Tesztek

F-Gas English Low GWP

Minden kérdés
HFC | Flammable | CO₂ | NH₃
Könnyű kérdések | Közepes nehézségű kérdések | Nehéz kérdések

1

Nehéz

Why do liquid and oil separators in CO₂ systems require particular attention?


Because CO₂ is flammable and separators must not be used
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

2

Nehéz

What is the critical pressure of CO₂ (R744)?


Approx. 120 bar
Approx. 31 bar
Approx. 74 bar (abs)
Approx. 10 bar

3

Nehéz

What characterises the transcritical process in CO₂ systems?


It is irrelevant to system operation
Conventional condensation occurs
It occurs only at low temperatures
Heat rejection takes place above the critical point – the refrigerant does not condense in the conventional way (gas cooler instead of condenser)

4

Nehéz

What characterises the subcritical CO₂ process?


Operation above the critical point
The conventional condensation process below the critical point
The absence of evaporation
The absence of condensation

5

Könnyű

What is the log(p)-h diagram for CO₂ used for?


To regulate pressure
For analysing thermodynamic processes in a refrigeration system
To measure temperature
To detect leaks

6

Könnyű

What can be read from the log(p)-h graph?


Only the temperature
Pressure, enthalpy, temperature and state of the refrigerant
Only the humidity
Only the amount of refrigerant

7

Könnyű

What are CO₂ saturation tables used for?


To measure humidity
To regulate the compressor
To determine the relationship between temperature and pressure at saturation
To determine the oil quantity

8

Könnyű

What does the CO₂ saturation state mean?


Equilibrium between the liquid and vapour phases
A state of high pressure
A state in which the refrigerant is only a gas
Dry ice state

9

Nehéz

When can dry ice (solid CO₂) form?


During normal operation of the system
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 temperature
At high pressure

10

Nehéz

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

11

Nehéz

How does CO₂ saturation pressure change as temperature increases?


It remains the same
It first decreases, then increases
It decreases
It increases rapidly

12

Könnyű

Why is understanding the log(p)-h graph particularly important for CO₂?


It is only used for design purposes
It only applies to NH₃
It doesn’t matter
As CO₂ systems operate in different modes (sub- and transcritical) and require detailed analysis

13

Nehéz

What happens to CO₂ above the critical point?


It transitions to a supercritical state (no distinction between liquid and gas)
It turns into a liquid
It turns into a solid
It ceases to conduct heat

14

Könnyű

Why do CO₂ systems require precise pressure control?


Because there are no valves
They do not require control
Due to low pressures
Due to significant pressure changes with temperature fluctuations

15

Nehéz

What is the main characteristic of R744 (CO₂) refrigeration systems?


Low energy efficiency under all conditions
Low operating pressures
Very high operating pressures
No need for control

16

Nehéz

What are the requirements for piping materials in CO₂ systems?


Only plastics
They must withstand high pressures (e.g. stainless steel, high-strength copper-alloy tubes such as K65)
Any material may be used
It doesn't matter

17

Nehéz

How does a booster system work in CO₂ installations?


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
It reduces evaporator pressure
It reduces the refrigerant quantity

18

Könnyű

What is the function of high-pressure control valves in CO₂ systems?


They are used to store the refrigerant
They lower the evaporator temperature
They regulate oil flow
They control the pressure on the high-pressure side and optimise system operation

19

Nehéz

Why is pressure optimisation in CO₂ systems crucial?


It enables the system to achieve maximum energy efficiency in transcritical mode
It reduces the ambient temperature
It reduces the amount of refrigerant
It doesn’t matter

20

Nehéz

What role does parallel compression play in CO₂ systems?


It has no significance
It lowers the ambient temperature
It increases energy consumption
It compresses flash gas from the intermediate receiver directly to high pressure, improving efficiency

21

Nehéz

How does an ejector work in a CO₂ system?


Stores the refrigerant
It lowers the evaporator temperature
It increases the amount of refrigerant
It uses the energy of the higher-pressure refrigerant to raise the pressure of the refrigerant from the lower stage, which improves efficiency

22

Nehéz

What is the function of liquid ejectors in CO₂ systems?


