VODA KAO RASHLADNI FLUID WATER AS REFRIGERANT

Transcription

1 VODA KAO RASHLADNI FLUID WATER AS REFRIGERANT Alexander Cohr PACHAI Sabroe Factory, Johnson Controls Christian X s Vej 201, 8270 Hoejbjerg, Denmark alexander.c.pachai@jci.com Mi koji smatramo činjenicom da je voda jedan od najčešće korišćenih sekundarnih rashladnih fluida, u poređenju sa vazduhom, nismo u većini. Radi upotrebe ispod tačke smrzavanja mi često dodajemo neko sredstvo za ublažavanje tačke smrzavanja, kao što su glikol ili razne soli. Voda je ranije korišćena u apsorpcionim sistemima kao jedan deo radnog para, dok su amonijak ili litijum-bromid bili drugi par. Ovaj rad će razmatrati upotrebu vode kao primarnog rashladnog fluida u kompresionom sistemu. Voda kao rashladni fluid nije novost, ali ju je većina kompanija godinama ignorisala kao primarni rashladni fluid. Nova tehnološka unapređenja i povećan pritisak na konvencionalne rashladne fluide na bazi fluora, zbog briga o klimatskim promenama, na amonijak zbog toksičnosti i zapaljivosti i na ugljovodonike zbog visoke zapaljivosti znače da je danas fokus pomeren na ugljen-dioksid i na vodu i vazduh zbog izuzetno niskih temperatura. Voda kao rashladni fluid u čistom obliku pogodna je samo za temperature iznad tačke smrzavanja vode, ili nekoliko stepeni iznad. Za specijalnu primenu, na primer kada se morska voda koristi kao toplotni izvor za proizvodnju ledene kaše zimi, to je izuzetak od opšteg pravila. Tehnologija ledene kaše može se primeniti za klimatizaciju, hlađenje u rudniku i prelazno hlađenje/smrzavanje hrane, npr. ribe. Ključne reči: voda; rashladno sredstvo; primena; kompresor; isparivač Not many of us consider the fact that one of the most commonly used secondary refrigerants is water, in competition with air. For use below the freezing point, we often add some freezing point suppressant, such as glycol or various salts. Water has previously been used in absorption systems as one part of the working pair, with ammonia or lithium bromide as the other. This paper will discuss the use of water as a primary refrigerant in a compression system. Water as refrigerant is not new, but the majority of companies have neglected it as a primary refrigerant for many years. New advances in technology and the increased pressure on conventional fluorine-based refrigerants due to climate concerns, on ammonia due to toxicity and flammability and on hydrocarbons due to high flammability, mean that the focus is now more on carbon dioxide, and on water and air for extremely low temperatures. Water as a refrigerant in its pure form is only for temperatures above the water freezing point, or some degrees over. For special applications, for example when 13

2 using seawater as a heat source to produce ice slurry in the winter, this is an exemption from the general rule. The ice slurry technology can be applied for air conditioning, mine cooling and for intermediate cooling/freezing of foods, such as fish. The advantages of water as refrigerant are many, including toxicity and flammability not being issues. The environmental benefits are evident, as it is not causing ozone depletion or global warming. Water is available in large parts of the world. In some parts, seawater can be used, and the technology can even help produce potable water as a side product. This enhances the benefits of the system. Even water that might have some micro algae can be used, because the temperatures can be as high as 150 C in the compressor, which will kill any bacteria. Key words: water; refrigerant; application; compressor; evaporator Introduction Water has a very high latent heat, higher than any other refrigerant: 2257 kj/kg, as indicated in Table 1. kj/kg Table 1 Properties of selected refrigerants Critical temperature C Critical pressure bar Atmospheric boiling point C Molar mass M kg/kmol Water Ammonia CO Propane ,097 R R134a R R600a Table 1 also shows that the critical temperature and critical pressure are higher than for other refrigerants. The atmospheric boiling point is 100 C under normal conditions. This indicates that, in the future, we may see heat pumps working well over the conditions we see today. Figure 1 shows how the pressure increases for water as the saturation temperature increases. Figure 2 shows the density of water vapour as the temperature increases. When the temperature is low, you will use a high swept volume. For higher temperatures, you will be able to change technology. When reaching temperatures around 350 C, the density is about the same as for other refrigerants often used with radial compressors. Figure 3 shows that the volume decreases drastically until about 60 C, and then the curve flattens out. If you use an axial compressor for the first stage, you 14

