F-Gas English Low GWP
All questions
HFC | Flammable | CO₂ | NH₃
Easy questions |
Medium questions |
Difficult questions
1
Difficult
Why do liquid and oil separators in CO₂ systems require particular attention?
| Because CO₂ needs no lubrication, so oil separators are unnecessary |
| Because of very high pressures – adequate PS and protection devices are required |
| Because sight glasses and level indicators must not be used in CO₂ systems |
| Because CO₂ is flammable and separators must not be used |
2
Difficult
What is the critical pressure of CO₂ (R744)?
| Approx. 31 bar |
| Approx. 10 bar |
| Approx. 120 bar |
| Approx. 74 bar (abs) |
3
Difficult
What characterises the transcritical process in CO₂ systems?
| It occurs only at low temperatures |
| It is irrelevant to system operation |
| Conventional condensation occurs |
| Heat rejection takes place above the critical point – the refrigerant does not condense in the conventional way (gas cooler instead of condenser) |
4
Difficult
What characterises the subcritical CO₂ process?
| The conventional condensation process below the critical point |
| The absence of condensation |
| The absence of evaporation |
| Operation above the critical point |
5
Difficult
When can dry ice (solid CO₂) form?
| During normal operation of the system |
| At high temperature |
| When the pressure drops suddenly below the triple point (approx. 5.2 bar abs, −56.6 °C), e.g. when venting liquid CO₂ or charging liquid into an evacuated system |
| At high pressure |
6
Difficult
What risks are associated with dry ice forming in a CO₂ system?
| Blocked pipes and valves and system damage (and a sharp pressure rise when it thaws) |
| Pressure reduction |
| Improved efficiency |
| No risk |
7
Difficult
How does CO₂ saturation pressure change as temperature increases?
| It first decreases, then increases |
| It increases rapidly |
| It decreases |
| It remains the same |
8
Difficult
What happens to CO₂ above the critical point?
| It transitions to a supercritical state (no distinction between liquid and gas) |
| It ceases to conduct heat |
| It turns into a liquid |
| It turns into a solid |
9
Difficult
What is the main characteristic of R744 (CO₂) refrigeration systems?
| Very high operating pressures |
| No need for control |
| Low energy efficiency under all conditions |
| Low operating pressures |
10
Difficult
What are the requirements for piping materials in CO₂ systems?
| Any material may be used |
| They must withstand high pressures (e.g. stainless steel, high-strength copper-alloy tubes such as K65) |
| It doesn't matter |
| Only plastics |
11
Difficult
How does a booster system work in CO₂ installations?
| It reduces evaporator pressure |
| It reduces the refrigerant quantity |
| It is used to store refrigerant |
| Low-temperature (LT) compressors discharge into the suction of medium-temperature (MT) compressors – two-stage compression in a single circuit |
12
Difficult
Why is pressure optimisation in CO₂ systems crucial?
| It reduces the ambient temperature |
| It enables the system to achieve maximum energy efficiency in transcritical mode |
| It doesn’t matter |
| It reduces the amount of refrigerant |
13
Difficult
What role does parallel compression play in CO₂ systems?
| It has no significance |
| It increases energy consumption |
| It compresses flash gas from the intermediate receiver directly to high pressure, improving efficiency |
| It lowers the ambient temperature |
14
Difficult
How does an ejector work in a CO₂ system?
| Stores the refrigerant |
| It increases the amount of refrigerant |
| It lowers the evaporator temperature |
| It uses the energy of the higher-pressure refrigerant to raise the pressure of the refrigerant from the lower stage, which improves efficiency |
15
Difficult
What is the function of liquid ejectors in CO₂ systems?
| They return liquid from the separator and allow partially flooded evaporator operation (better evaporator feed) |
| They reduce atmospheric pressure |
| They increase the amount of oil |
| They raise the refrigerant temperature |
16
Difficult
What are the characteristics of partially flooded systems in CO₂ systems?
