Refrigeration and Air Conditioning 3 E Part 2 potx

Low-pressure refrigerant gasAbsorber Pressure reducing valve Weak liquor High-pressure refrigerant gas Generator Pump Strong liquor Expansion valve Condenser High-pressure refrigerant li

5 Compatibility with materials of construction, with lubricatingoils, and with other materials present in the system

6 Convenient working pressures, i.e not too high and preferablynot below atmospheric pressure

7 High dielectric strength (for compressors having integral electricmotors)

2.7 Total loss refrigerants

Some volatile fluids are used once only, and then escape into theatmosphere Two of these are in general use, carbon dioxide andnitrogen Both are stored as liquids under a combination of pressureand low temperature and then released when the cooling effect isrequired Carbon dioxide is below its critical point at atmosphericpressure and can only exist as ‘snow’ or a gas Since both gasescome from the atmosphere, there is no pollution hazard Thetemperature of carbon dioxide when released will be – 78.4°C.Nitrogen will be at –198.8°C Water ice can also be classified as atotal loss refrigerant

2.8 Absorption cycle

Vapour can be withdrawn from an evaporator by absorption (Figure2.11) into a liquid Two combinations are in use, the absorption ofammonia gas into water and the absorption of water vapour intolithium bromide The latter is non-toxic and so may be used for air-conditioning The use of water as the refrigerant in this combinationrestricts it to systems above its freezing point Refrigerant vapourfrom the evaporator is drawn into the absorber by the liquidabsorbant, which is sprayed into the chamber The resulting solution(or liquor) is then pumped up to condenser pressure and the vapour

is driven off in the generator by direct heating The high-pressurerefrigerant gas given off can then be condensed in the usual wayand passed back through the expansion valve into the evaporator.Weak liquor from the generator is passed through another pressure-reducing valve to the absorber Overall thermal efficiency is improved

Low-pressure refrigerant gas

Absorber

Pressure reducing valve Weak liquor

High-pressure refrigerant gas Generator

Pump Strong liquor

Expansion valve

Condenser

High-pressure refrigerant liquid

Evaporator

Expansion valve

suction-to-(a)

(Solar radiation can also be used.) The overall energy used is greaterthan with the compression cycle, so the COP (coefficient ofperformance) is lower Typical figures are as shown in Table 2.2.

2.9 Steam ejector system

The low pressures (8–22 mbar) required to evaporate water as arefrigerant at 4–7°C for air-conditioning duty can be obtained with

a steam ejector High-pressure steam at 10 bar is commonly used.The COP of this cycle is somewhat less than with the absorptionsystem, so its use is restricted to applications where large volumes ofsteam are available when required (large, steam-driven ships) orwhere water is to be removed along with cooling, as in freeze-dryingand fruit juice concentration

2.10 Air cycle

Any gas, when compressed, rises in temperature Conversely, if it ismade to do work while expanding, the temperature will drop Use

is made of the sensible heat only (although it is, of course, the basis

of the air liquefaction process)

The main application for this cycle is the air-conditioning andpressurization of aircraft The turbines used for compression andexpansion turn at very high speeds to obtain the necessary pressureratios and, consequently, are noisy The COP is lower than withother systems [15]

The normal cycle uses the expansion of the air to drive the firststage of compression, so reclaiming some of the input energy (Figure2.12)

Compressor

Heat exchanger

Expander

Cold air

to process

Cooling air in Fan

2.11 Thermoelectric cooling

The passage of an electric current through junctions of dissimilarmetals causes a fall in temperature at one junction and a rise at theother, the Peltier effect Improvements in this method of coolinghave been made possible in recent years by the production of suitablesemiconductors Applications are limited in size, owing to the highelectric currents required, and practical uses are small cooling systemsfor military, aerospace and laboratory use (Figure 2.13)

Cooled surface Heat

sink

P type –

+

15 V d.c.

N type

or air-conditioning refrigeration applications because of its toxicity,flammability and attack by copper.

This chapter is about the new refrigerants and the new attitudeneeded in design, maintenance and servicing of refrigerationequipment

3.2 Ideal properties for a refrigerant

It will be useful to remind ourselves of the requirements for a fluidused as a refrigerant

• A high latent heat of vaporization

• A high density of suction gas

• Non-corrosive, non-toxic and non-flammable

• Critical temperature and triple point outside the working range

• Compatibility with component materials and lubricating oil

• Reasonable working pressures (not too high, or belowatmospheric pressure)

• High dielectric strength (for compressors with integral motors)

• Low cost

• Ease of leak detection

• Environmentally friendly

No single fluid has all these properties, and meets the newenvironmental requirements, but this chapter will show thedevelopments that are taking place in influencing the selection andchoice of a refrigerant.

