REFRIGERATION CYCLES PART 2

THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

The vapor-compression refrigeration cycle is the ideal model for refrigeration systems, air conditions and heat pumps.

It consists of four processes:

1-2 Isentropic compression in compressor.

2-3 Constant-pressure heat rejection in a condenser.

3-4 Throttling in an expansion devise.

4-1 Constant-pressure heat absorption in an evaporator.

The  process  in  ideal  vapor  compression  refrigeration cycle:

The  refrigerant  enters  the  compressor  at  state 1 as saturated vapor and is compressed isentropically to the condenser pressure. The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of the surrounding medium.

The refrigerant then enters the condenser as superheat vapor at state 2 and leaves as saturated liquid at state 3 as a result to the heat rejection to the surrounding. 

The saturated liquid at state 3 enters an expansion valve or capillary tube and leaves at  evaporator pressure.  The temperature of refrigerant drop below the temperature of  refrigerated  space  during  this  stage.  

The  refrigerant enters the evaporator at stage 4 as saturated mixture and it completely evaporate by absorbing the heat from the refrigerated space. The refrigerant leaves the evaporator as saturated vapor and reenters the compressor, completing the cycle.

ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

An actual vapor-compression refrigeration cycle differs from the ideal one owing mostly to the irreversibilities that occur in various components, mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings.

The COP decreases as a result of irreversibilities.

DIFFERENCES

• Non-isentropic compression

• Superheated vapor at evaporator exit

• Subcooled liquid at condenser exit

• Pressure drops in condenser and evaporator 

In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor.  In practice, however, it ·may not be possible to control the state of the refrigerant so precisely.  Instead, it is easier to design the system so that the refrigerant is slightly superheated at the compressor inlet.

This slight overdesign ensures that the refrigerant is completely vaporized when it enters the compressor. Also, the line connecting the evaporator to the compressor is usually very long; thus the pressure drop caused by fluid friction and heat transfer from the surroundings to the refrigerant can be very significant. The result of superheating, heat gain in the connecting line, and pressure drops in the evaporator and the connecting line is an increase in the specific volume, thus an increase in the power input requirements to the compressor  since steady-flow work is proportional to the specific volume.

The compression process in the ideal cycle is internally reversible and adiabatic, and thus isentropic.  The actual compression process, however, involves frictional effects, which may increase or decrease the entropy, depending on the direction.  

Therefore, the entropy of the refrigerant may increase or decrease during an actual compression process depending on which effects dominate.  The compression process may be even more desirable than the isentropic compression process since the specific volume of refrigerant and thus the work input requirement are smaller in this case.  

Therefore, the refrigerant should be cooled during the compression process whenever it is practical and economical to do so.

In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at the compressor exit pressure. In reality, however, it is unavoidable to have some pressure drop in the condenser as well as in the lines connecting the condenser to the compressor and to the throttling valve. 

Also, it is not easy to execute the condensation process with such precision that the refrigerant  is a saturated  liquid at the end, and it is undesirable  to route the refrigerant  to the throttling valve before the refrigerant is completely condensed. 

Therefore, the refrigerant is subcooled somewhat before it enters the throttling valve. We do not mind this at all, however, since the refrigerant in this case enters the evaporator with a lower enthalpy and thus can absorb more heat from the refrigerated space. 

The throttling valve and the evaporator are usually located very close to each other, so the pressure drop in the connecting line is small.

REFRIGERATION CYCLES PART 1

In this article we are reviewing the factors involved in selecting the right refrigerant for an application.

REFRIGERATORS

The transfer of heat from a low- temperature region to a high-temperature one requires special devices called refrigerators.

Refrigerators are cyclic device, and the working fluids used in the refrigeration cycles are called refrigerants. 

HEAT PUMPS

Another   device   that   transfers heat from a low-temperature medium to a high-temperature one is the heat pump

Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. 

The objective of a refrigerator is to remove heat (QL) from the cold medium.

The objective of a heat pump is to supply heat (QH) to a warm medium.

The cooling capacity of a refrigeration system is the rate of heat removal from the refrigeration space  is often expressed in terms of ton of refrigeration.

