To achieve proper refrigerant distribution, the liquid portion of the mixture must be dispersed equally to each evaporator circuit. This task is accomplished by maintaining a homogeneous mixture of liquid and vapor from the thermal expansion valve to the evaporator circuits.
The mixture is uniform in composition when it exits the thermal expansion valve. It then enters the distributor nozzle, where the velocity of the refrigerant is increased so that the liquid and vapor components can be mixed. The nozzle is positioned so that the flow is focused directly on the dispersion cone, which equally divides the mixture into evenly spaced passages around the cone. The distributor tubes then transport the refrigerant to each evaporator circuit. The various components of a distributor assembly are shown in figure 1.
The distributor, distributor tube and nozzle size are key parameters in ensuring proper refrigerant distribution. Three principles - all related to pressure drop - explain why these components are so important:
- The pressure drop across the nozzle focuses the flow and
provides the necessary liquid-vapor mixture.
- The pressure drop across the distributor creates
the high velocity necessary to distribute the refrigerant flow
- The pressure drop across the distributor tubes assists in balancing the flow as it enters the evaporator.
Unobstructed airflow also is critical to proper evaporator performance. Because the distributor meters an equal amount of the homogeneous refrigerant mixture to each evaporator circuit, the heat load of each circuit also must be equal. If the heat load is uneven, the lightest load circuit will have the lowest outlet superheat and will affect control of the expansion valve feed. At the same time, heavier-loaded circuits will be starved for refrigerant, an occurrence that will reduce the evaporator’s rated capacity and performance significantly.
However, perhaps the most important parameter in the distributor assembly is the nozzle size. The distributor performance is linked directly to the performance of the compressor, expansion valve and evaporator. An incorrectly sized nozzle will cause poor distribution, contributing to expansion valve hunting (alternate overfeeding and starving of refrigerant flow to the evaporator) that can cause occasional liquid flood-back to the compressor. This situation, in turn, reduces evaporator capacity and also might cause higher operating temperatures, ice buildup and uneven frost patterns.
Effects of Liquid TemperatureNozzles and expansion valves typically are sized for the warmest liquid temperature possible. Normal parameters are 115°F (46°C) condensing and 95°F (35°C) liquid. However, a change in ambient conditions can cause a significant swing in liquid temperature. Liquid subcooling, in which the refrigerant is cooled below the saturation point at a given pressure, can help minimize an undesirable temperature rise. The three methods through which liquid subcooling can occur are mechanical subcooling, subcooling circuits and heat exchangers, and ambient subcooling.
Mechanical Subcooling.Usually found on rack refrigeration systems where the medium-temperature suction group also serves a liquid heat exchanger to increase the capacity of the low-temperature suction group, mechanical subcooling provides a constant subcooled liquid temperature.
Subcooling Circuits and Heat Exchangers.A section of the air-cooled condenser known as the subcooling circuit can be used to cool the liquid refrigerant below the saturation temperature. In some cases, a liquid-to-vapor heat exchanger mounted at the evaporator’s outlet is used to accomplish this task.
Ambient Subcooling.Ambient subcooling usually is unintentional and involves a liquid temperature drop due to exposure to the elements or air-conditioned space. It can be eliminated by insulating any liquid lines that run through air-conditioned areas or by eliminating exposure to cold ambient conditions.
Regardless of how subcooling is achieved, it is important to note that a subcooled 50°F (10°C) liquid requires a lower mass flow rate (the movement of mass per unit time) than a 90°F (32°C) liquid to absorb the same amount of heat (energy).
To understand mass flow, it is important to understand the relationship between mass, temperature and pressure. Refrigerants roughly follow the perfect gas law, which states that the pressure, temperature and volume of a gas are interrelated:
PV = nRT
P is pressure
V is volume
n is the number of molecules or mass expressed in moles
R is the ideal gas constant
T is temperature.
The perfect gas law defines the volume a mass of a specific gas will occupy at defined conditions. For example, a fixed mass of a certain gas occupies a one-liter container at room temperature at sea level. If the pressure is increased while the temperature is held constant, the volume will decrease, resulting in a slight compression of the gas. If the temperature is increased with the volume held constant, the pressure will increase. The only constant variable is the mass of the gas; it does not change. This concept also is evident in elevation changes.
Basic thermodynamic principles explain why the nominal capacity of a nozzle increases as liquid temperature decreases. Enthalpy, also known as heat content, is a term of measurement for how much heat a gas or liquid can hold, and for how much heat is needed to change the temperature. A gas can have the same heat content at different temperatures, although other properties such as pressure and volume will vary, according to the perfect gas law. Thus, subcooled liquid has a greater capacity to absorb heat, and changes in enthalpy and mass flow rate also affect capacity.
Refrigerant-specific nozzles supplied with each evaporator coil typically are sized for a maximum 15°F (-9°C) evaporator temperature difference (TD) and a 95°F (35°C) liquid temperature. However, if mechanical subcooling is used, a different nozzle and expansion valve will be required to ensure proper refrigerant distribution
Selecting a NozzleNozzle sizing is based on the following information:
- Evaporating temperature.
- BTU per hour (BTU/hr).
- Highest liquid temperature.
Step 2.If you do not have a 100°F (38°C) liquid temperature, you will need to use a correction factor (table 1). Keep in mind the significant effect liquid temperature can have on nozzle sizing.
Step 3.Because the ideal nozzle is sized between 135 and 180 percent of the nominal rating, divide the tonnage by these percentages to find the appropriate nozzle size range. (Note: 135 percent = 1.35; 180 percent = 1.8) Find the proper column on table 2 based on your liquid temperature and refrigerant, and find a value in that column based on the range calculated in step 3. That value will allow you to determine the correct nozzle orifice needed for the application.(See the sidebar “Putting Theory into Practice” [scroll down] for an example of these calculations.)
You should consult the refrigeration system manufacturer’s application engineering department or a representative for distributor nozzle sizing to verify your nozzle selection. However, by following the steps above, you can take the guesswork out of nozzle sizing and optimize the performance of your refrigeration system.
SidebarFollowing are the basic parameters for a sample
Putting Theory into Practice: A Nozzle Sizing Example
- Refrigerant is R22.
- Evaporation temperature is 20°F (-7°C).
Step 2.Use a correction factor (table 1) if the highest liquid temperature is above or below 100°F. In this example, because the highest liquid temperature is 100°F, the correction factor is 1.00.
Corrected tonnage = Nominal tons X Correction factor
Corrected tonnage = 5.58 tons X 1.00
Corrected tonnage = 5.58 tons
Step 3.Divide the tonnage by 135 and 180 percent to find the appropriate nozzle size range.
Find the proper column in table 2 based on the evaporating temperature and refrigerant - in this case, 20°F and R22. Find a value in that column based on the range of 3.1 to 4.13 calculated in step 3. There is only one value in the column that falls into the calculated range. This value is 3.84, which corresponds to a size 4 nozzle orifice.