3.3 Ozone depletion potential

The ozone layer in our upper atmosphere provides a filter forultraviolet radiation, which can be harmful to our health Researchhas found that the ozone layer is thinning, due to emissions intothe atmosphere of chlorofluorocarbons (CFCs), halons and bromides.The Montreal Protocol in 1987 agreed that the production of thesechemicals would be phased out by 1995 and alternative fluidsdeveloped From Table 3.1, R11, R12, R114 and R502 are all CFCsused as refrigerants, while R13B1 is a halon They have all ceasedproduction within those countries which are signatories to theMontreal Protocol The situation is not so clear-cut, because thereare countries like Russia, India, China etc who are not signatoriesand who could still be producing these harmful chemicals Table3.2 shows a comparison between old and new refrigerants

R22 is an HCFC and now regarded as a transitional refrigerant,

in that it will be completely phased out of production by 2030, asagreed under the Montreal Protocol A separate European Com-munity decision has set the following dates

1/1/2000 CFCs banned for servicing existing plants

1/1/2000 HCFCs banned for new systems with a shaft input power

greater than 150 kW

1/1/2001 HCFCs banned in all new systems except heat pumps

and reversible systems

1/1/2004 HCFCs banned for all systems

1/1/2008 Virgin HCFCs banned for plant servicing

Table 3.2 Comparison of new refrigerants

3.4 Global warming potential (GWP)

Global warming is the increasing of the world’s temperatures, whichresults in melting of the polar ice caps and rising sea levels It iscaused by the release into the atmosphere of so-called ‘greenhouse’gases, which form a blanket and reflect heat back to the earth’ssurface, or hold heat in the atmosphere The most infamousgreenhouse gas is carbon dioxide (CO2), which once released remains

in the atmosphere for 500 years, so there is a constant build-up astime progresses

The main cause of CO2 emission is in the generation of electricity

at power stations Each kWh of electricity used in the UK produces

about 0.53 kg of CO2 and it is estimated that refrigeration compressors

in the UK consume 12.5 billion kWh per year

Table 3.3 shows that the newly developed refrigerant gases alsohave a global warming potential if released into the atmosphere.For example, R134a has a GWP of 1300, which means that theemission of 1 kg of R134a is equivalent to 1300 kg of CO2 Thechoice of refrigerant affects the GWP of the plant, but other factorsalso contribute to the overall GWP and this has been represented

by the term total equivalent warming impact (TEWI) This term shows

the overall impact on the global warming effect, and includesrefrigerant leakage, refrigerant recover y losses and energyconsumption It is a term which should be calculated for eachrefrigeration plant Figures 3.1 and 3.2 show the equation used and

an example for a medium temperature R134a plant

TEWI = (GWP × L × n) + (GWP × m [1 – α recovery ] + (n × Eannual× β )

TEWI = TOTAL EQUIVALENT WARMING IMPACT

Leakage Recovery losses Energy consumption

direct global warming potential

GWP = Global warming potential [CO2-related]

α recovery = Recycling factor

Eannual = Energy consumption per year [kWh]

β = CO2-Emission per kWh (Energy-Mix)

indirect global warming potential

One thing that is certain is that the largest element of the TEWI

is energy consumption, which contributes CO2 emission to theatmosphere The choice of refrigerant is therefore about the efficiency

of the refrigerant and the efficiency of the refrigeration system.The less the amount of energy needed to produce each kW ofcooling, the less will be the effect on global warming

3.5 Ammonia and the hydrocarbons

These fluids have virtually zero ODP and zero GWP when releasedinto the atmosphere and therefore present a very friendly environ-mental picture Ammonia has long been used as a refrigerant forindustrial applications The engineering and servicing requirementsare well established to deal with its high toxicity and flammability.There have been developments to produce packaged liquid chillerswith ammonia as the refrigerant for use in air-conditioning insupermarkets, for example Ammonia cannot be used with copper

or copper alloys, so refrigerant piping and components have to besteel or aluminium This may present difficulties for the air-conditioning market where copper has been the base material forpiping and plant One property that is unique to ammonia compared

to all other refrigerants is that it is less dense than air, so a leakage

of ammonia results in it rising above the plant room and into theatmosphere If the plant room is outside or on the roof of a building,the escaping ammonia will drift away from the refrigeration plant

tcm Mean condensing temperature

tom Mean evaporating temperature

blends

The safety aspects of ammonia plants are well documented andthere is reason to expect an increase in the use of ammonia as arefrigerant.

