refrigeration with ammonia

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 1 ( 2 0 0 8 ) 5 4 5 – 5 5 1

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Review

Refrigeration with ammonia

Andy Pearson*

Star Refrigeration Ltd., Glasgow G46 8JW, UK

a r t i c l e i n f o

Article history:

Received 11 August 2007

Received in revised form

22 November 2007

Accepted 23 November 2007

Published online 16 January 2008

Keywords:

Ammonia

Refrigeration system

Survey

Refrigerant

Safety

Regulations

Development

* Tel.: þ44 141 638 7916; fax: þ44 141 638 8E-mail address: [email protected]

0140-7007/$ – see front matter ª 2007 Elsevidoi:10.1016/j.ijrefrig.2007.11.011

a b s t r a c t

Ammonia is widely used as a refrigerant in industrial systems for food refrigeration, distri-

bution warehousing and process cooling. It has more recently been proposed for use in

applications such as water chilling for air-conditioning systems but has not yet received

widespread acceptance in this field. This review paper assesses the reasons why ammonia

is so popular in industrial systems, the reasons why it is deemed less suitable for other ap-

plications and the possible benefits at local, national and international levels that might be

gained by more general acceptance of ammonia as a refrigerant. The paper also considers

other possible applications which might benefit from the use of ammonia as refrigerant.

ª 2007 Elsevier Ltd and IIR. All rights reserved.

Le froid a ammoniac

Mots cles : Ammoniac ; Systeme frigorifique ; Enquete ; Frigorigene ; Securite ; Reglementation ; Developpement

1. Introduction

The continued refinement of our understanding of climate

science combined with increased concerns on many levels

about energy consumption has created an unprecedented

requirement for the development of efficient refrigeration sys-

tems with minimal impact on the environment. If HFC refrig-

erants are to be substituted, as seems to be the case in several

European countries, or even if their use is to be constrained to

111.

er Ltd and IIR. All rights

applications where there is no technically and economically

viable alternative, then it is essential that the chemicals

used in their stead satisfy some fundamental requirements.

They must be no less energy efficient than the HFCs that

they replace. They must be proven to be safe, both for the

immediate neighbourhood and for the global environment.

They must be simple and cost-effective to use, they must be

readily available and ideally they must not require any signif-

icantly new or unfamiliar technology.

reserved.

mailto:[email protected]

http://www.iifiir.org

http://www.elsevier.com/locate/ijrefrig

Nomenclature

a moles per unit volume at 25 �C: 40.87 (mol m�3)

A dangerous toxic load (DTL) (–)

c speed of sound in gas (m s�1)

C gas concentration in atmosphere (ppmv or

mg m�3)

CoP coefficient of performance (kW/kW)

M molar mass (g mol�1)

n toxicity exponent (–)

P pressure (bar gauge)

R universal gas constant: 8.314472 (J mol�1 K�1)

T temperature (K)

t exposure time (min)

Vi volume ratio (–)

Greek symbols

DP differential pressure (bar)

Pi pressure ratio (–)

g index of compression (–)

4 refrigerant concentration limit (mass) (mg m�3)

c refrigerant concentration limit (volume) (ppmv)

Subscripts

c condenser

crit critical

e evaporator

i ratio

ideal for an ideal gas


Ammonia presents several challenges in this respect, if it is

truly to be considered to be an alternative to HFCs. The max-

imum charge permitted in an occupied space is defined by

the practical limit, also known as the practical charge limit,

for the refrigerant (EN-378:2007, 2007). If the charge exceeds

the practical limit, and in the majority of cases for ammonia

plant it does, then restrictions must be placed on the location

of the compressor equipment and gas detection must be

fitted, linked to appropriate emergency ventilation. Consider-

ation must also be given to the flammable limits for ammonia.