They increase the amount of oil
They return liquid from the separator and allow partially flooded evaporator operation (better evaporator feed)
They reduce atmospheric pressure
They raise the refrigerant temperature

23

Nehéz

What are the characteristics of partially flooded systems in CO₂ systems?


Lack of regulation
The presence of liquid in the evaporator, which improves heat transfer and efficiency
Low operating pressure
The absence of liquid in the evaporator

24

Nehéz

What is the main risk associated with standstill pressure in CO₂ systems?


Increased efficiency
A pressure rise as the refrigerant warms up during standstill (possibly exceeding PS and lifting the relief valves)
A drop in temperature
A decrease in refrigerant quantity

25

Nehéz

How is standstill pressure limited in CO₂ systems?


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 increasing the refrigerant charge
By raising the plant-room temperature
By completely shutting down the system and power supply

26

Nehéz

What does standstill (stagnation) of a CO₂ system mean, and why is it important?


A drop in system temperature
Improved efficiency
A state with no compressor operation or refrigerant flow – pressure rises with ambient temperature and may exceed PS
Normal system operation

27

Könnyű

Why do CO₂ systems require more advanced automation than traditional systems?


Because there are no valves
Due to low pressures
Due to the need for precise control of pressure and temperature
They do not require automation

28

Nehéz

Which safety class applies to R744?


A1
B2L
A2.
A3

29

Nehéz

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
…saturated vapour at intermediate pressure
…a subcooled liquid
…a supercritical fluid

30

Nehéz

In R744 transcritical systems, the gas cooler outlet valve controls:


The gas cooler fans
The inlet temperature to the gas cooler
The pressure in the gas cooler
The outlet temperature of the gas cooler

31

Nehéz

What is the main reason for using K65 copper tubing in some R744 systems?


They have good low-temperature performance
They bend easily
They come in a wider range of diameters
They can withstand higher pressures

32

Nehéz

Why should an evacuated R744 system initially be charged with refrigerant in the vapour phase?


To prevent the drain valve on the receiver from opening
To charge R744 more slowly and measure it more accurately
To avoid damaging the compressor with liquid CO₂
To prevent dry ice forming (until approx. 5.2 bar abs is exceeded)

33

Nehéz

If moisture remains in an R744 system because it was not properly evacuated (dehydrated), what is the likely consequence?


Reduced cooling capacity
Formation of hydrogen fluoride and hydrofluoric acid
Formation of carbonic acid, which damages the system
Excessive discharge pressure

34

Nehéz

The pressure of R744 at a saturation temperature of 20 °C is approx.


14 bar g
56 bar g
25 bar g
90 bar g

35

Nehéz

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…


…higher than the maximum allowable pressure (PS) of the low-pressure stage
…the same as the high-pressure switch setting
…approx. 27.5 bar g
…the same as the relief valve set pressure

36

Nehéz

What is the correct definition of critical temperature?


The temperature above which there are no distinct liquid and gaseous phases
The temperature at which the vapour pressure of a liquid is equal to the ambient pressure surrounding the liquid
The temperature at which a substance changes from a gaseous to a solid state
The temperature at which, for certain substances, electrical resistance is zero

37

Nehéz

In the gas cooler of a transcritical system, the supercritical working fluid…


…rejects heat at constant temperature and pressure
…rejects heat as its pressure falls
…rejects heat as its temperature falls
…rejects heat during a phase change

38

Közepes

What hazard does a large CO₂ leak in an enclosed room pose to people?


Accumulation near the floor and asphyxiating/toxic effects at high concentrations
Explosion of the mixture with air
Formation of hydrogen fluoride
CO₂ is completely safe

39

Nehéz

Why does the efficiency of transcritical CO₂ systems fall in warm climates, and how is this mitigated?


CO₂ freezes in the gas cooler – heaters are used
At high ambient temperatures the system operates transcritically with large throttling losses – parallel compression and ejectors are used, among others
Efficiency does not depend on climate
Gas cooler pressure drops – extra charge is added