How tonnage is calculated

Cooling capacity is the product of refrigerant weight flow times the enthalpy change across the evaporator, or refrigerating effect per pound. Refrigerating effect is usually taken as the difference between suction gas and liquid enthalpies:

Cooling Capacity = Flow x Refrigerating Effect

The flow of refrigerant is proportional to the square root of pressure drop and refrigerant density, times the valve effective flow area. In equation form:

Flow = Density x Pressure Drop x Flow Area

The above relation always holds for liquid and also for gas, as long as the pressure drop doesn’t exceed about 10% of the pressure at the valve inlet.

For gas, when the pressure drop across the valve is high in relation to inlet pressure, a more complex calculation of flow is necessary due to compressibility effects. All of these calculations are detailed in ARI Standards 750, 760, and 770.

It will be seen that a valve produces more tonnage on R-410A primarily because more pressure drop is available. Also, on suction-side applications the gas is denser.

Expansion valves

The ratio of R-410A to R-22 capacity for a given physical size of expansion valve is easily calculated by the above methods, if one decides what pressure drops to use for each of the refrigerants.

The basic assumption is that condensing and evaporating temperatures are unchanged from R-22. With this in mind, and making allowance for coil and distributor pressure drop, the valve pressure drops shown by Figure 1 will probably be adopted by the industry as standard rating points.

As Figure 1 shows, a given size valve will produce about 20% higher tonnage using R-410A. Looking at it another way, a valve can produce the same tonnage on R-410A with about 83% of the flow area of the R-22 valve.

The table also shows that the size factor is about the same at all evaporator temperatures.

Liquid-line solenoid valves

It is common practice to select liquid solenoids for a pressure drop equivalent to a 1° change in saturation temperature. For R-22 at 100°F, this is about 3 psi.

If the same criterion is used for 410A, that is a 1° loss in subcooling; the valve would be sized for about 4.75-psi pressure drop.

When tonnage is calculated as illustrated earlier, the same valve will produce about 20% more tonnage on 410A. Again, looking at it another way, it would require only about 83% of the flow area as for R-22.

As a practical matter, some people may feel more comfortable sizing liquid solenoids for the same pressure drops they have traditionally used for R-22. If this were done, the valve would actually require about 4% more flow area than for R-22 to produce the required tonnage.

This is because the density of 410A liquid is significantly less than R-22, and other things being equal, mass flow is proportional to the square root of liquid density. Figure 2 (page 8) illustrates the influence of refrigerant type on capacity.

Reversing valves and other suction solenoids

The suction-line valve situation is a little more complicated, since suction pressure drop has such a great influence on system efficiency and capacity. Reversing valves fall into this category. Suction line pressure drop is the primary sizing criteria for reversing valves. The following will explore relative flow passage areas needed for R-410A versus -22 for two scenarios. The first is that pressure drop is maintained at 2 psi for both R-410A and -22 at conditions of 120° liquid and 45° evaporating, with 10° superheat. These are the conditions at which the manufacturers now rate reversing valves.

The second scenario is that a higher pressure drop is allowed for 410A, so that the same system losses are taken.

Theoretical calculations reveal that at the above high-temperature cooling conditions, a 2-psi suction pressure drop produces about 2% loss of efficiency and 2.4% loss of capacity in an R-22 system. But in an R-410A system, the suction pressure drop can be allowed to go as high as 3.1 psi before the same losses in performance are seen. The R-410A system is simply not as sensitive to pressure drop.

Figure 3 shows the suction capacity of a reversing valve when applied to an R-410A system plotted versus the capacity of the same valve when applied to an R-22 system. There are two lines on the figure. One shows the result if a valve is selected for the same pressure drop, and the other if it is selected to produce the same system losses. The following example will show how to interpret the figure.

Assume that a valve is needed for a 4-ton, 410A system and that the same system losses are to be accepted as for an R-22 system with a 2-psi pressure drop. Read across from 4 tons on the vertical scale to the “same loss” line, then downward to read the equivalent tonnage for an R-22 rated valve. This says that a valve of the physical size to provide 2.8 tons on R-22 will give 4 tons on R-410A at the same loss, due to suction pressure drop.

At the cooling conditions noted above, a valve for R-410A requires only 88% of the flow passage area if the same pressure drop is to be maintained, and only 71% of the flow passage area if the same system performance decrement is to be accepted.

These factors are approximately the same for heating performance at 47° outdoor conditions. For heating, flow area need be only 84% for the same pressure drop and 68% for the same performance loss.

Liquid line filter-driers

It was important that driers be included in this article, even though they are not valves. These are the only components that will actually need to be larger.

Pressure drop and flow capacity are not the issue. R-410A can hold about 50% more water than R-22, and POE lubricants can hold about 10 times the water of mineral oil. For this reason, a drier should be selected that has about twice the water capacity than would be chosen for R-22. With fittings that match the line size, pressure drop should be acceptable.

In summary, liquid solenoids and expansion valves can, typically, be downsized to the next smaller port size within a valve family. This usually won’t make a difference in external dimensions, since the same family or design series will be used.

Downsizing of reversing valves can be more dramatic, particularly if a higher pressure drop is allowed. In many cases a smaller body size can be used, greatly reducing the dimensional envelope.

It must be emphasized that components suitable for R-410A service are different in many ways:

• A higher maximum working pressure rating is required.

• Solenoids require a higher rated operating pressure differential.

• Expansion valves require a different charge.

• And material changes may be needed for compatibility.

For these reasons, never apply a component to an R-410A system unless the manufacturer recommends it. The sizing discussion in this article is for general information, and to provide insight into future system design.

Use only components rated for R-410A, and size them from the product information provided by the manufacturer.