If it is possible for the concentration in air in the machinery

room to reach a level of 20% of the lower flammable limit

then additional precautions must be taken. In some interna-

tional safety codes, for example the European Standard

EN-378, automatic isolation of the electrical supply to the ma-

chinery room is required. In other safety codes, for example

ASHRAE-15 (2001), very high emergency ventilation rates, of-

ten exceeding one air change per minute, are mandated. The

safety codes are generally conservative in their approach,

but sensible. For example they recognise that, provided access

to the area containing ammonia plant is restricted to person-

nel with some basic knowledge of emergency safety proce-

dures and provided occupants are not restricted in their

movements there is no need to install gas detectors for human

safety, because the strong smell of ammonia provides suffi-

cient warning of its presence at concentrations well below

the danger zone. Ammonia is both flammable and toxic, but

despite the hazards implicit in its use as a refrigerant, over

100 years of experience and refinement in the industrial

refrigeration field has produced a clear understanding of

what needs to be done to avoid accidents. In general, if the

requirements of the existing safety codes are followed,

ammonia systems are very efficient, reliable and safe. This

makes them more attractive for large industrial refrigeration

systems than fluorocarbon alternatives for which the costs

of installation and operation are likely to be higher.

2. Implications of the properties ofammonia for system efficiency

The properties which make ammonia so attractive as a refrig-

erant are well documented and clearly understood. It has an

extremely high latent heat, second only to water in commonly

recognised fluids, and therefore provides more refrigerating

effect per unit mass flow than any other refrigerant used in

traditional vapour compression systems. The relatively low

gas density of ammonia, which is a result of its low molecular

weight, predicates increased compressor swept volume in

comparison with the heavier fluorocarbon refrigerants, but

the combination of latent heat and density mean that the

volumic refrigerating effect of ammonia is almost identical

to that of HCFC-22 at typical operating conditions, namely

about 60% higher than that of HFC-134a and 60% lower than

that of R-410A. Ammonia also has a very high critical temper-

ature, comparable to HC-600a and only exceeded by CFC-11

and HCFC-123. This makes ammonia, unlike all of the HFC

refrigerants and refrigerant blends, particularly well suited

to use in air-cooled equipment in high ambient temperatures.

The speed of sound in an ideal gas is given by the equation

cideal ¼ffiffiffiffiffiffiffiffiffigRTM

r: (1)

It follows that cideal will be a higher value for a low molar

mass. The acoustic velocity for ammonia is much higher

than for all other refrigerants. This means that higher gas

velocities can be used in the design of pipe, valves and fittings

without incurring excessive losses. At �10 �C the acoustic

velocity for ammonia is 397.5 m s�1 whereas for HFC-134a it

is 146.9 m s�1 and for R-404A it is 143.4 m s�1. This also has im-

plications for compressor design, where the efficiency losses

associated with inlet and discharge valves are much lower

for ammonia (Anon., 2007).

The combination of high acoustic velocity and high latent

heat results in remarkably small liquid pipe sizes for ammonia

compared to HFCs. Likewise the size of the expansion orifice

required to control the refrigerant flow is very small. This

can create a challenge in designing low capacity ammonia

systems because the very small diameter expansion orifice

is easily blocked. If a capillary tube expansion device is used

then it is better to make it longer than the equivalent for

HFCs, rather than to reduce the diameter and risk blockages.

The effect on theoretical system efficiency can be clearly

seen in Table 1 (Pearson, 2005a). The efficiency is based on

a theoretical cycle described in the ASHRAE Handbook of

2001 with compressor efficiencies and pipeline pressure

losses assumed to be equal in all systems. The key difference

Table 1 – Comparative refrigerant performance

No. Name CoP

R-717 Ammonia 4.84

R-290 Propane 4.74

R-600 Butane 4.68

R-22 Chlorodifluoromethane 4.65

R-134a Tetrafluoroethane 4.60

R-407C R-32/R-125/R-134a (23/25/52) 4.51

R-410A R-32/R-125 (50/50) 4.41

R-404A R-125/R-143a/R-134a (44/52/4) 4.21

R-744 Carbon dioxide 2.96

Based upon a standard operating cycle of 258 K evaporating

temperature, 303 K condensing temperature, 0 K subcooling and

0 K superheat.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 1 ( 2 0 0 8 ) 5 4 5 – 5 5 1 547

between the refrigerants listed is in the size of the irreversibil-

ity losses in the expansion process, which in turn is a function

of the reduced pressure (the ratio of operating pressure to

critical pressure). Table 2 gives the reduced pressures for the

evaporating and condensing conditions of the refrigerants

listed in Table 1 and also gives the pressure ratio, the ideal ra-

tio of suction to discharge volume and the pressure difference.