| The presence of liquid in the evaporator, which improves heat transfer and efficiency |
| Lack of regulation |
| Low operating pressure |
| The absence of liquid in the evaporator |
17
Difficult
What is the main risk associated with standstill pressure in CO₂ systems?
| A pressure rise as the refrigerant warms up during standstill (possibly exceeding PS and lifting the relief valves) |
| A decrease in refrigerant quantity |
| A drop in temperature |
| Increased efficiency |
18
Difficult
How is standstill pressure limited in CO₂ systems?
| By completely shutting down the system and power supply |
| By increasing the refrigerant charge |
| By using an auxiliary standstill (stagnation) cooling unit, e.g. cooling the receiver, with back-up power, and by selecting an adequate PS and relief valves |
| By raising the plant-room temperature |
19
Difficult
What does standstill (stagnation) of a CO₂ system mean, and why is it important?
| A state with no compressor operation or refrigerant flow – pressure rises with ambient temperature and may exceed PS |
| A drop in system temperature |
| Normal system operation |
| Improved efficiency |
20
Difficult
Which safety class applies to R744?
| A3 |
| A2. |
| B2L |
| A1 |
21
Difficult
In a simple single-stage R744 system without an intermediate receiver (refrigerant flows from the gas cooler directly to the expansion valve), operating transcritically at 90 bar with a gas-cooler outlet temperature of 35 °C, the refrigerant upstream of the expansion valve is…
| …a two-phase mixture at intermediate pressure |
| …a subcooled liquid |
| …a supercritical fluid |
| …saturated vapour at intermediate pressure |
22
Difficult
In R744 transcritical systems, the gas cooler outlet valve controls:
| The inlet temperature to the gas cooler |
| The outlet temperature of the gas cooler |
| The pressure in the gas cooler |
| The gas cooler fans |
23
Difficult
What is the main reason for using K65 copper tubing in some R744 systems?
| They bend easily |
| They come in a wider range of diameters |
| They have good low-temperature performance |
| They can withstand higher pressures |
24
Difficult
Why should an evacuated R744 system initially be charged with refrigerant in the vapour phase?
| To prevent dry ice forming (until approx. 5.2 bar abs is exceeded) |
| To prevent the drain valve on the receiver from opening |
| To avoid damaging the compressor with liquid CO₂ |
| To charge R744 more slowly and measure it more accurately |
25
Difficult
If moisture remains in an R744 system because it was not properly evacuated (dehydrated), what is the likely consequence?
| Formation of carbonic acid, which damages the system |
| Excessive discharge pressure |
| Reduced cooling capacity |
| Formation of hydrogen fluoride and hydrofluoric acid |
26
Difficult
The pressure of R744 at a saturation temperature of 20 °C is approx.
| 90 bar g |
| 14 bar g |
| 56 bar g |
| 25 bar g |
27
Difficult
At an ambient temperature of 25 °C, the standstill pressure in the low-pressure stage of an R744 cascade (without standstill cooling) will typically be…
| …the same as the high-pressure switch setting |
| …approx. 27.5 bar g |
| …the same as the relief valve set pressure |
| …higher than the maximum allowable pressure (PS) of the low-pressure stage |
28
Difficult
What is the correct definition of critical temperature?
| The temperature at which the vapour pressure of a liquid is equal to the ambient pressure surrounding the liquid |
| The temperature at which, for certain substances, electrical resistance is zero |
| The temperature at which a substance changes from a gaseous to a solid state |
| The temperature above which there are no distinct liquid and gaseous phases |
29
Difficult
In the gas cooler of a transcritical system, the supercritical working fluid…
| …rejects heat during a phase change |
| …rejects heat at constant temperature and pressure |
| …rejects heat as its pressure falls |
| …rejects heat as its temperature falls |
30
Difficult
Why does the efficiency of transcritical CO₂ systems fall in warm climates, and how is this mitigated?
| Gas cooler pressure drops – extra charge is added |
| Efficiency does not depend on climate |
| At high ambient temperatures the system operates transcritically with large throttling losses – parallel compression and ejectors are used, among others |
| CO₂ freezes in the gas cooler – heaters are used |