Metering Device and the Evaporator

Metering Device and the Evaporator

In previous installments we discussed some characteristics of the refrigerant and how it is converted into its liquid form.  The liquid refrigerant leaves the condenser and flows through a small pipe, called the liquid line, usually through a filter drier and on to the metering device and evaporator. This is where the system will absorb heat from the air passing over it, making the air cooler and drier. 

The compressor pumps refrigerant out of the evaporator, reducing the pressure in the evaporator.  When refrigerant is at a reduced pressure, its saturation temperature is lowered.  The metering device is a restriction to refrigerant flow.  This maintains a pressure difference between the relatively lower pressure evaporator and the relatively higher pressure liquid line.  As the refrigerant is exposed to the lower pressure in the evaporator, it expands and becomes a vapor at the low side saturation temperature, also known as the evaporating temperature. The refrigerant requires energy to evaporate and it gets that energy by taking it from the air passing over the evaporator.  This is what cools the air, and when the evaporating temperature is below the dew-point of the air going over the evaporator, it condenses the water vapor from the air and dries it out some.

The majority of the heat absorbed from the air by the refrigerant happens as the refrigerant changes from the liquid state to its vapor state.  This is called latent heat.  When the water is condensed from the air, it is called latent load.  Latent means that the energy is being used in the process of changing state and there is no temperature change because of latent heat in the refrigerant or latent load in the air.  When a change in temperature happens, it is called “sensible” meaning able to be sensed by touch and measured by a thermometer. All latent processes must be worked out before any sensible work is done. This means that the liquid refrigerant must evaporate before the refrigerant can become warmer than its saturation temperature, or superheated. It also means that once the air is cooled to the dew point, enough water must condense out to lower the dew point before the air will get cooler. 

Air is sometimes very humid and sometimes very dry.  For a given amount of refrigeration capacity, the amount the air is cooled is related to how much water needs to be condensed out of it.  It is why measuring the temperature drop of the air across an evaporator, while a favorite measurement of many technicians, is not diagnostic of the refrigeration capacity or the airflow through the evaporator unless there is some calculation available that considers the humidity of the air.

The metering device

There are two basic kinds of metering devices.

Fixed orifice devices include cap tubes, piston actuators, short orifices, and other devices with a variety of names. They all however are fixed restriction devices.

A thermostatic expansion valve (TxV) and an electronic expansion valve (ExV) are variable restriction devices. These devices actively control superheat by adjusting the mass flow rate of the refrigerant through the device by modulating the size of the orifice.

Mass flow

In order to understand metering devices we must become familiar with mass flow. Mass flow is something many technicians grasp intuitively, but for this discussion we must be explicit. The mass flow rate is the rate at which the refrigerant flows through the system. It is expressed in pounds per minute.

This is important because, from the technician’s standpoint, mass flow can be understood as refrigeration capacity. Engineers define refrigeration-side capacity as the change in enthalpy of the refrigerant multiplied by the mass flow rate. The change in enthalpy of the refrigerant is the difference in the amount of heat contained in a quantity of refrigerant in the liquid line as compared to the amount of heat in the same quantity of refrigerant in the suction line.

Service technicians can simplify the idea for their purposes by assuming that the difference between the enthalpy of the refrigerant in the liquid line and suction line remains relatively constant. A change in the flow of refrigerant through the metering device creates a significant change in the refrigeration capacity of the system.

What determines the refrigerant mass flow rate through a metering device?

There is the size of the orifice, and the difference in pressure between the high side and the low side of the refrigeration cycle or ΔP (Meaning change in pressure). The size of the orifice is a design issue and assuming the system is properly designed and constructed, it is not a service issue. It becomes a service issue when we cannot assume the proper metering device is installed. This is usually not the case in package equipment but in field erected equipment, like split systems, this might be a problem.

From a service standpoint the range of ΔP or the pressure difference across a fixed orifice device accounts for the entire range of mass flow rates experienced by the refrigeration cycle without a flow restriction or compressor fault.

There is a reason why fixed orifice units have very low superheat expectations when it’s hot outside and high superheat expectations when it’s cold outside. When it’s hot outside, the high side pressure often rises and since the low side pressure usually remains essentially constant, the flow of refrigerant through the metering device increases because of the greater ΔP. The result of that is more capacity. More capacity produces a colder suction line and lower superheat. This is why superheat charging charts use ambient temperature to determine the superheat goal.

Some charging charts also use return air wet bulb temperature in their calculation.  The higher the return air wet bulb temperature, the more humidity is in the air and the greater the load there is on the evaporator.  When the load increases the superheat expectation rises.