It can be seen that carbon dioxide works closer to the critical

point than any other refrigerant, with high pressure differ-

ences and low pressure ratios. Ammonia shows lower reduced

pressures, implying good prospects for high efficiency, than

all the others except butane, which is exceptionally low.

Pressure ratio is a key consideration in the efficiency of

reciprocating compressors because it determines, in conjunc-

tion with the clearance volume, the amount of re-expansion

that occurs as the piston draws gas into the cylinder, and

hence the volumetric efficiency of the compressor. In compar-

ison leakage past the piston rings is at least an order of

magnitude less significant. In screw compressors the pressure

ratio indicates what the ideal volume ratio is, which in turn for

a given compressor geometry will determine whether the gas

is over- or under-compressed. However in screw compressors,

provided the geometry is suitable for the pressure ratio, the

pressure difference is a more significant factor in establishing

the volumetric efficiency as it affects the quantity of gas pass-

ing from discharge to suction past the rotor tip seals and

through the ‘‘blowhole’’ (the gap between the rotors). Suitable

compressor geometry is achieved either through correct

Table 2 – Reduced pressures and pressure ratio forTe [ 258 K and Tc [ 303 K

No. Pe/Pcrit Pc/Pcrit Pi Vi DP (bar)

R-717 0.021 0.103 4.95 3.22 9.3

R-290 0.068 0.253 3.71 2.97 7.9

R-600 0.015 0.074 5.03 3.16 2.3

R-22 0.059 0.238 4.03 3.02 8.9

R-134a 0.040 0.189 4.71 3.79 6.0

R-407C 0.055 0.247 4.52 3.56 8.4

R-410A 0.102 0.399 3.92 2.95 13.4

R-404A 0.097 0.381 3.93 3.15 10.6

R-744 0.309 0.974 3.15 1.97 49.1

compressor selection, or within limits through the use of an

automatically variable volume ratio (Vi) mechanism. It should

be noted that for most compressors the limits of range of

automatic Vi adjustment are less than can be achieved in

customised machines. For example the range of variable Vi

might be from 2.8 to 4.8 for a given model of compressor,

but by specifying a very small, fixed discharge port a Vi of 5.8

can be achieved. Likewise by modifying the profile of the slide

valve to increase the area of the discharge port the volume

ratio can be reduced to as little as 2.1. It is therefore important

for compressors operating over a large temperature lift to se-

lect as high a ratio as possible, particularly as the ideal volume

ratio for a compressor operating at these conditions is far

higher than the maximum that the compressor geometry

will allow. The pressure ratio for an ammonia compressor op-

erating at�30 �C suction and 35 �C discharge is 11.34, suggest-

ing that the ideal volume ratio is 6.04. For the same conditions

with R-134a the pressure ratio is 10.46, indicating an ideal vol-

ume ratio of 7.57 when the maximum that the geometry will

allow is 5.8 as stated previously, or 4.8 if variable Vi is used.

There is not such an obvious connection between refriger-

ant choice and compressor isentropic efficiency although the

following points should be noted. As previously stated the

high acoustic velocity of ammonia reduces the irreversible

losses in compressor valves so a smaller port size can be

used for a given refrigerating duty. The relatively high volumic

refrigerating effect for ammonia, comparable with R-22 and

exceeded only by R-410A and R-744 in the common refriger-

ants, means that frictional losses and parasitic loads are rela-

tively low for ammonia compressors. However comparison

between refrigerants is difficult because lower pressure refrig-

erants such as R-123 tend to use different compressor types,

and the most important conclusion is that a compressor

optimised for one refrigerant type is unlikely to give as good

performance with another. For example a reciprocating

compressor will require different valve designs for ammonia

and fluorocarbons in order to maximise the isentropic

efficiency in each case.

Ammonia tends to cope better with contaminants such as

water and oil than fluorocarbon refrigerants. Water will accu-

mulate in the low pressure side of the system and will have an

adverse effect on system efficiency (Cotter et al., 2007), but in

general it will not prevent the plant from operating, whereas

in an R-22 plant excess water will freeze at the expansion

valve and block it. Gigiel and Evans (2007) report that the

combined effect of oil contamination, water and compressor

wear on a large ammonia freezer plant which had been

extended several times over many years, had been to increase

the energy consumption by 43%. However they noted that

even in this poor condition the plant was more efficient than

an equivalent R-22 system with electric defrost would be

when new.

