This Climate Control blog is to review the basic refrigeration cycle and the interaction between the four basic components. The four basic components are the compressor, condenser, expansion device, and evaporator. Let's look at each component and its function and then look at what happens when we do not properly match these components.
The compressor is the transition point from the system's low-pressure side to its high-pressure side. Its purpose is to compress the cool low-pressure gas/vapor at the evaporator pressure up to the condenser pressure. In the compression process, the heat from compression and possibly motor heat increases the gas's temperature. The vapor entering the compressor is also superheated, which further increases during the compression process. At the discharge of the compressor, the refrigerant is a high-temperature, high-pressure superheated gas. Figure 1 below shows the ideal refrigeration cycle graphically on a Pressure-Enthalpy Diagram. The vertical axis is pressure, and the horizontal axis is enthalpy, or the heat content of the refrigerant per pound of refrigerant circulated. The compressor is the sloped line on the right side. The upper horizontal line is the condenser, the lower horizontal line is the evaporator, and the vertical line on the left is the expansion valve. Please note this is an ideal refrigeration diagram. There is no superheat exiting the evaporator or subcooling exiting the condenser. For a more detailed explanation of the P-H diagram, please refer to Sporlan Form 5-200 in our website's literature section under HVACR Educational Material.
We task the condenser with removing the heat absorbed in the evaporator, the heat of compression, and heat added by the compressor motor (as long as we use a hermetic or semi-hermetic compressor). This heat removal comes in two forms. First, the gas is desuperheated since the gas exiting the compressor increases in temperature above the saturated condensing temperature. Desuperheating is a sensible temperature change that can be measured as a decrease in temperature as the heat is removed from the gas. After the gas is desuperheated, it is in saturated conditions. At this point, additional heat removal condenses the gas into a liquid and is where the condenser gets its name. This heat removal occurs at a constant temperature and pressure until all the gas is condensed into a liquid, referred to as latent heat change. Keep in mind that the pressure is still at the same pressure when it exited the compressor, minus any small pressure loss from flow losses. Also, anytime there is liquid and vapor present, you are in saturated conditions, such as in the condenser or evaporator. You can use a Pressure-Temperature chart to determine the corresponding temperature from the measured pressure. Be aware of the difference and when to use dew point or bubble point when using a blended refrigerant. After the gas completely condenses into a liquid, any additional heat removal results in a temperature drop or sensible heat change. When the temperature drops below the condensing temperature or the saturated condensing temperature, the liquid is subcooled. Subcooling means we cool the refrigerant below the saturated temperature or, in this case, the condensing temperature. Subcooling is beneficial to prevent the refrigerant from flashing or reaching saturated conditions before the expansion device. Flashing can occur due to pressure drop from pressure losses in the tubing and accessories ahead of the expansion device. Any gas bubbles (flashing) can severely affect the TEV flowrate by reducing the refrigerant volume that can pass through the expansion device orifice.
The expansion device is the transition point from the system's high side to its low side. The expansion device drops the pressure from the high side pressure, referred to as the discharge pressure or condenser pressure, to the low side pressure. You may also refer to the low side pressure as the suction pressure or evaporator pressure. For this example, we use the Thermostatic Expansion Valve (TEV) as the expansion device. Other expansion devices could be a capillary tube, fixed restrictor, automatic expansion valve, or an electric expansion valve. These devices all have their benefits when weighing cost and performance or efficiency. The thermostatic expansion valve or TEV controls the superheat exiting the evaporator. In doing so, it controls the proper amount of refrigerant flows into the evaporator under all load conditions. As the load increases, the superheat increases driving the TEV further open to match the amount of refrigerant boiled off in the evaporator. Vice versa, if the load decreases, the superheat decreases driving the TEV in a more closed position.
