When taking a simple and direct approach to understanding the fundamentals of HVACR, a technician evaluating the operation of a comfort cooling system needs to understand the operation of the refrigeration and air flow systems in the equipment individually and collectively. They are considered as being in balance with one another, simply because it’s universally understood that a refrigeration system cannot accomplish heat transfer at optimum efficiency without the proper amount of air flow throughout the ductwork, and the air handling system cannot provide maximum comfort if the refrigeration system isn’t operating as designed.
In Figure One, the TXV, DX system shown is accomplishing the maximum level of heat transfer possible.
Consider first that the metering device in this system is capable of delivering the proper volume of refrigerant to the evaporator coil due to the synchronicity of the three pressures in the valve (evaporator pressure, spring pressure and bulb pressure), and, second, because of the correct volume and velocity of air flow through the coil. Note that the saturation temperature of the refrigerant is 38-degrees, and that the temperature drop through the coil is 20-degrees. And, we can also see that the evaporator superheat in this system is 12-degrees when we consider the last point of liquid in the 38-degree coil, and the 50-degree temperature reading at a point on the suction line directly ahead of the TXV sensing bulb.
Figure Two shows an example of the air handling system that makes the temperatures above achievable.
The first factor we want to mention regarding this illustration is that the equipment capacity is 3 tons, due to the application of the fundamental rule that an efficient system will operate with an air flow of 400 CFM per ton. (The total air flow shown in the return ductwork and the supply plenum is 1200 CFM.)
Note also the reduction of the supply plenum after the first four supply registers of 100 CFM each are served; then again after the next set of registers, which involves three at 100 CFM each and the bathroom registers at 50 CFM each, and then the third reduction in plenum size at the final segment of the supply duct system. This design of the supply ductwork, along with the turning vanes, allows the air handling system to operate with minimum noise and the necessary velocity required for proper throw from the supply registers into each room in the building. And, it also ensures that the static pressure in the system will be correct.
The factors to consider here are the slightly negative -0.03 WC (Water-Column-Inch) pressure in the return system, which allows for the free flow of air through the return, and the fan static pressure of 0.4, which is achieved due to the -.02 pressure at the fan inlet and the 0.2 pressure at the fan outlet. The proper static pressure in this system is achieved through the application of a manufacturer’s table for airflow characteristics, such as the one shown in Figure Three.
This table shows that the indoor fan has a maximum capacity of 1360 CFM while operating against a 0.4 static pressure, which means that the blower will easily achieve the required 1200 CFM for our three-ton system.
With the duct system and blower allowing for the proper return and delivery of air throughout the building, the balance between it, and the refrigeration system, will be achieved, resulting in the proper operation of the equipment from a fundamental perspective.
Until next week…
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