3. Safety considerations

The toxic effect of ammonia is dependent upon the level of

concentration in the atmosphere and on the length of time

the exposure lasts. A chart is shown in Fig. 1 which enables

the time and concentration to be assessed in the event of an

10 100 1000 10000 100000Atmospheric concentration (mg m

-3)

1

10

100

1000

Exp

osu

e tim

e (m

in

utes)

5% probability of damage5% lethal probability50% lethal probabilityService Technician thresholdMiljokontrollen request

Fig. 1 – Ammonia concentration exposure limits, adapted

from Lindborg (2006).


industrial accident (Lindborg, 2006). The chart was created by

the Swedish National Defence Research Establishment for use

in toxic release scenario modelling and is based on the litera-

ture review, tests and experience. The line on the left of the

diagram corresponds to the maximum concentration that

can be sustained by a susceptible population (the elderly, in-

firm and very young) without sustaining injury and the two

lines on the right correspond to various risks of fatality in

the same susceptible population. The shorter line in the

centre of the graph is used to determine the risk for techni-

cians engaged in service activity. If the concentration exceeds

that indicated by the line then it is probable that some form of

medical treatment will be required. It should be noted that the

concentration figures used in Fig. 1 are in mg/m3.

The following equation (adapted from EN-378:2007, 2007)

can be used to convert the atmospheric concentration from

mass per unit volume, g/m3, to a volumetric ratio, ppm by

volume, assuming a temperature of 25 �C:

c ¼ 4� 103

aM: (2)

For ammonia, with a molar mass of 17 g/mol, this simplifies

to

4 ¼ 0:7c: (3)

In a study for the Carlsberg Brewery in Copenhagen

(Lindborg, 2006) the Swedish Environment Agency (Miljo-

kontrollen) required confirmation that a concentration of

1200–1500 ppm would not be exceeded for more than 5 min.

This range is shown by the black horizontal line in Fig. 1, which

is slightly lower than the level used for risk assessment for

technical staff.

Ammonia is not considered suitable for use in domestic

refrigerators or air-conditioners because it is not compatible

with the materials commonly used in these systems, particu-

larly copper, but also because the practical limit results in

extremely low quantities for the maximum charge. The

practical limit is derived from the IDLH value published by

the United States National Institute for Occupational Safety

and Health (NIOSH, 1994), which for ammonia was reduced

from 500 ppm to 300 ppm in 1996. It should be noted however

that the practical limit for ammonia commonly quoted in

safety standards of 0.00035 kg/m3 is based upon the old IDLH

of 500 ppm under ‘‘grandfather clauses’’ in the standards.

These are clauses where a previously agreed value is retained

because it is in common usage, even after the basis for the

original calculation of the value has changed. IDLH is defined

as the maximum concentration at which escape will not be

impaired by 30 min exposure, which is deemed to be repre-

sentative of conditions found in the workplace for industrial

installations. However in the domestic context for typical

small kitchen dimensions of 2 m wide� 4 m� long� 3 m

high this definition for IDLH seems excessive. In this typical

room, which has a gross volume of 24 m3, the maximum

charge of ammonia permitted would be 8.4 g. It seems

unlikely that it would require 30 min to escape from this

room in the event of a bad smell of ammonia: 1 min seems

a more reasonable estimate, and 5 min would be a very

conservative allowance.

The effect of large doses of toxic chemicals on humans is

difficult to study directly. Values for lethal doses for humans

based on laboratory tests need to be extrapolations from tests

on other species or from lower doses on humans. Field expe-

rience from incidents is never sufficiently well documented

and the estimates of exposure levels suffered are subject to

wide margins of error. In particular it has been noted that

humans have a higher respiratory rate than animals in acci-

dent or emergency situations. It is therefore right to take a con-

servative approach to these values. In the method proposed by

Fairhurst and Turner (1993) a ‘‘Specified Level of Toxicity’’

