1) List the refrigerant characteristics that are shown on a pressure-enthalpy diagram (PH).
PH diagrams show the relationships between a refrigerant's PRESSURE, HEAT, TEMPERATURE, VOLUME, and STATE. (Pg 280)
2) What is the SATURATION CURVE?
The Large hump in the middle of the PH chart is the saturation curve. Every point on or inside the saturation curve is saturated. The critical point is at the very top of the hump. (Pg 280)
3) Where are the pressure lines, and how are they marked?
The pressure lines run horizontally from the right side to the left side of the diagram. The pressure scales are given in POUNDS PER SQUARE INCH ABSOLUTE, PSIA. (Pg 280 & 282)
4) Define ENTHALPY.
Heat content of a refrigerant, usually with respect to a reference point (Btu/lb). Enthalpy is the amount of heat in something; specific enthalpy is the amount of heat per pound, (Pg 293 & 1490)
5) Explain the difference between enthalpy and specific enthalpy.
Enthalpy is the amount of heat in something, without regard to weight. Specific enthalpy is the amount of heat per pound. Enthalpy is represented by the uppercase H; specific enthalpy is represented by the lowercase h. (Pg 293)
6) Where are the specific enthalpy lines, and how are they marked?
The specific enthalpy lines are vertical lines going from top to bottom; the scale is shown along the top and bottom of the PH diagram. The units are given in BTU per pound of refrigerant, BTU/lb. (Pg 282)
7) Why are the temperature lines horizontal inside the saturated curve?
The temperature lines run horizontally inside the saturated region because the temperature of a saturated mixture stays the same as it changes state. (Pg 283)
8) What are constant-quality lines?
The constant-quality lines start at the bottom and extend upward toward the critical point located in top center of the dome. These lines represent the percent age of vapor in the saturated refrigerant. (Pg 280)
9) As you move to the right or left along a constant pressure line in the saturated liquid area, what happens to the temperature?
The temperature remains the same. (Pg 283)
10) As you move to the right or left along a constant pressure line, what happens to the specific enthalpy?
The specific enthalpy increases as you move from the left to the right and decreases moving from right to left. (Pg 282)
11) As you move up or down along a constant enthalpy line, what happens to the pressure?
The pressure increases as you move up and decreases as you move down. (Pg 282)
12) As you move up along a constant enthalpy line in the constant-quality area, what happens to the temperature?
The temperature increases as you move up along a constant-enthalpy line in the constant-quality area. (Pg 28
13) In refrigeration, how do we define entropy?
In refrigeration, entropy is defined as the ratio of the heat content of the refrigerant to its absolute temperature in degrees Rankine. On a PH diagram, entropy is expressed in terms of BTU per pound per degree (BTU/lb/R degrees) (Pg 284)At
14) Why does some of the refrigerant liquid flash-vaporize in the metering device?
Beause the pressure has dropped to the saturation point. The small amount of liquid flashing cools the remaining liquid. (Pg 289)
15) At what point in the refrigeration cycle does the last of the liquid vaporize?
All the liquid refrigerant has changed to vapor by the end of the evaporator. (Pg 287)
16) What happens to the refrigerant pressure during desuperheating?
Desuperheating decreases the sensible temperature and heat, but the pressure remains the same. (Pg 288)
17) When does liquid refrigerant first begin forming in the condenser?
Condensation begins after all the refrigerant superheat has been removed. (Pg 288)
18) What is subcooling?
Subcooling is the lowering of a liquid's temperature below its saturated liquid pressure
19) How much heat enters the refrigerant in the metering device when some of it flashes to a vapor, cooling the remaining liquid to the evaporator temperature.
No heat is gained or lost in the metering device, but the pressure drops dramatically. The refrigerant increases in volume through the metering device. (Pg 287)
20) What can bubbles of refrigerant vapor do to the metering device's operating efficiency?