3 could continue with a radial compressor in two or more stages. For water recompression or heat recovery, a heat pump is a good solution. Water is one of the very few refrigerants that can be used at temperatures above 160 C, potentially going up to 360 C, or maybe even higher? Pressure Pressure [bar] Temperature [ C] Figure 1. The X axis is the saturation temperature, and the Y axis is the pressure for water (Refprop 9.1) Density [kg/m3] Vapor Density [kg/m3] Temperature [ C] Figure 2. Vapour density of water, a parameter used when selecting compressor technology (Refprop 9.1). Vapor volume (m³/kg) Volume [m3/kg] Temperature [ C] Figure 3. Vapour volume of water vapour: high volume at low temperatures, and low at high temperatures (Refprop 9.1). 15

4 Currently, a chiller unit is being field-tested in a factory where it will be operating for a year to verify that the concept can operate automatically for many hours under actual production conditions. The preliminary test results are promising. The concept shows that it is possible to achieve the same efficiency as an R134a chiller under the same conditions. The unit has a cooling capacity about 850 kw@9/16 C under Danish weather conditions. The compressor and the concept The compressor is of an axial type. It is specifically designed to work with water as refrigerant, and it is using water for the required lubrication. The compressor is sensitive to water droplets, and it is therefore very important to have proper droplet separation before the vapour reaches the compressor impellers; otherwise, there will be erosion, especially on the first impeller. One might think that the compressor from a jet engine could be used, but they are designed for air compression meaning that they are only tolerant to water entering the impellers, but not optimised for it. Besides, the materials used in a jet engine are more expensive than would be acceptable in a refrigeration compressor. Figure 4. Compressor suction side. Figure 5. The unit. The vessel to the right/far end is the evaporator vessel. Figure 6. Boiling in the water. The vapour that is boiled away represents about 1% of the water. There is not a big difference between a normal chiller circuit and the water vapour system. In principle, the water vapour system is more simple than a system with conventional refrigerants because there are less components. Figure 7 shows a blue evaporator and a red condenser. In the evaporator, about 1% of the circulated water is evaporated. The vapour travels through the compressor and enters the red condenser. There is a negative pressure in both the evaporator and the condenser, but in the condenser, it is slightly higher to reflect the condensing temperature. The water then runs to the cooling tower where it evaporates to the surroundings to release the heat. All top-up water resulting from this way of cooling is now entered on the low side of the system instead of the cooling tower, as is normally the case. This, however, requires a possibility to drain what may collect 16

5 at the bottom of the evaporator. Through a sight glass in the vessel, you can see the water boiling in the surface (Figure 6), but most is boiling from the falling water. Figure 7. In principle, the water vapour system is very simple. The unit is relatively compact. The field test unit (Figure 5) is equipped with many measure points, which will not be on the commercialised model. There are many nice-to-know things, which will not be needed on the final product. It is important that water droplets do not get in contact with the impellers, Figure 4. With the speed (14,500 rpm) and the pressure, there will be cavitation and material erosion, and the damage shows very quickly. The droplets can be prevented in various ways, but there is not room for any significant pressure drop as the pressure is already very low. The water returning from the cooling tower is saturated with air, and this air must be removed to keep the pressure down in the condenser. Otherwise, the condensing pressure will be pushed up like in any other refrigeration circuit. Figure 6 shows boiling in the water in the lower part of the vessel. It is harder to see the boiling that takes place in the drops injected at the top of the evaporator. When the water enters the evaporator (Figure 8), it expands before it goes through a pressure reduction that again makes the droplets smaller. In this way, we get a very large surface enabling heat to boil out and cool the water. Between the falling droplets (Figure 8) and the compressor suction opening, there is a droplet separation filter. This is to avoid droplets reaching the impeller. 17

6 Figure 8. The first water entering the evaporator. There are many ways to further increase the surface of the water with smaller drops. Capacity and other details Table 2. Test data for water vapour unit at the design point for the demonstration site. Cooling capacity including lubricant cooling kw Compressor shaft power kw Power input to compressor and vacuum system 153,3 kw Total power input to unit including water pumps, etc kw Evaporator inlet (plate heat exchanger in) 12 (20) Evaporator out, Te (plate heat exchanger out) 7.5 (9.2) C Condenser in 25.5 C Condenser out, Tc 31.2 C EER (Power input to compressor and vacuum system) 5.29 The cooling capacity of the test unit is approximately 800 kw at 12/7 C (in/out temperatures) (Table 2) i. At full speed, the compressor rotates at 14,500 rpm. The electric motor is water-cooled with water from the warm side. Condensation in 18

