Refrigeration cycle, gauges & leaks: EPA 608 Practice Questions

In what state does refrigerant normally leave the evaporator?

The refrigerant boils in the evaporator and picks up a little extra heat, so it leaves as a low-pressure, superheated vapor on its way to the compressor.

In what state does refrigerant normally leave the condenser?

The condenser removes heat until the vapor turns to liquid and cools a little below saturation, so it leaves as a high-pressure, subcooled liquid.

What leaves the compressor discharge?

The compressor squeezes low-pressure vapor into high-pressure, high-temperature (superheated) vapor that goes to the condenser.

Which component divides the high-pressure side from the low-pressure side at the evaporator inlet?

The metering device (TXV, EEV, piston or capillary tube) drops the liquid's pressure so it can boil in the evaporator. The compressor is the other dividing point.

On a standard manifold gauge set, what is the blue compound gauge used for?

The blue compound gauge connects to the low side and reads both positive pressure and vacuum in inches of mercury. The red gauge is for the high side; deep vacuum needs a micron gauge.

The suction line is at 52°F and the evaporator saturation temperature from the PT chart is 40°F. What is the superheat?

Superheat is the vapor temperature minus the saturation temperature at the same pressure: 52°F − 40°F = 12°F.

The condensing (bubble point) temperature is 105°F and the liquid line is at 95°F. What is the subcooling?

Subcooling is the saturation temperature minus the actual liquid temperature: 105°F − 95°F = 10°F.

A recovery cylinder of R-134a has been sitting at 75°F, but its pressure is well above the PT chart value for 75°F. What is the most likely cause?

A pure refrigerant with liquid present follows its PT relationship. Pressure above that value at a stable temperature points to non-condensable gases such as air.

Which leak detection method is best for pinpointing the exact location of a small leak at a joint?

Bubble solution shows exactly where gas escapes. A standing vacuum or pressure test can show that a leak exists but not where it is.

Which gas should be used to pressurize a system for a standing pressure leak test?

Dry nitrogen is inert and contains no moisture. Oxygen or compressed air with oil can explode, and shop air adds moisture.

When an electronic leak detector needs something to sense, what may be added to the nitrogen used for a pressure test?

A trace amount of refrigerant mixed with nitrogen lets an electronic detector find leaks. The refrigerant should not be the main pressurizing gas, and the system must already be evacuated; nitrogen cannot be used to avoid recovering refrigerant.

Which sign often points to a refrigerant leak at a fitting?

Refrigerant carries oil with it, so an oily, dirty spot at a joint is a common sign of a leak.

A gauge reads 100 psig at sea level. What is the absolute pressure?

Absolute pressure is gauge pressure plus atmospheric pressure (about 14.7 psi at sea level): 100 + 14.7 = 114.7 psia.

How does an ultrasonic leak detector find a leak?

Gas forced through a small opening makes ultrasonic noise; the detector converts it to sound the technician can hear, so it works with any gas, including nitrogen.