Bubbles of refrigerant vapor decrease the amount of refrigerant the metering device feeds, reducing the system's operating efficiency. (Pg 288) TECH TIP
21) When does a refrigerant pick up most of the heat as it circulates through a system?
Refrigerant flows into the evaporator, where it can begin to pick up heat. Heat is added to the refrigerant, causing it to evaporate, increasing the percentage of vapor to 100 percent.. The refrigerant gains the most heat in the portion of the evaporator
22) DEFINE ADIABATIC.
WITHOUT HEAT. (Pg 286)
23) DEFINE ISOTHERMAL.
Isothermal is the process where the temperature remains the same while the refrigerant changes state from a gas to a liquid. (Pg 286)
24) Explain how a system's refrigeration effect can be determined.
The refrigeration effect is the difference in the enthalpy of the refrigerant entering the evaporator and the enthalpy of the refrigerant leaving the evaporator. (Pg 289)
25) Determine the refrigeration effect of the following system: the liquid entering the evaporator has an enthalpy of 64 BTU/lb, and the vapor leaving the evaporator has an enthalpy of 121 BTU/lb.
121 BTU/lb minus 64 BTU/lb = 57 BTU/lb (Pg 289)
26) What is the net refrigeration effect for the system in question 25 if the system is circulating 4 lb of refrigerant each minute?
57 BTU/lb x 4 lb/min = 228 BTUs/min (Pg 290)
27) When drawing a PH diagram of an operating system, how is placement of the condenser and evaporator lines determined?
The first step in plotting a refrigeration cycle on a PH diagram is to establish the condensing and evaporating lines. Take the system operating pressures and convert them to absolute pressure by adding 15 These pressures will be used to establish the eva
28) When drawing a PH diagram of an operating system, how is placement of the compressor line determined?
Measure the suction-line temperature entering the compressor. Find where the evaporator line crosses this suction-line temperature in the superheated area. This is where the compressor line starts. Follow the entropy lines up until they cross the condense
29) When drawing a PH diagram of an operating system, how is placement of the metering device line determined?
Measure the temperature of the liquid line entering the metering device. Follow the condenser line until it crosses this liquid-line temperature in the subcooled area. This is where the metering device lines starts. Draw a line straight down until it cros
30) Why is the condenser line wider than the evaporator line in a PH diagram of a refrigeration cycle?
Notice that the condenser must dissipate more heat than the evaporator absorbs-the heat of compression as well as the heat picked up in the evaporator. (Pg 288)
31) How can a PH diagram be used to select the system compressor?
Determine the amount of heat picked up in the evaporator by subtracting the enthalpy of the saturated mix entering the evaporator from the enthalpy of the gas leaving the evaporator. divide this into the required system capacity to get lbs/min of refriger
32) How can a PH diagram be used to determine the RCOP for a particular system?
A refrigerant's coefficient of performance (RCOP) may be calculated by dividing the Refrigeration Effect by the heat of compression: RCOP = Refrigeration Effect / Heat of Compression. The refrigeration effect is the difference in the enthalpy of the refri
33) How can a PH diagram be used to determine the heat of compression?
Draw vertical lines from the points where the refrigerant enters and leaves the compressor to determine the enthalpy entering and leaving the compressor. The heat of compression is the difference in the enthalpy.
34) How many pounds of refrigerant must flow through a 4-ton evaporator each minute if each pound has an enthalpy of 50 BTU/lb?
1 ton = 200 BTU / minute. 4 x 200 BTU / minute = 800 BTUs / minute
800 BTUs / minute / 50 BTUs / lb = 12.6 lbs / minute
35) What will happen in the evaporator if the compressor capacity is not large enough to remove all the vapor?
Some of the vapor will remain in the evaporator. This will cause an increase in evaporator pressure and temperature and a decrease in system capacity.
On a blank sheet of paper, sketch a typical PH diagram for a refrigeration system. You do not need to duplicate all the scales on the PH diagram. Just show the saturation curve and the four cycle lines.
PAGE 281 SEE Figure 19-1