(SLOT) is determined from available data extrapolated to

humans. The SLOT dangerous toxic load (DTL or SLOT DTL)

is denoted as A. This is usually calculated from the concentra-

tion that would result in the onset of fatality in the most

vulnerable members of the exposed population and can be

correlated to exposure at a given concentration for a specified

time by the simple equation

A ¼ Cnt; (4)

where C is the atmospheric concentration, n is a substance-

specific exponent and t is the exposure time. In the case of

ammonia the SLOT DTL is 3.78� 108 and the exponent is 2

(Anon., 2006). It follows that the maximum concentration

related to a 5 min exposure for this toxic load is 8695 ppm. A

higher toxic load, termed as the Significant Likelihood of

Death (SLOD) can also be calculated. For SLOD the DTL is

equivalent to the concentration likely to result in fatality for

50% of the exposed population. A value of 1.03� 109 is given

for ammonia, which results in a calculated maximum concen-

tration for 30 min exposure of 5859 ppm. Using the concept of

the dangerous toxic load for the original IDLH values gives

a value for A of 7.5� 106. When these values are considered

in the domestic context two things are clear. Firstly, it seems

unlikely, given the high latent heat of ammonia, that the total

system charge would be able to transfer from within the

system as liquid to vapour distributed throughout the room

in 1 min, or even 5 min. The more likely scenario is that

a sudden leak behind the refrigerator would diffuse through

the room at a slower rate. Secondly applying Eq. (4) to the

30 min ‘‘grandfathered’’ practical limit value for 1 min and

5 min exposures gives values of 2740 ppm and 1225 ppm,


respectively. If these values were adopted for domestic

circumstances, characterised by small charge systems in rela-

tively small rooms, then the maximum charge allowed would

be 45 g and 20 g, respectively. These ammonia charge values

seem low compared to the typical charge of refrigerant in a do-

mestic refrigerator with R-134a as the refrigerant, but when

the liquid densities are compared the actual difference in re-

frigerant quantity is considerably less than it appears at first

sight. The density of liquid R-134a at 20 �C is 1219 kg/m3,

whereas for ammonia it is almost exactly half, at 609.7 kg/

m3. If the maximum charge of a large domestic refrigerator

is taken to be 100 g of R-134a (Clodic et al., 1999) then the

equivalent design would require 50 g of ammonia. It is not un-

reasonable to conclude that minor alterations to the con-

denser, liquid line and evaporator could achieve a charge of

45 g and to speculate that further optimisation might achieve

a charge of 20 g. Furthermore the choice of material would not

present a problem provided a hermetic design of unit was

used and the ammonia was completely free of moisture

when the unit was charged, so that there would be no reaction

between the ammonia and the copper in the system (Anon.,

2005; Hansen, 2006).

The comparison of the Specified Level of Toxicity, Signifi-

cant Likelihood of Death and Immediately Dangerous to Life

and Health concentrations over a 120 min time period is

shown in Fig. 2. It is clear that it would be necessary to remain

in the concentration level calculated for a 1 min IDLH for

50 min in order to reach the SLOT value. It is highly improba-

ble that this concentration level could be sustained for 50 min.

For comparison of Figs. 1 and 2 the conversion factor given in

Eq. (3) must be used. This shows that the levels used by the

Swedish National Defence Research Establishment are gener-

ally conservative, and equate to an exposure concentration for

a 30 min period of 240 ppm, 20% lower than the NIOSH revised

IDLH and less than half of the older figure.

It is unlikely that there will be any move towards the

adoption of ammonia in domestic refrigerators in the near

future although it would appear to be technically feasible

and of low risk, because the use of isobutane and propane/

isobutane mixtures has been comprehensively demonstrated

to be safe, with millions of units now in use. However with in-

creasing interest in heat pumps there might be a role for the

unique properties of ammonia in domestic heat pumps

1

10

100

1000

10 100 1000 10000 100000

Atmospheric concentration (ppm)

Exp

osu

e tim

e (m

in

utes)

C SLODC SLOTC IDLH 500ppmC IDLH 300ppm

Likely to cause injury

new IDLH old IDLH

onset offatality insusceptiblemembersof thepopulation 50% fatality rate

Fig. 2 – SLOT and SLOD concentrations with IDLH values,

calculated values.

alongside the propane and carbon dioxide systems already

on the market. This high efficiency system would be more

likely to be successfully adopted if the practical limit for in-

stallations were based on the pragmatic approach outlined

above, giving a significant energy advantage in the domestic

heat pump market compared to fluorocarbon systems.

