Testy

F-Gas English Low GWP

Všetky otázky
HFC | Flammable | CO₂ | NH₃
Ľahké otázky | Stredne ťažké otázky | Ťažké otázky

1

Ťažká

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


Because of very high pressures – adequate PS and protection devices are required
Because CO₂ needs no lubrication, so oil separators are unnecessary
Because CO₂ is flammable and separators must not be used
Because sight glasses and level indicators must not be used in CO₂ systems

2

Ťažká

What is the critical pressure of CO₂ (R744)?


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

3

Ťažká

What characterises the transcritical process in CO₂ systems?


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)
It is irrelevant to system operation
Conventional condensation occurs

4

Ťažká

What characterises the subcritical CO₂ process?


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

5

Ťažká

When can dry ice (solid CO₂) form?


At high pressure
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

6

Ťažká

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)
No risk
Improved efficiency
Pressure reduction

7

Ťažká

How does CO₂ saturation pressure change as temperature increases?


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

8

Ťažká

What happens to CO₂ above the critical point?


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

9

Ťažká

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

Ťažká

What are the requirements for piping materials in CO₂ systems?


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

11

Ťažká

How does a booster system work in CO₂ installations?


Low-temperature (LT) compressors discharge into the suction of medium-temperature (MT) compressors – two-stage compression in a single circuit
It reduces the refrigerant quantity
It reduces evaporator pressure
It is used to store refrigerant

12

Ťažká

Why is pressure optimisation in CO₂ systems crucial?


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

13

Ťažká

What role does parallel compression play in CO₂ systems?


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

14

Ťažká

How does an ejector work in a CO₂ system?


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

15

Ťažká

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

Ťažká

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


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

17

Ťažká

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


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

18

Ťažká

How is standstill pressure limited in CO₂ systems?


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

19

Ťažká

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


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

20

Ťažká

Which safety class applies to R744?


A3
A1
A2.
B2L

21

Ťažká

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
…saturated vapour at intermediate pressure
…a supercritical fluid

22

Ťažká

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


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

23

Ťažká

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


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

24

Ťažká

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


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

25

Ťažká

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


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

26

Ťažká

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


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

27

Ťažká

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

28

Ťažká

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

Ťažká

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


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

30

Ťažká

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


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
Gas cooler pressure drops – extra charge is added
CO₂ freezes in the gas cooler – heaters are used