calculate cooling & heating capacity with the free online calculator
Two typical applications in detail: the SZE2 PC computer cabinet — suitable for cooling units thanks to IP 54, for production and industry — and server rack cooling for 19″ racks, from roof fans to the CVE cooling unit. The complete range of units with factory fitting is shown in the overview enclosure cooling units & air conditioners.
Step 2: ambient temperature vs. desired internal temperature
This is the most important fork in the whole selection. Cooling without a refrigeration circuit (fans, heat exchangers) can only cool the enclosure down as far as the surroundings allow — it “pushes” the heat outside. A compressor cooling unit, on the other hand, actively generates cold and keeps the internal temperature independent of the surroundings.
- Surroundings considerably cooler than the target internal temperature → a solution without refrigeration technology (filter fan or heat exchanger) is usually sufficient.
- Surroundings just as warm or warmer than the target internal temperature → only an active compressor cooling unit can hold the temperature.
Step 3: choose the right type of cooling
The type of cooling follows from the heat load, the temperature ratio and the ambient conditions (dust, humidity, oil, outdoor installation). The table shows when each solution is suitable — together with the matching ZPAS component:
| Type of cooling | When suitable | ZPAS component |
|---|---|---|
| Filter fan | Surroundings cooler than the target internal temperature, clean ambient air, low to medium power loss. Inexpensive and low-maintenance. | LV series: 15–930 m³/h, IP 54 |
| Air/air heat exchanger | Surroundings still cooler than inside, but dusty/humid — no outside air may enter the enclosure. Separates the internal and external air circuits. | Wall-mounted or roof-mounted unit, IP 54 |
| Compressor cooling unit | Surroundings just as warm or warmer than inside, high power loss. Keeps the internal temperature constant. Requires an enclosure of IP 54 or higher. | Side-mounted unit CVE (310–5,800 W), roof-mounted unit ETE (330–2,700 W) |
| Air/water heat exchanger | Very high heat loads and an existing cooling water circuit (e.g. data centre, machine cooling). | On request / custom build |
| Outdoor air conditioner / heat pump | Outdoor installation with sun and frost — cools in summer, heats in winter, prevents condensation. | CVO (550–5,800 W), AHP 7.0 (−15 to +50 °C) |
How the degree of protection and the installation site are related is explained in the guide IP ratings explained. For the calculation alone, including a recommended solution, use the cooling capacity calculator in the overheating guide.
Filter fan, heat exchanger or cooling unit — a direct comparison
| Criterion | Filter fan (LV series) | Air/air heat exchanger | Compressor cooling unit (CVE/ETE/CVO) |
|---|---|---|---|
| Capacity range | 15–930 m³/h air flow | on request | 310–5,800 W cooling capacity |
| Ambient requirement | considerably cooler than the target internal temperature, clean air | cooler than inside — dust/humidity permissible | also just as warm or warmer than inside |
| IP rating maintained | up to IP 54 (with filter insert) | maintained — separate air circuits | maintained — enclosure of IP 54 or higher required |
| Energy demand | very low (fan only) | low | higher (compressor operation) |
| Costs (rough) | € | €€ | €€€ |
Compressor cooling unit: capacity, limits and waste heat
A compressor cooling unit (refrigeration unit based on the direct expansion principle) is the solution when the surroundings are too warm for passive cooling. Two points are decisive in the selection:
1. The enclosure must be sealed. The unit cools the air inside the closed enclosure. If the enclosure is leaky, the cold air escapes, warm and humid ambient air enters, condensation forms and the cooling capacity is wasted. The rule is therefore: enclosure of at least IP 54.
2. The waste heat has to go somewhere. As a rough rule of thumb, up to about 6 kW per enclosure can usually be covered without problems by a single compressor cooling unit. More powerful units exist and can be used — the real challenge then lies not in the unit itself but in two side effects:
- Heating of the room: a cooling unit does not “destroy” heat, it pumps it — plus its own electrical power — out of the enclosure into the installation room. With high cooling capacity or several enclosures, a small or closed technical room heats up noticeably. Room ventilation or room air conditioning must then be planned in.
- Airflow inside the enclosure: at high capacity, the cold air must be directed specifically to where the heat is generated. Poor airflow leads to hot spots, even though the unit is sufficient on paper. Cable management, air baffles and the arrangement of the components are therefore part of the design.
In other words: the limit is not the unit capacity, but the overall concept of enclosure, room and airflow. That is exactly what we clarify together with you at ZPAS.
Server rack or electrical enclosure — differences in cooling
The selection principle is the same, but practice differs: a server rack in an air-conditioned server room usually works with perforated doors and cold aisle/hot aisle routing — the room air conditioning does the cooling, the rack only ensures the airflow. Only at very high density (GPU/AI racks) are in-row heat exchangers and precision cooling added. An electrical enclosure in a hall or outdoors, by contrast, is usually closed (IP 54/55) and carries its cooling unit directly on or in the enclosure.
Find out more: cooling for server racks · data centre and server room cooling · server racks for AI & GPU.
Common mistakes in selection
- Power loss estimated instead of added up — the total from the data sheets is a must.
- Calculated with the average instead of the maximum temperature — what counts is the hottest summer day at the installation site.
- Cooling unit on a leaky enclosure — without IP 54, a compressor unit works against the ambient air that gets in.
- Waste heat into the room forgotten — especially in small technical rooms with several enclosures.
- Reserve forgotten — plan a 20–25 % margin for expansion and filter clogging.
Frequently asked questions — choosing a cooling unit
Filter fan or cooling unit — which is better?
That depends on the ambient temperature. If the ambient air is cooler than the desired internal temperature and sufficiently clean, a filter fan is usually enough — inexpensive and low-maintenance. If the surroundings are just as warm or warmer, a fan can no longer remove the heat; an active compressor cooling unit is then needed, which cools independently of the outside temperature.
When do I need a compressor cooling unit?
As soon as the ambient temperature is equal to or higher than the desired internal temperature — or when the power loss is so high that ventilation and heat exchangers can no longer remove it. A compressor cooling unit keeps the internal temperature at a defined value regardless of the surroundings. The cooling capacity calculator gives you the exact threshold.
Why must the enclosure have at least IP 54 for a cooling unit?
A compressor cooling unit cools the air inside the closed enclosure. To prevent the cooled air from escaping and warm, humid ambient air from entering, the enclosure must be sufficiently sealed — at least IP 54. In leaky enclosures, condensation forms and cooling capacity is lost.
Can I use a cooling unit with more than 6 kW?
Yes. Compressor cooling units well above 6 kW exist and can be used. The challenges then lie elsewhere: the waste heat is released into the installation room and can heat it up too much — room ventilation or air conditioning is then needed. In addition, the airflow inside the enclosure must ensure that the cold air is directed specifically to the hot components.
How ZPAS finds the right cooling unit for you
You do not have to do the sizing on your own. First calculate the required capacity in the cooling & heating capacity calculator, select the climate control components directly in the electrical enclosure configurator or server rack configurator — or tell us the power loss, ambient temperature and installation site, and we will check the sizing free of charge.
Not sure which cooling unit is right?
Tell us the power loss, ambient temperature and installation site — you will receive a specific recommendation and a quotation within 2 working days at most. No obligation.
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