4. Regulatory considerations

Safety regulations concerning refrigeration can be traced back

to the early years of the 20th century. Since then the codes and

standards have diversified. There is now such a wide range of

national regulatory constraints on the use of ammonia, even

within the European Union, that it must be concluded that

they all cannot be correct. In France the use of ammonia in

refrigeration systems is governed by the government agency

DRIRE (Direction Regionale de l’Industrie de la Recherche et

de l’Environnement). This stipulates that systems must be

notified to the regional authorities if they are expected to

contain more than 150 kg of ammonia and the design must

be subject to third party examination and approval if the

charge is greater than 1500 kg. One of the most unattractive

aspects of the French regulations is the requirement that

any modification to the plant must also be approved by the

regional authority. This gives sufficient doubt as to the ease

with which the plant may be amended to suit future require-

ments to persuade many users to avoid ammonia completely.

As a result the plants selected, typically using secondary

refrigerants or fluorocarbons, are less efficient in operation,

and so the users are placed in an uncompetitive position

compared to other manufacturers in Europe. As energy prices

rise this discrepancy becomes more significant.

In the United States the use of ammonia in industrial

facilities is governed by the Occupational Safety and Health Ad-

ministration (OSHA) and the Environmental Protection Agency

(EPA) through various sub-sections of OSHA 29 CFR part 1910

and EPA 40 CFR part 68. In addition the Department of Home-

land Security (DHS) is currently formulating additional regula-

tions to cope with the threat of terrorist attack on industrial

facilities. It is not yet known how many industrial refrigeration

facilities will be affected by these new rules, although it is likely

that only the very largest facilities will be covered. It is surpris-

ing, given the highly toxic nature of the products of combustion

of fluorocarbons (Pearson, 2007), that large plants using HCFC

and HFC refrigerants are also not covered.

In the United Kingdom the use of ammonia in industrial

refrigeration facilities is not covered by specific rules, but

rather is included in various sections of general Health and

Safety at Work regulations. There is a much greater scope

for end-users to develop their own methodology for providing

a safe system of working to their employees and there is much

less bureaucracy than in either the French or the American

systems. In other countries there are various additional

requirements, for example, the need for permanent operator

presence on sites in Canada with more than 25 kW connected

shaft power on the compressors.

A review of national accident statistics shows that there is

no appreciable difference in the fatality rate in these

countries, despite the large variation in approaches to safety


legislation. There are so few fatal accidents, and such wide

variations in system size and type that it is difficult to draw

any conclusion, however the fatality rate due to ammonia

refrigeration in the United States of America and the United

Kingdom is in the range 0.5–2.5 deaths per billion people per

year (Pearson, 2007). It appears that this level is much the

same in other European countries, and is similar to the level

of fatalities due to accidents with fluorocarbon refrigerants,

with liquid nitrogen and with carbon dioxide in the food

industry, although this includes non-refrigeration uses such

as CO2 recovery from brewing and CO2 injection in soft drinks.

By comparison the rate of deaths by lightning strikes in the

United States is in the range 32–48 deaths per billion per year.

According to EN-378:2007 (2007) ammonia is not permitted

for use in direct air-conditioning systems for human comfort

(where the refrigerant-containing parts are in contact with the

air being cooled), but it can be used as the refrigerant in chillers

where the chilled water is pumped to air handling units. The

standard requires that any such system with an ammonia

charge greater than 500 kg must have ammonia detectors fitted

to the watercircuit.There is alsoa requirement for automaticair

purgers on all secondary circuits, but this fails to discriminate

between the soluble refrigerants like ammonia or carbon diox-

ide and the insoluble, such as fluorocarbons and hydrocarbons.

When the wide range of legal positions from country to

country is contrasted with the homogeneity of the fatality rates

across all industries it seems that there is no strong correlation

between strict regulation and fatality rate. It is more likely that

there is a certain rate at which the population as a whole will do

something unforeseen and inadvertently dangerous. This leads

to the conclusion that regulations which discourage the use of

ammonia, as is the case in France, the Netherlands and Italy,

provide no safety benefit, but penalise the manufacturers in

these countries by leading them to adopt more expensive, less

efficient systems for their refrigeration plant, for no benefit.