The evaporator is the component that the other components are supporting. The evaporator removes the heat from the space you want to cool, whether it is a walk-in cooler or freezer, supermarket case, or an A/C unit. When the refrigerant leaves the TEV, it is a refrigerant liquid and vapor mixture. The vapor exiting the TEV is due to the refrigerant boiling at the lower pressure and cooling the liquid down to the desired evaporator temperature. When the refrigerant mixture enters the evaporator, it continues to boil at constant pressure and temperature. As it changes from a liquid to a vapor, it absorbs the heat flowing across the evaporator at the desired temperature. Evaporators may also be used as heat sinks to cool computer chips, machinery, or other items.
The secret to an optimally performing refrigeration or air conditioning system is matching all the components to balance the system. If one or more of the components are oversized or undersized, poor performance could result and higher energy costs.
For example, let us start with a 3 ton or 36 MBH(36,000 BTU/hr) system with all the components properly matched for the load. When we match the components correctly, we maintain the desired evaporator temperature at design conditions. Let us analyze what would happen if we replaced the 36 MBH compressor with a 48 MBH compressor, with everything else remaining the same. The first issue is the higher cost for a larger compressor, but we selected a larger compressor in this case. Since the compressor is now larger, the evaporator's pressure with a larger displacement compressor operates at lower suction pressure and lower evaporator temperature. Lower suction pressure results in a compressor that can cause short cycling, higher energy cost, and a shortened compressor life. Lower suction also causes a higher TD (temperature difference) across the evaporator coil, increasing its BTU capacity. It can also result in a different relative humidity than desired and possible coil frosting or icing since the TD is now higher across the coil. The increase in TD also causes an increase in the evaporator capacity and results in a higher flow rate than design through the expansion valve. If not sized for the new compressor, the TEV is not large enough for the system and starves or operates at a higher superheat. The compressor EER or Energy Efficiency Ratio also decreases due to the lower operating suction pressure meaning less BTU removed per Watts of energy consumed.
The result of installing a smaller compressor would be the obvious lower system capacity and not meeting the required design conditions or comfort for space. Also, the TEV would be oversized and could hunt. Also, the evaporator would operate at a higher pressure/temperature resulting in possible poor humidity control.
Moving around the system, if everything else is equal, what happens if the condenser is oversized? The drawbacks to an oversized condenser would be increased system refrigerant charge and increased equipment cost, and possibly operating issues during cold ambient conditions. However, there are benefits to an oversized condenser when properly evaluated by equipment manufacturers. Everyone has noticed that condensers are much larger than in past years on residential air conditioning units. The condensers in these systems have increased in size to meet the new SEER ratings, and a larger condenser helps increase the SEER rating of the system and reduce energy consumption.
An undersized condenser results in higher discharge temperatures, added stress on the compressor, and higher energy cost. It could also cause oil and refrigerant breakdown and result in a premature compressor failure.
It is always wise to properly size the TEV to match the compressor and evaporator capacity. Undersized TEVs starve the evaporator resulting in low suction pressure and poor system performance and temperature control. A starving valve (high superheat) can also cause high discharge temperatures and compressor overheating. An oversized TEV can result in TEV hunting because it overshoots its superheat setpoint due to the oversized valve port. Additionally, a hunting oversized valve could cause flood back to the compressor and damage to the compressor. A hunting valve also causes poor system performance as the valve overfeeds and underfeeds.
Evaporators need to be correctly sized to extract the correct amount of heat to meet the design load. If undersized, they operate at a lower suction pressure affecting/reducing the space's humidity and causing the compressor to operate at a lower pressure than design, resulting in higher energy costs. If oversized, it could result in better energy efficiency, but it could adversely affect the humidity, which could be undesirable if used for comfort cooling. Sizing the evaporator becomes a balancing act like the other components between comfort or desired result and energy efficiency.
These are just some examples to look for when installing or replacing equipment components. As well as these examples, there are other considerations when diagnosing a system; a change in entering air temperature, relative humidity, outdoor air temperature, dirty filters or condensers, and many other factors. When troubleshooting, look at the basics and check temperatures and pressures. If these numbers are not correct, check and verify what could be influencing the pressures and temperatures.
HVACR Tech Tip Article contributed by Pat Bundy, application engineer, Sporlan Division of Parker Hannifin
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