Blends, Glide and Flooded Evaporators

Systems with a flooded evaporator pose a unique challenge when it comes to retrofitting, but one alternative appears to have a low enough glide to make a retrofit simple enough for the equipment owner.

BY GUS ROLOTTI All images courtesy of Coolgas Inc.

wners of equipment using R-22 are aware of the refrig- erant’s current phaseout. Many have taken measures Oto mitigate supply shortages and equipment downtime by saving R-22 for service by reclaiming it, replacing some of their equipment with systems using HFCs or by using alter- natives to R-22. A combination of these measures makes the basis for a well-thought-out -management plan, which equipment owners should have in place. However, owners of equipment that use flooded evaporators have seen that their options for alternatives are very limited. This article explores some of the issues affecting flooded evaporators, in particular those dealing with blends and glide. Definitions Let’s start by defining the terminology to ensure we are all Figure 1 Flooded evaporator schematic. speaking the same language. Flooded evaporators: Typically used in large applications, used and their quantity in the composition will determine they come in several types and technologies but essentially, the final behavior. For example, when creating a blend to a flooded evaporator is a container for liquid refrigerant. It replace R-22, manufacturers will look for components that on becomes cold because of of the refrigerant as it average have similar pressure-temperature relationships, add a boils off and exits. Bundles of tubes containing water, brine particular component to reduce the flammability of another or other solutions of secondary are submerged in or to improve oil miscibility. Of course, there are many ways this pool of liquid and get cold as a result. The refrigerant to do this, which explains why there are so many replace- and secondary exchange heat, causing a phase ments for R-22. change (refrigerant) or temperature change (secondary coolant), Glide: Since the components of a blend do not chemically but never really come in contact with each other. Figure 1 react with each other, but rather remain more or less inde- shows a schematic of a flooded evaporator. Please note that pendent, when a blended refrigerant goes through a phase only the refrigerant and secondary fluid paths are shown for change at one of the heat exchangers, the composition of the simplicity without controls, oil return lines, etc. and liquid portions will be different from each other, Blends: When two or more single-component refrig- not only at a particular point, but also all along the length of erants (those made up of the same molecules) are mixed the exchanger. In practical terms, the refrigerant will not boil together, the do not chemically react to form a off at a single temperature, as a single-component refrigerant new refrigerant, but rather behave as an average of how the would, but over a range of temperatures. This range is called components would behave. Each individual refrigerant will “glide.” The higher the glide in a blend, the higher the dif- retain its pressure-temperature relationship, boiling point, etc. ferences will be in composition between the vapor and liquid Depending on how close the different components behave, for a given temperature and pressure. Glide plays a significant the blend will more or less closely resemble a single-compo- role on what applications are suitable for a blend and its nent refrigerant. The type of single component refrigerants behavior on different types of evaporators.

14 RSES Journal JULY 2014 www.rsesjournal.com Blends, Glide and Flooded Evaporators

Figure 2 Temperature vs. composition for a blend of two components at constant pressure.