The capacity of a refrigeration system that can freeze 1 ton of liquid water at 0 °C into ice at 0 °C in 24 h is said to be 1 ton.

VAPOR-COMPRESSION REFRIGERATION CYCLE

Two modes of operations:

1. Ideal vapor-compression refrigeration cycle

2. Actual vapor-compression refrigeration cycle

THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

The vapor-compression refrigeration cycle is the ideal model for refrigeration systems, air conditions and heat pumps.

It consists of four processes:

1-2 Isentropic compression in compressor.

2-3 Constant-pressure heat rejection in a condenser.

3-4 Throttling in an expansion devise.

4-1 Constant-pressure heat absorption in an evaporator.

The  process in ideal vapor compression  refrigeration

cycle:

The  refrigerant  enters  the  compressor  at  state 1 as

saturated vapor and is compressed isentropically to the

condenser pressure. 

The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of the surrounding medium.

The refrigerant then enters the condenser as superheat

vapor at state 2 and leaves as saturated liquid at state 3

as a result to the heat rejection to the surrounding.

The saturated liquid at state 3 enters an expansion valve or

capillary  tube  and  leaves  at  evaporator  pressure.  The

temperature of refrigerant drop below the temperature

of  refrigerated  space  during  this  stage.  

The  refrigerant enters the evaporator at stage 4 as saturated mixture and it completely evaporate by absorbing the heat from the refrigerated space. The refrigerant leaves the evaporator as saturated  vapor and reenters the compressor, completing the cycle.  

Notice that the ideal vapor­ compression refrigeration  cycle is not an internally reversible cycle since  it involves an irreversible (throttling) process. This process is maintained in the cycle to make it a more realistic model for the actual vapor­ compression refrigeration cycle. If the thronling device  were replaced by an isentropic turbine, the refrigerant would enter the evaporator at state 4´ instead of state 4. As a result, the refrigeration capacity  would increase and the net work input would decrease (by the amount of work output of the turbine). Replacing the expansion valve by the turbine is not practical since the added benefits cannot justify the added cost and complexity

SELECTING THE RIGHT REFRIGERANT

• Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).

•    R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.

•    The industrial and heavy-commercial sectors use ammonia (it is toxic).

•    R-11 is used in large-capacity water chillers serving A-C systems in buildings.

•    R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and

freezers, as well as automotive air conditioners.

• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.

• R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.

• CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to the ozone layer have been developed.

• Two important parameters that need to be considered in the selection of a refrigerant are the temperatures of the two media (the refrigerated space and the environment) with which the refrigerant exchanges heat.

REPAIR A AIR CONDICIONER - MAKE A SYSTEMATIC ANALYSIS

When an air conditioner failure, changing parts might be the first reaction, BUT...

1. May not be necessary and...
2. Does not always solve the problem

SUPERHEAT AND SUCTION PRESSURE

SYMPTOMS CAN PROVIDE THE REAL CAUSE.

1.       Moisture, dirt, wax

2.       Undersized valve

3.       High superheat adjustment

4.       Gas Charge condensation

5.       Dead thermostatic element charge

6.       Wrong thermostatic charge

7.       Evaporator pressure drop – no external equalizer

8.       External equalizer location.

9.       Restricted or capped eternal equalizer

10.   Low refrigerant charge

11.   Liquid line vapor

a.       Vertical lift

b.       High friction loss

c.       Long or small line

d.       Plugged drier or strainer

12.   Low pressure drop across valve

a.       Undersized distributor nozzle or circuits

b.       Low condensing temperature

1.       Oversized valve

2.       TEV seat leak

3.       Low superheat adjustment

4.      Bulb installation

a.       Poor thermal contact

b.       Warm location

5.       Wrong thermostatic charge

6.       Bad compressor – low capacity

7.       Moisture, dirt, wax

8.       Incorrectly located external equalizer

1.       Low load

a.       Not enough air.

b.       Dirty air filters

c.       Air too cold.

d.       Cool icing

2.       Poor Air distribution

3.       Poor Refrigerant distribution

4.       Improper compressor-evaporator balance

5.       Evaporator oil logged

6.       Flow from one TEV affecting another´s bulb

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