7 the motor must be avoided by keeping up the temperature of the cooling water on the pump. Notice in Table 2 that the vacuum pumps are included in the absorbed power consumption. Total absorbed power includes water pumps, which is unusual when discussing chiller COP/EER. The EER value here includes the compressor power and the vacuum system. Table 1. Comparison between the water vapour unit and a standard propane chiller. H 2 O prototype industrial chiller Evaporator Open Closed Closed Condenser Closed (dry cooler) Closed (dry cooler) To in To out Tc in Tc out SABlight Propane chiller Closed (dry cooler) Cooling capacity 1170 kw 1030 kw 477 kw Power input inverter, vacuum, etc. COP (compressor, vacuum, etc.) *) Redesigned thrust bearing 240 kw 229 kw 123 kw 4.88 (5.27*) 4.50 (4.84*) Table 3 compares two types of water vapour with a standard propane chiller under the same working conditions. The capacity is quite different, but this is not relevant when discussing COP/EER. The table shows that the water vapour system is approximately 14% better than the propane system at worst-case, and about 20% better at best-case scenarios. During the field test, we found that some modifications of the thrust bearing could increase efficiency to 20% and 23% respectively. Naturally, these modifications will be implemented in the final commercial version. The water vapour chiller can be configured in various ways, which will result in different efficiency levels. In many air conditioning applications, the trend is to increase the water temperatures from the current 12/7 C water to 18/13, or, in some cases, higher. This is the main difference between refrigeration for food and vegetables and air conditioning. Table 4 shows systems in different configurations and at different running conditions. As for any other chiller, regardless of the refrigerant, you will notice that the lower the condensing temperature and higher evaporating temperatures, the higher COP/EER. The sub-conclusion is that water is just another refrigerant

8 Table 4: Chiller unit performance at various system configurations and temperature level Evaporator Open Closed Open Closed Open Closed Condenser Open (cooling tower) Open (cooling tower) Closed (dry cooler) Closed (dry cooler) Open To in To out Tc in Tc out Open Cooling capacity 1170 kw 1030 kw 1170 kw 1030 kw 820 kw 715 kw Compressor shaft power 115 kw 115 kw 220 kw 210 kw 155 kw 150 kw Power input inverter, vacuum etc. 127 kw 127 kw 240 kw 229 kw 170 kw 164 kw Total power input 160 kw 160 kw 272 kw 262 kw 203 kw 197 kw COP (compressor, vacuum, etc.) COP (total input) COP* (re-design thrust bearing) Other applications Another example could be an application using water from either a river or open sea. In many countries, water is plentiful and, even though it may not be immediately potable, it is still useful as a heat source. The advantage of using a water vapour compressor is that the condensed water can be used as potable water after adding salt to it. This is important to remember, because the water is both deaerated and desalinated. During the compression process, it is possible to reach a temperature well over the 120 C required to effectively kill bacteria, and this is therefore a way to ensure potable water while having a never-ending heat source. This helps to make a better business case for the system. 20

9 Figure 9. For a district heating system in Denmark, seawater is used as heat source. During cold winters, the water will produce ice slurry. The condensing takes place in a heat exchanger connected to an ammonia heat pump, which can compress the gas to a higher leaving temperature. The trend for district heating in Denmark is moving towards 60 C. Conclusion and further work After many years of research and development, the concept of a water chiller, using water as refrigerant and lubricant, is close to reaching market readiness. This does not stop here. There are many other areas in which water can make an excellent replacement for high GWP refrigerants. To begin with, the obvious main areas to focus on are cooling of processes in the industrial segment and air conditioning systems in high ambient temperature regions, where you can benefit from the thermodynamic properties of water. This technology opens up to a wealth of opportunities in other applications than the ones described in this paper. The future as a refrigerant has only just begun for water, and there is a lot of work to do in the years to come, both for onecompressor systems and for systems with two or more compressors in serial connection. i Madsboell, H., Pachai, A.C.: First field-test results from a full-scale industrial water vapour chiller. 12 th Gustav Lorentzen 2016 conference, Edinburgh, August

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