Incidentally, while the glide of a blend depends on tem- temperature of the vapor begins increasing. Keep in mind perature, pressure and composition, the HVACR industry has that when referring to a constant composition, it is the total not settled on a standard condition to report it. This leads to amounts of components A and B and not how much of each misinformation and incorrect comparisons. When comparing component there is in the vapor or liquid phases. the glide of different refrigerants, be sure to compare them at It should be pointed out that for the illustration, a blend similar conditions, especially in the case of higher-glide blends. was chosen with two separate lines that do not cross each Fractionation: Just as the components of a blend can other, which is typical of many refrigerant combinations. be put together, they can also separate or “fractionate.” The However, many other configurations are possible. In one higher the glide of a blend, the greater its tendency is to fractionate. The amount of fractionation depends on the pressures of each component for a given temperature, avail- able volume for vapor and liquid in a tank or system, relative quantities of each phase, turbulence (whether the system is running or at rest) and many other factors. Blends, glide and fractionation In order to visualize the complex phenomena taking place when blends evaporate, we will use a simpler two-component blend made of refrigerants A and B. Adding a third (or a fourth, fifth, etc.) component will simply increase the number ECOLINE VARISPEED of interactions, but in essence will be similar to what is cov- ered in the following paragraphs. In Figure 2, the saturation temperature of the blend has been plotted against the composition for a constant pressure. The x axis shows the amount of component A, therefore at x=0, pure B is at its boiling point, and at x=1, pure A is at its boiling point. The first thing to notice is that there are two lines. The area above the top line, called the “dew-point curve,” repre- sents an area of all vapor. The area below the bottom line, called the “bubble-point curve,” represents an area of all liquid. ECOLINE ECOLINE VARISPEED TRANSCRITICAL CO2 The area between the two lines is the glide area and repre- In and systems, efficiency and environmental protection play an increasingly sents the transition of the components from liquid to vapor. decisive role. Innovative reciprocating from the market leader will prepare you today for the demands of tomorrow. Whether it is natural refrigerants, precise capacity control or optimized If we follow a vertical straight line up (increasing temperature technology, with BITZER you will find the best eco-efficient solution for every application. at a constant composition for the given pressure), the refrig- erant will be all liquid until we reach the bottom line. At that point, the first bubble will form as the refrigerant begins BITZER US, Inc. // www.bitzerus.com // +1 (770) 503-9226 to evaporate. This will continue until the last drop of liquid evaporates when the top line is reached. Beyond that point, Circle Reader Service No. 78 www.rsesjournal.com JULY 2014 RSES Journal 15 the same as the composition at the exit. In other words, if you were to follow a discrete amount of refrigerant, it would enter, evaporate and exit in its totality. While the individual compositions of vapor and liquid might be different from each other along the evaporator path, all the refrigerant will evaporate upon exit. While this would cause evaporator glide, the circulating composition in the system is the intended composition of the refrigerant. In the case of a flooded evaporator, that is not necessarily the case. The mass of liquid refrigerant entering R-407C R-32 R-125 R-134a the evaporator is the same as the mass of vapor leaving 23 25 52 (otherwise the evaporator would be filling up with refrigerant) Nominal 23 25 52 but their compositions are not the same. The reason for this 90% 22.9 24.9 52.1 is fractionation. The vapor phase inside the evaporator Liquid 32.6 31.6 35.9 will be different from the liquid phase and richer in the higher-pressure components. As the suction of the compressor 50% 22.6 24.7 52.7 removes vapor from the evaporator, more and more of the Liquid 32.2 31.4 36.4 higher-pressure components are removed from the liquid and 20.4 23.1 56.5 10% evaporated. This will continue until a balance for the specific Liquid 29.9 30.1 40 conditions of the running system are reached and equilibrium is established. This means that more of the higher-pressure R-434A R-125 R-143a R-134a Isobutane components will be in circulation, and more of the lower- 63.2 18 16 2.8 pressure ones will stay behind in the evaporator. If we were Nominal 63.2 18 16 2.8 to follow a discrete mass of refrigerant, we would see that it would enter the evaporator and separate. A portion of all the 90% 63.2 18 16 2.8 Liquid 68.8 17.7 10.8 2.7 components would evaporate, but the portion of the higher- pressure component evaporating would be larger. 62.9 18 16.3 2.8 50% Figure 3 shows the changes in composition for both the Liquid 68.5 17.8 11 2.7 liquid and vapor phases of the refrigerants from the nominal. 10% 61.1 18 18 2.8 While the compositions are those at static equilibrium, Liquid 67.1 17.9 12.2 2.7 which is not what would be happening inside the flooded Figure 3 These tables show the liquid and vapor compo- evaporator, they illustrate to some degree the changes that sitions for a vessel filled with 90%, 50% and 10% liquid could be expected. Notice also that while some components compared to the nominal composition (due to rounding off only change a small amount, others have a more significant decimals, some numbers may not add up to 100%). change as in R-134a reducing by 12% in R-407C. The problem in this case is that the composition of the possible configuration the two lines could actually cross each circulating refrigerant is not that of the nominal refrigerant other. The crossing point denotes an azeotropic blend for itself. This will cause the system to behave differently since that particular pressure, temperature and composition, and the blend will behave as a single-component refrigerant. The vertical distance between the bubble-point and dew- point lines is in fact the glide of the blend at that particular condition. Notice how much the glide can vary as the con- stant composition line is moved horizontally. Blends and the evaporator Now that the behavior of blends and the construction of a flooded evaporator have been explained, it is time to look at how blends behave in them. When refrigerant passes through a direct-expansion evaporator, all of it will change from liquid to vapor. The Figure 4 Comparison of glide for common R-22 alternatives composition of the blend at the inlet of the evaporator is at the same conditions.

16 RSES Journal JULY 2014 www.rsesjournal.com pressures will be higher, especially at the condenser and the Gus Rolotti is the Vice President of Technical Sales and Marketing system will typically lose capacity and efficiency or experi- for Coolgas Inc., a division of A-Gas Americas. He has worked ence other performance issues. in the HVACR industry on refrigerant-related issues for more than 30 years, now with Coolgas Inc., and previously with Solutions refrigerant and auto A/C manufacturers. He can be reached via Owners of systems with flooded evaporators have few e-mail at [email protected]. For more information, visit options if they want to retrofit their R-22 equipment. www.coolgas.com. Mechanical changes may offer some improvement. In general, when high- glide blends are used, depending on Mastercool is the leading manufacturer of the application and running condi- professional quality A/C manifold gauge sets. Our manifold gauge sets are constructed of tions, they have typically limited suc- the finest materials and machined with cess. The best approach is to use a precision to the highest quality standards. refrigerant with low glide that will is... minimize the issues described above. EVERY MANIFOLD BUILT Figure 4 shows a comparison of the IS B glide of some of the different R-22 BUILT TO LAST alternatives in the market today.

Out of the long list of R-22 alterna- A tive refrigerants, only one has a glide that appears low enough to be used in

• COLOR CODED GAUGES C flooded evaporator systems, R-434A. FOR EASY REFRIGERANT It has been used successfully in many IDENTIFICATION

flooded evaporator retrofits in Europe. K While it is difficult to predict exactly what glide is low enough to operate properly, it is clear that the lower the glide, the better the chances are in T avoiding these problems and achieving

optimal performance. R-434A has a O glide of 3°F, while the closest refrig- • TRUE THERMOCOUPLE erant in the list is at almost double VACUUM SENSOR

that. The most common and best-per- DOWN TO 1 MICRON forming R-22 alternative refrigerants (R-407A and R-407C) have a glide B about three times that value, mak-

ing them clearly unsuitable for the A application. There are other low-glide refrigerants that could be used, such as R-404A or R-507A, but their use S would require significant and costly • FREE FLOATING system changes due to their operating PISTON VALVE conditions. PROVIDES I

MAXIMUM C Summary SEAL Systems with a flooded evaporator pose a unique challenge when it comes to S retrofitting, due to the potential for fractionation. One alternative, R-434A, 1 Aspen Drive • Randolph, NJ 07869 appears to have a low enough glide to w w w . m a s t e r c o o l . c o m make a retrofit simple enough for the equipment owner. Circle Reader Service No. 79 www.rsesjournal.com JULY 2014 RSES Journal 17