Refrigerants and Refrigeration Cycles
Refrigerants and refrigeration cycles - this domain tests refrigerant identification and safety classification, the pressure–enthalpy diagram, and how changing operating conditions move capacity, power and efficiency.
Identity and safety group
Refrigerants carry an ASHRAE number: R-717 is ammonia, R-134a and R-410A are halocarbons, R-744 is carbon dioxide. They are also classed by safety group - A for lower toxicity, B for higher toxicity, with a number for flammability. Ammonia sits in the higher-toxicity B group, which drives machinery-room ventilation, detection and emergency equipment. Zeotropic blends such as R-410A and R-507A have temperature glide: they boil and condense over a range, so you read dew point for superheat and bubble point for subcooling.
Reading the cycle
On a pressure–enthalpy diagram the four points are: 1, evaporator outlet (saturated vapour plus superheat); 2, discharge after compression; 3, condenser outlet (saturated liquid minus subcooling); 4, after the expansion valve, which is isenthalpic. Then refrigeration effect = h1 − h4, compressor work = h2 − h1, and COP = (h1 − h4) ÷ (h2 − h1).
What conditions do
Raising condensing temperature raises both h2 and h4 and cuts capacity and COP. Lowering suction pressure also cuts capacity and raises discharge temperature. Subcooling adds capacity with no extra compressor work; superheat adds work and gives nothing back. A dirty condenser therefore costs money twice - lost capacity and higher kW per tonne.
Worked example
An R-134a plant runs h1 = 400, h2 = 430, h3 = h4 = 250 kJ/kg. COP = (400 − 250) ÷ (430 − 400) = 150 ÷ 30 = 5.0. Condensing temperature then rises 10 K: h4 becomes 272 and h2 becomes 440. COP = (400 − 272) ÷ (440 − 400) = 128 ÷ 40 = 3.2 - a 36 percent loss from one operating change, which is why condenser cleaning pays.
Traps
Using gauge pressure in a P–T table instead of absolute (add about 101 kPa). Using average temperature instead of dew point on a blend. Treating COP as a fixed property of the refrigerant. Calling flash gas at the expansion valve a fault - it is normal.