5. Implications for the design of ammoniasystems for new markets

The benefits of using ammonia for water chilling applications

have been described by several authors in recent years

(Pearson, 2004). Apart from the efficiency improvement, said

to be in the range 9–17% (Tychsen, 2003), there is also a signifi-

cant improvement in heat transfer, both in the evaporator and

the condenser (Hrnjak and Park, 2007). This offers the opportu-

nity to make efficient chillers in smaller footprints, particularly

when air-cooled condensers are used. The major constraint

identified by Palm (2007) was that components for small am-

monia systems are difficult to source. Continued development

of these components, including electronic expansion valves,

low charge evaporators and hermetic compressors would

make it much easier to use ammonia in small systems. In larger

systems there is significant benefit in air-cooled systems if

a method of evaporative cooling is used to lower the dry bulb

temperature in very warm weather. Such systems use a sparge

on the condenser air inlet to pre-cool the air. It would be possi-

ble, with an air-cooled ammonia condenser fitted with such

a system, to arrange for the fans to run in reverse and the

sparge to be activated in the event of a leak on the condenser.

This would prevent the loss of ammonia to the neighbourhood,

although the resultant ammonia solution would need to be

trapped rather than allowing it to run to drainage.

As ammonia-based water chillers are adopted for use in

commercial buildings, there is likely to be an increased use

of ammonia in commercial scale heat pumps. The high latent

heat and high critical temperature of ammonia relative to all

other refrigeration fluids make it particularly suited to the

heating of low pressure hot water (LPHW) systems for building

heating applications, where there has been no natural

successor to R-12 as a heat pump fluid.

The concept of a semi-hermetic ammonia compressor has

been proved in several ways, but at present there is very little

demand principally because most ammonia systems are

installed on site using welded steel pipework, and do not

achieve sufficient levels of cleanliness to permit the use of

semi-hermetics. If the stator is kept out of the refrigerant

flow, for example using a canned motor, then the overall

motor efficiency is low. As ammonia is introduced to new

markets which lend themselves more readily to packaged

systems, for example smaller water chillers, packaged air

handling units for process cooling and heat pumps, there

will be an increased use of factory built, sealed systems. In

this style of equipment there would be no disadvantage in

using a good semi-hermetic design.

6. Implications for the use of ammoniain traditional markets

There is very little motivation to do research on the use of

ammonia in existing applications because it is believed that

it is already well understood, so there is very little to learn.

This is a rather simplistic view, and in fact there are many

aspects of traditional systems that would benefit greatly

from further enhancement. The adaptation of the recently

commercialised technology of electromagnetic bearings, cur-

rently only applied to centrifugal compressors with R-134a

(Pearson, 2005b), would enable oil-free ammonia systems to

be constructed. This could improve the overall heat transfer

performance of air coolers by up to 50% (Shen and Groll,

2003) and would reduce the risk of performance degradation

over time. There would also be significant advantages to be

gained in evaporator design to minimise refrigerant charge.

There are currently no commercial applications of microchan-

nel heat exchangers to ammonia evaporator duties, partly be-

cause the available size of heat exchanger elements is rather

small for the current ammonia market, and partly because

the combination of ammonia, water and lubricant could cause

blockage of the microchannels. It is also likely that ammonia

will lose ground in some of these traditional applications to

carbon dioxide; particularly when the evaporating tempera-

ture is lower than �40 �C. Under these conditions a cascade

carbon dioxide/ammonia installation is likely to be more effi-

cient than a two stage ammonia plant, and in plate freezers

particularly the high heat transfer and low pressure drop

combination offered by carbon dioxide is unbeatable (Pearson,

2005a). There is still a place for ammonia, as the high temper-

ature side of the cascade, where the charge can be greatly

reduced.


7. Conclusion

It seems counterintuitive to suggest that established safety

limits in the industrial sector should be relaxed for the domes-

tic market, but in the case of practical limits for ammonia the

use of a common standard for all sizes of equipment results in

an excessively cautious approach to allowable system charge.