Operation of the TxV

The TxV is a superheat regulator. In theory, a system with an expansion valve should have constant superheat once it achieves steady-state operation. It will however only have something like constant superheat in steady state operation when the expansion valve is within its throttling range.  When an expansion valve is at either at its full open position or its minimum position, it acts as a fixed orifice and superheat varies with the ΔP and the load on the evaporator.

There are three forces that act upon the expansion valve to position it:

1.    Spring pressure closes the valve; if the valve is adjustable, spring pressure is what is adjusted

2.    Suction pressure closes the valve.

3.    Diaphragm pressure opens the valve; the pressure generated in the sensing bulb is transmitted to the diaphragm.

How does the sensing bulb pressure change?

The bulb contains refrigerant. The refrigerant in the bulb has to follow its temperature-pressure correlation. The bulb is strapped to the suction line, very often with 2 copper straps. When the suction line gets warm the heat is transferred to the refrigerant in the bulb. That raises the pressure in the bulb. The pressure is transmitted through a capillary tube to the diaphragm in the “power head” of the expansion valve. When the pressure on the diaphragm exceeds the sum of the suction pressure and the spring pressure, it pushes on the valve to open and it lets more refrigerant through.

The heat transfer between the bulb and the pipe occurs mostly through the straps. The straps are the only things that have much surface area in contact with both the bulb and the suction line. The bulb and the pipe are both round; they have very little surface area in contact with each other. Copper straps conduct heat. That is why copper straps are important. Some other materials are far less effective at conducting heat. That’s why wire ties and duct tape are not acceptable materials to secure a sensing bulb to a suction line.

The bulb must be insulated when the surrounding air is much warmer than the suction line so that the bulb temperature is not strongly influenced by the air temperature.

When the expansion valve opens, there is more refrigerant mass flow through it because the orifice became larger. When more refrigerant flows, there is more refrigeration capacity in the evaporator and at some point the suction line will become cooler because of it.

The cooler suction line cools the bulb. The cooled refrigerant in the bulb is reduced in pressure and the expansion valve closes a little because of the effects of the spring and suction pressures on the expansion valve. The smaller orifice restricts refrigerant flow.  Soon the suction line starts to warm and the process starts again. That’s how the expansion valve modulates.  Waiting for the expansion valve to find its place is one of the reasons steady state operation is required for testing.

Does the expansion valve modulate from open to closed? 

Have you ever seen a system pump down into a vacuum because the expansion valve closed? You probably have not in direct expansion (dx) comfort cooling, if the system has a mechanical expansion valve and it is not restricted by some foreign substance. The expansion valves that are commonly used in constant volume units used for comfort cooling, modulate from a minimum position to fully open. Many expansion valves have a minimum position; some valves have smaller minimum positions than others.

Assuming the minimum position of the valve is about 50% of nominal capacity, the mass flow rate through the valve can vary from about 25% to about 125% of nominal mass flow.

How does that work?

The difference between positioning and percent mass flow is accounted for by the varying ΔP. When the valve is fully open and head pressure is high the valve may pass 125% of its nominal mass flow. When the valve is at its minimum position and the head pressure is at its low limit the mass flow of the refrigerant may be only 25% of nominal mass flow.

What would that look like and why should anyone care?

When there isn’t much load on a unit and the head pressure is low, there is a real danger of not maintaining the minimum required mass flow. In comfort cooling about 25% of the nominal mass flow is required to provide adequate cooling for the compressor motor.

Since the compressor continues to pump refrigerant out of the evaporator. There needs to be enough mass flow through the metering device to maintain sufficient refrigerant in the evaporator to pressurize it to a saturation temperature above water freezing, or the condensate on the evaporator coil may freeze.

Expansion valves will have problems controlling superheat when the ΔP is less than the minimum specified by the manufacturer. This may cause the expansion valve to “hunt” or have the inability to achieve steady-state operation. This situation, low ΔP and unstable expansion valve operation, most commonly happens when it’s cold outside.  Other potential causes include over-sized expansion valves and when compressors with unloaders remain loaded under low load conditions.

Technicians deal with a low head pressure problem by controlling the speed of the condenser fans with pressure switches or electronic speed controllers. These are called “low ambient controls” When low ambient controls are installed, the problems that are being solved include the risk of burning out the compressor from running with little capacity to cool the compressor motor and freezing the evaporator coil.

Our first objective criteria was condensing temperature is high when the condensing temperature over ambient is high. When is head pressure low? Head pressure is low when the ΔP across the metering device is low.