Hydrocarbons such as propane and butane are being successfullyused as replacement and new refrigerants for R12 systems Theyobviously have flammable characteristics which have to be takeninto account by health and safety requirements However, there is amarket for their use in sealed refrigerant systems such as domesticrefrigeration and unitary air-conditioners

3.6 Refrigerant blends

Many of the new, alternative refrigerants are ‘blends’, which havetwo or three components, developed for existing and new plants ascomparable alternatives to the refrigerants being replaced Theyare ‘zeotropes’ with varying evaporating or condensing temperatures

in the latent heat of vaporization phase, referred to as the

‘temperature glide’ Figure 3.3 shows the variation in evaporatingand condensing temperatures

To compare the performance between single componentrefrigerants and blends it will be necessary to specify the evaporatingtemperature of the blend to point A on the diagram and thecondensing temperature to point B

The temperature glide can be used to advantage in improvingplant performance, by correct design of the heat exchangers Aproblem associated with blends is that refrigerant leakage results in

a change in the component concentration of the refrigerant However,tests indicate that small changes in concentration (say less than10%) have a negligible effect on plant performance

The following recommendations apply to the use of blends:

• The plant must always be charged with liquid refrigerant, or thecomponent concentrations will shift

• Since most blends contain at least one flammable component,the entry of air into the system must be avoided

• Blends which have a large temperature glide, greater than 5K,should not be used for flooded-type evaporators

3.7 Lubricants

Choosing the right lubricating oil for the compressor has becomemore complex with the introduction of new refrigerants Table 3.4gives some indication as to the suitability of the traditional and newlubricating oils Compressor manufacturers should be consultedwith regards to changing the specified oil for a particular compressor

Table 3.4 Choice of compressor lubricant

Those lubricants marked ‘M’ easily absorb moisture and greatcare must be taken to prevent exposure to air when adding new oil.The moisture in the air will be absorbed into the oil and will lead

to contamination of both refrigerant and oil With hermeticcompressors this can lead to motor winding failure

3.8 Health and safety

When dealing with any refrigerant, personal safety and the safety ofothers are vitally important Service and maintenance staff need to

be familiar with safety procedures and what to do in the event of anemergency Health and safety requirements are available frommanufacturers of all refrigerants and should be obtained and studied.Safety codes are available from the Institute of Refrigeration inLondon, for HCFC/HFC refrigerants (A1 and A2), ammonia (B2)and hydrocarbons (A3)

In the UK and most of Europe, it is illegal to dispose of refrigerant

in any other way than through an authorized waste disposal company.The UK legislation expects that anyone handling refrigerants iscompetent to do so and has the correct equipment and containers.Disposal must be through an approved contractor and must be fullydocumented Severe penalties may be imposed for failure toimplement these laws

4 Compressors

4.1 General

The purpose of the compressor in the vapour compression cycle is

to accept the low-pressure dry gas from the evaporator and raise itspressure to that of the condenser

Compressors may be of the positive displacement or dynamictype The general form of positive displacement compressor is thepiston type, being adaptable in size, number of cylinders, speedand method of drive It works on the two-stroke cycle (see Figure4.1) As the piston descends on the suction stroke, the internalpressure falls until it is lower than that in the suction inlet pipe, andthe suction valve opens to admit gas from the evaporator At thebottom of the stroke, this valve closes again and the compressionstroke begins When the cylinder pressure is higher than that in thedischarge pipe, the discharge valve opens and the compressed gaspasses to the condenser Clearance gas left at the top of the strokemust re-expand before a fresh charge can enter the cylinder (see

Suction

inlet

Discharge outlet

(b) Discharge stroke

Figure 4.2 and also Chapter 2, for theoretical and practical cycles

on the Mollier chart and for volumetric efficiency)

Clearance volume

Compression

The first commercial piston compressors were built in the middle

of the last century, and evolved from the steam engines whichprovided the prime mover Construction at first was double acting,but there was difficulty in maintaining gas-tightness at the pistonrod, so the design evolved further into a single-acting machine withthe crankcase at suction inlet pressure, leaving only the rotatingshaft as a possible source of leakage, and this was sealed with apacked gland

throw of the crankshaft to give a short, rigid machine (see Figure4.4).

This construction gives a large number of common parts – pistons,connecting rods, loose liners and valves – through a range ofcompressors, and such parts can be replaced if worn or damagedwithout removing the compressor body from its installation.Compressors for small systems will be simpler, of two, three orfour cylinders (see Figure 4.5)

4.3 Valves

Piston compressors may be generally classified by the type of valve,and this depends on size, since a small swept volume requires aproportionally small inlet and outlet gas port The smallestcompressors have spring steel reed valves, both inlet and outlet inthe cylinder head and arranged on a valve plate (Figure 4.6) Above

a bore of about 40 mm, the port area available within the head size

is insufficient for both inlet and outlet valves, and the inlet is moved

to the piston crown or to an annulus surrounding the head Theoutlet or discharge valve remains in the central part of the cylinderhead In most makes, both types of valve cover a ring of circular gasports, and so are made in annular form and generally termed ringplate valves (Figure 4.7) Ring plate valves are made of thin springsteel or titanium, limited in lift and damped by light springs toassist even closure and lessen bouncing

Although intended to handle only dry gas, liquid refrigerant ortraces of oil may sometimes enter the cylinder and must pass out

(Courtesy of Vilter Manufacturing Corporation)