There are grounds for adopting a more suitable approach for

the domestic market, without compromising on safety, in

order to facilitate the beneficial application of ammonia to

certain types of equipment such as air-to-air heat pumps. It

also seems that the application in national legislation of

particularly onerous constraints on the design and use of

ammonia systems does not deliver increased safety. In the

drive for increased efficiency it may also be appropriate to

consider a unified approach to safety legislation, at least at

a European level, to ensure that the best combination of

efficiency, safety, reliability and ease of use is achieved. It

seems that this combination is most likely to be delivered by

increased adoption of ammonia as a refrigerant.

r e f e r e n c e s

ASHRAE Standard 15, 2001. Safety Standard for RefrigerationSystems. American Society of Heating, Refrigeration and Air-Conditioning Engineers, Atlanta.

Anon., 2007. Ammonia as a Refrigerant, third ed. InternationalInstitute of Refrigeration, Paris.

Anon., 2006. Methods of Approximation and Determination ofHuman Vulnerability for Offshore Major Accident HazardAssessment. SPC/Tech/OSD/30. Health and Safety Executive,London.

Anon., 2005. Innovation in Small Capacity AmmoniaRefrigeration Plants, EU Contract No ENK6-CT-2002-30020,Final Report. Institut fur Luft-und Kaltetechnik GmbH,Dresden.

Cotter, D., Missenden, J., Maidment, G., 2007. Contaminants inammonia refrigeration systems. In: IIR Conference: AmmoniaRefrigeration Technology for Today and Tomorrow, Ohrid.

Clodic, D., Chang, Y.S., Pougin, A.M., 1999. Evaluation des fluidesfrigorigenes a faible GWP pour le froid domestique et

commercial, les transports refrigeres et la climatisationautomobile. Ecole de Mines, Paris.

EN-378:2007, 2007. Refrigerating Systems and Heat Pumps –Safety and Environmental Requirements. Comite Europeen deNormalisation, Geneva.

Fairhurst, S., Turner, R.M., 1993. Toxicological assessments inrelation to major hazards. Journal of Hazardous Materials 33,215–227 (Elsevier).

Gigiel, A., Evans, J., 2007. Experience of operating an olderammonia plant and the energy consumption. In: IIRConference: Ammonia Refrigeration Technology for Todayand Tomorrow, Ohrid.

Hrnjak, P., Park, C.Y., 2007. In-tube heat transfer and pressuredrop characteristics of pure NH3 and CO2 in refrigerationsystems. In: IIR Conference: Ammonia RefrigerationTechnology for Today and Tomorrow, Ohrid.

Hansen, S., 2006. Experience with R723 in small refrigerationsystems. In: IIR Gustav Lorentzen Conference on NaturalRefrigerants, Trondheim.

Lindborg, A., 2006. Risk assessment on a large industrialammonia refrigeration system in Central Copenhagen,Denmark. In: IIAR Annual Meeting, Reno.

Documentation for Immediately Dangerous to Life or HealthConsiderations (IDLH). NTIS Publication No. PB-94-195047,1994. National Institute for Occupational Safety and Health,Washington DC.

Pearson, A., 2005a. The Optimisation of Carbon DioxideRefrigeration Systems. University of Strathclyde, Glasgow.

Pearson, A., 2005b. Chillers for Building Retrofit – Low Charge,Leakproof, R-134a. Institute of Refrigeration Conference,London.

Pearson, A., 2007. Designing and operating safe ammoniarefrigeration systems. In: XIIth European Convention onRefrigeration, Milan.

Pearson, A., 2004. Assessment of life cycle climate performancefor chillers. In: IIR Gustav Lorentzen Conference on NaturalRefrigerants, Glasgow.

Palm, B., 2007. Ammonia in small capacity refrigeration and heatpump systems. In: IIR Conference: Ammonia RefrigerationTechnology for Today and Tomorrow, Ohrid.

Shen, B., Groll, E., 2003. Critical Literature Review of LubricantInfluence on Refrigerant Heat Transfer and Pressure Drop.Report 21CR 611-20080. Air-conditioning and RefrigerationTechnology Institute, Arlington.

Tychsen, H., 2003. Comparing R-134a Chillers v PackagedAmmonia Chillers for Air Conditioning Applications.International Institute of Ammonia Refrigeration,Albuquerque.

refrigeration with ammonia

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