Evaporating temperature (ET)

The evaporating temperature is the low side saturation temperature.

The suction pressure converts to the saturation temperature on the T-P chart.

Increase the suction pressure by increasing the refrigerant flow through the metering device, by increasing the heat transfer into the refrigerant in the evaporator or by decreasing the pumping capacity of the compressor with unloading.

When is evaporating temperature low?

Many technicians have seen equipment running with low suction pressure. How do we know that it’s low?  What are our criteria for judging if suction pressure is low?  Remember, suction pressure converts to evaporating temperature on the T-P chart.

Water freezes at 32°, so when the evaporating temperature remains below that temperature for an extended time, ice forms on the evaporator coil. Ice is an obstruction to airflow as well as insulation against heat transfer. One problem we are trying to avoid in air conditioners by keeping the evaporating temperature above a minimum temperature is icing of the evaporator coil.

Low evaporating temperatures and especially low superheat might mean that the refrigerant isn’t absorbing enough heat from the air passing over the evaporator coil. When this problem occurs, the diagnosis is a low-side heat transfer problem. We care about this because the heat absorbed by the refrigerant in the evaporator is needed to evaporate the liquid refrigerant. When there is either too much refrigerant in the evaporator or too little heat available to evaporate the liquid refrigerant, it remains liquid. Liquid refrigerant entering the compressor leads directly to pre-mature compressor failures.

A low-side heat transfer problem may have one or more of several root causes. Some of the causes of low-side heat transfer problems are dirty evaporator coils, dirty filters, or a slow blower fan.  A slow blower fan may be caused by a worn belt, a worn or improperly adjusted motor sheave or a multi-speed fan motor set at the wrong speed. Other times obstructed or under-sized duct work will cause low air flow. Whatever the problem is, it should be corrected, or the technician will be forced to reduce the capacity of the unit to compensate when possible or the unit will operate with a low side heat transfer problem. Operating with a low side heat transfer problem increases the system’s energy use and it increases the risk of “slugging” the compressor, meaning allowing liquid refrigerant to enter the compressor.  Slugging is a primary cause of premature compressor failure.

Another possible cause of low evaporating temperatures is an insufficient refrigerant charge. High superheat or low subcooling along with low suction pressure will characterize this problem, as we will see in the diagnostics section. Low evaporating temperatures cause low capacities and low efficiencies.

There are a wide range of evaporating temperature goals depending on the driving conditions, the design efficiency of the unit, and the metering device type. A 35° evaporating temperature can be high and a 45° evaporating temperature can be low depending on the design efficiency (EER) or the unit, the metering device used and the driving conditions meaning the ambient temperature and the return are wet bulb temperature.

When is evaporating temperature high?

The issue is, how much are you concerned about latent capacity?

Humidity control is an important aspect of comfort in some places. The answer to this question depends in a large measure on your level of concern about controlling humidity. If you work in areas where controlling humidity is important, limiting the evaporating temperature to below the dew point of the air must be a priority for you. You may wish to make your evaporating temperature low to maximize latent capacity

If humidity control is not your issue because you work in a dry environment, you may want to make your evaporating temperature high to maximize sensible capacity and total capacity. If humidity control is of average importance, make your evaporating temperature in the normal range to balance latent and sensible capacity.

There is a considerable amount of judgment required to arrive at an evaporating temperature goal. Higher efficiency equipment usually has a higher evaporating temperature expectation. Different operating conditions experienced in various parts of the country result in people having differing concerns when considering evaporating temperature.

Some things however remain true everywhere. Higher evaporating temperatures will always produce more refrigeration capacity than lower evaporating temperatures, everything else remaining equal. This is because high pressure vapor is denser, meaning there are more refrigerant molecules in a given volume of gas at higher pressures. Since the compressor pumps at a fixed rate, denser gas entering the compressor will result in the compressor pumping more mass of refrigerant with every rotation. Remember more mass flow means more capacity. Additionally higher suction pressures may produce lower compressor ΔP. That reduces the load on the compressor motor, saving energy.

Higher evaporator temperatures often produce benefits. Can we say that more is always better?  Is there a limit to how high evaporating temperatures should go?  The answer is of course, yes. But what is the upper limit of acceptable evaporating temperatures for comfort cooling? That depends on how much you value humidity control. However, if the evaporating temperature is above 50°F, it’s getting high.  If the evaporating temperature is above 60°F, it might be time to start thinking about an inefficient compressor or improper unloader control.