How do water-to-water and water-to-air heat exchangers compare?
Medium and high power variable frequency drives (VFD) predominantly use liquid cooling. For such purpose either a water-to-water or water-to-air heat exchanger is used as part of the cooling system. There is sometimes a bit of confusion about how the heat exchanger looks like, where it is located, what temperature ranges to be expected and so on. Hopefully this blog post sheds some light into the topic.
VFD internal cooling circuit
Liquid cooling is very efficient and therefore used as a preferred cooling method in the higher power range. Liquid cooled medium voltage VFDs usually have an internal cooling circuit. The cooling liquid is either a deionized¹ water or a mixture of deionized water with glycol based antifreeze. The cooling medium must have low conductivity (typically below 0.3 μS/cm). For this purpose, the water or water-glycol mixture is treated with a deionizer. The internal circuit shall maintain the values of pressure and flow rate within the design limits. Internal cooling pumps are used. Those pumps can be either fix speed with constant flow or utilizing variable speed. The later option helps to optimize the internal power consumption at partial load.
¹ demineralized, purified
The cooling medium must have low conductivity (typically below 0.3 μS/cm). For this purpose, the water or water-glycol mixture is treated with a deionizer. It continuously purifies the water in the internal circuit and maintains the conductivity level in the specified range.
Types of heat exchanger
The internal cooling circuit transports the heat from the source towards outside of the VFD and is coupled to a heat exchanger. Such heat exchanger is principally either of a water-to-water or water-to-air type. The choice is usually quite simple: If there is a cooling water available on site then water-to-water heat exchanger is used. In the absence of cooling water, a water-to-air heat exchanger is selected.
A) Water-to-water heat exchanger
This heat exchanger couples the internal water circuit in the VFD with the plant cooling water circuit. It is characterized by high compactness and consequent small footprint. The heat exchanger is mostly part of the cooling unit. Apart from the heat exchanger it contains cooling pump(s), valves and pipes, expansion vessel, deionizer or various sensors to supervise the pressure, temperature or conductivity. The heat exchanger itself is mostly of a plate heat exchanger (PHE) type. Tubular heat exchanger is principally possible as well but rather seldom and typically not seen in variable speed rive systems.
PHE consists of relatively few components: multiple plates, gaskets, carrying bar, guiding bar, clamping (tightening) bolts, fixed cover (frame plate) and movable cover (pressure plate). When the plates are compressed together, they form a plate stack. Guiding bar ensures proper alignment. Inlets and outlets are attached to the fixed plate only. The hot and cold flow medium are separated by gaskets and plates and they do not mix. In the VFD, hot medium represents the internal cooling liquid while cold medium is the external (raw) cooling water. Due to the proximity of internal and external water, heat is exchanged between them.
The solution with water-to-water heat exchanger is also called “double loop”. That is because there are two water circuits: the internal pure (deionized) water circuit cooling directly the power electronics and other VFD components and the external raw water circuit that is typically part of a larger water cooling system on site.
The site water is mostly a water of industrial quality. As long as it is not corrosive, a stainless steel heat exchanger can be used. In case of “aggressive” water (sea water, water containing aggressive chemical substances) a special material of the heat exchanger might be required, such as e.g. titanium. The external water circuit can also be called raw water in contrast to the pure water in the VFD internal cooling loop.
Further in this post we will abbreviate the water-to-water heat exchanger as W/W HEX.
B) Water-to-air heat exchanger
High power VFDs are almost exclusively liquid cooled. But what if there is no cooling water on site? That is not a problem. In fact, it is quite common to use a re-cooler with water-to-air heat exchanger. Such type of cooler is known under multiple names such as:
- Air cooled cooler
- Dry cooler
- Fin-fan cooler
Dry coolers are heat exchangers that use air to transfer the heat into the atmosphere. The principle is that the internal cooling water is cooled down by using an ambient air that is blown through the cooling fins. The fans direct the air flow through the heat exchanger. The air temperature must be cooler than the internal water.
The shape of the dry cooler is either a “table-shape” or a “V-shape”. Table shaped cooler stands on the legs. The height of the lengths shall be sufficient to allow unrestricted air flow and to consider other ambient conditions, such as e.g. snow height during winter period (if applicable).
Nowadays the cooling fans are mostly speed controlled, so called EC fans (electronically commutated). A common feature is n+1 fan redundancy. It means that there is an additional cooling fan. Thus, full cooling capacity can be maintained even after a failure of one of the fans.
Depending on the location of the cooling fans we distinguish:
- Induced draft
- Forced draft
They come with their specific pros and cons.
The size of the dry cooler always depends on the combination of cooling capacity and delta T (difference between air inlet temperature and water outlet temperature).
Table 1: Technical data of selected dry coolers
The solution with water-to-air heat exchanger is also called “single loop”. That is because there is only one cooling water loop.
Further in this post we will abbreviate the water-to-air heat exchanger as W/A HEX.
Comparison of the heat exchangers
Both types of heat exchangers can be compared based on multiple criteria.
1. Footprint
W/W HEX is very compact due to high thermal capacity of cooling liquid and corresponding relatively small volumetric flow.
W/A HEX is significantly larger. The volume of the cooling air to be blown through the heat exchanger is quite high. The speed of the cooling fans and their diameter is limited in order to achieve low noise level. Most air-cooled coolers have a modular design where the number of modules is scaled with the required cooling capacity and temperature difference between air inlet temperature and water outlet temperature (the smaller the temperature difference the larger the cooling surface).
2. Location and installation
W/W HEX is mostly located indoors. Many VFDs have the water-cooling unit directly built inside the VFD enclosure on a common base frame. Alternatively, the cooling unit can be placed as stand-alone unit (typically IP00 execution), mounted on a skid. The cooling unit basically includes all relevant components (heat exchangers, pumps, internal piping and valves, filter, deionizer, expansion vessel, sensors etc).
W/A HEX is part of the dry cooler. As it uses ambient air, W/A HEX is normally located outdoors. The installation can be on the ground or optionally on the roof. Another option is to install the cooler on the wall. The cooler can be installed in a horizontal position with vertical air flow (on the ground / on the roof) or in a vertical position with horizontal air flow (e.g. on the ground or mounted on a wall).
3. Anti-freeze additive
Whether an anti-freeze additive (glycol) is required in the internal circuit or not depends on the minimum temperature that can occur during operation. W/W HEX is mostly located indoor in the electric room. Such environment is normally non-freezing. Therefore, glycol is not required (unless small amount is added for other purpose, e.g. to prevent an organic growth). In contrast, W/A HEX is located outdoors. It means that the pipes with internal water are partially located outdoors. Since the outdoor temperature in many regions drops below 0°C throughout the year, some portion of glycol is required.
Other criteria for comparison, such as internal volume of the liquid, pump rating, control method or investment cost are available in the full article for premium subscribers.
Table 2: Comparison of W/W HEX and W/A HEX in liquid cooled drives
Summary
Liquid cooled VFDs use either water-to-water or water-to-air heat exchangers. Both solutions are state of the art, being used since decades. W/W HEX is a clear choice when suitable cooling water is available on site. W/A HEX is a complementary solution when site water is not available. The internal cooling water circuit is practically identical for both cases. When using water-to-air cooling there is an additional component in the system: dry cooler with W/A HEX, cooling fans and control.
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/
