Ammonia Refrigerant Hazards

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Introduction

The phasing out of CFCs and HCFCs(chlorofluorocarbons andhydrochlorofluorocarbons), and uncertaintyabout the environmental acceptability of thenewer HFC (hydrofluorocarbon) refrigerants,has led to growing use of ammonia in airconditioning systems in buildings. Ammoniahas excellent refrigeration properties and a lowenvironmental impact. The high efficiency ofammonia-based refrigeration systems couldreduce the energy consumption and CO2

emissions of building cooling systems.However, ammonia is highly toxic and mildlyflammable which means that more demandingsafety measures are required than with themuch safer HFC refrigerants and older CFCsand HCFCs.

Until recently ammonia refrigerationsystems were mainly used for industrialapplications, especially in the food processingand brewing industries where the advantagesof higher efficiency offset the higher cost ofsafety and maintenance.

The environmental benefits of the use of

ammonia must be weighed against the highersafety risks. The Health and Safety Executive(HSE) is concerned[1]that deficiencies in theunderlying management of industrial ammoniasystems cause unnecessary incidents leading toinjury, deaths and sometimes widespreadevacuation of the vicinity. HSE is alsoconcerned that wider use of ammonia couldincrease the incidence of such occurrences.

Designers and consultants currently believethat a safe location for ammonia refrigerationsystems is in the open air on a building roof,

based on the premise that because ammonia

vapour is less dense than air it should dischargesafely skywards. This paper shows that this isan incorrect assumption which can lead tounsafe installations, and it gives preliminaryguidance on how to overcome this problem.

The hazards of ammonia:toxicity and flammability

Toxicity

Ammonia is highly toxic. However, itscharacteristic pungent odour makes it readily

detectable by smell at a concentration of

CI/SfB (57)(L7)

Ammonia refrigerantin buildings:

minimising the

hazardsD J G Butler* and D J Hall

*BRE Centre for Environmental EngineeringEnvirobods Ltd

IP 18/00

Ammonia is being

increasingly used as

refrigerant for building air

conditioning systems on

account of its lowenvironmental impact and

excellent refrigeration

properties. However,

ammonia is highly toxic

which means that greater

care is needed in its use.

This paper shows that the

design of safe ammonia

systems must also take

into account dispersal

mechanisms.

constructing the future

inform

ation

paper


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around 5 parts per million (ppm), well belowthe level for toxic effects, giving early warningof its presence and making inadvertent and

voluntary exposure to high concentrationsunlikely. Industrial experience is that exposure

to concentrations between 400 and 700 ppmcauses immediate eye and throat irritation, and

a level of 1700 ppm causes tissue damage andrepeated coughing which can prove fatal after30 minutes exposure. A significant fraction of

the general populace is far more sensitive thanthe working populace, and would show muchmore severe reactions to these levels ofexposure.

The HSEs occupational exposure standards(OESs) have been set to protect workers from

both long-term and short-term effects. The8-hour time-weighted average is 25 ppm and

the short-term limit measured over a10-minute reference period is 35 ppm. Thiscompares with an 8-hour time-weightedaverage of 1000 ppm for the majority of HCFCand HFC refrigerants.

Although HCFCs and HFCs have lowtoxicities they can also be dangerous. Becausethey are practically odourless and are heavierthan air, they can accumulate undetected inconfined spaces. High concentrations can resultin a risk of asphyxiation and other toxic effectsincluding narcosis and cardiac sensitivity.

Flammability

Ammonia is flammable in air at concentrationsbetween 16 and 27%, although ignition is moredifficult than for fuel gases.

Current ammonia safety standards,

codes and other guidance

Current UK safety codes and standards on thesafety of ammonia refrigeration systemsinclude:q

BS 4434:1995 Safety and environmental aspectsin the design, construction and installation ofrefrigerating appliances and systems[2]

q Health and Safety Executive Guidance NotePM81 Safe management of ammoniarefrigeration systems[1]

q Institute of Refrigeration Safety code forcompression systems utilising ammonia[3]

A European CEN standard is in preparation(prEN 378) which will eventually replaceBS 4434:1995. Although these documents give

detailed guidance on the design, installation,commissioning, maintenance and operation ofammonia refrigeration systems, none of themgives guidance on the discharge and dispersionof ammonia.

Accidental releases of ammonia

There is little published information on thecauses and frequency of ammonia dischargesfrom refrigeration plant, or on the rates ofdischarge. The rate of discharge is important asit will strongly influence both the behaviour of

an ammonia leak and the ambientconcentrations that it produces. A small leakfrom a leaking seal or gland may be in the orderof grams per second or lower. A catastrophicrelease, caused by a fractured or severed pipeor from a pressure relief device, could result inmost of the machine charge being released in a

very short time, perhaps at a rate of a 0.1 to1 kg/s. The total discharge is determined by

the machine charge, which depends on thecooling capacity of the machine and its design.

Table 1 gives details of some recently installedammonia chillers in air conditioned buildingsin the UK. Specific refrigerant charge varies

between 0.055 and 0.45 kg/kW of nominalcooling capacity. Clearly chiller selection is aneffective way of minimising the quantity ofammonia used and the consequential hazardsof accidental release.

A crucial feature of ammonia discharges isthe ambient density of the dischargedammonia. Ammonia vapour has a density

between 0.6 and 0.7 that of air (depending onits discharge temperature), so that a cloud ofreleased ammonia will be positively buoyant.Liquid ammonia releases behave differentlyand when released from a part of the plant

which is at high pressure can produce flashingclouds of boiling liquid droplets and vapour.

The cooling effect of the continuouslyevaporating droplets cools the cloud so that itbecomes denser than air. Very large dischargesfrom industrial plant, which sometimes contain

tonnage quantities of ammonia, can therefore

2

Table 1 Ammonia charge of some recently

installed ammonia chillers for air conditioning in

UK buildings

Nominal cooling Ammonia charge Specific charge

duty (kW) (kg) (kg/kW)

250 17 0.068

175 48 0.270

316 30 0.095

1000 55 0.055

200 48 0.240

550 250* 0.450

730 49 0.067

* The high charge was due to this chiller being a

thermosyphon chiller with a flooded shell and tube

evaporator


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be very dangerous. The hazard from themajority of the building systems detailed in

Table 1, which generally have total chargesbelow 60 kg, would be less. Some liquidreleases produce less flashing and can lead torelatively stable pools of liquid ammonia on theground. The high latent heat of ammonia can

mean that the ground cannot supply sufficientheat for rapid evaporation. Sudden input ofheat, such as by spraying with water to clear

the discharge (see Containment of accidentalspillage and leakage of liquid ammonia), wouldcause violent boiling and formation of vapourclouds. The high latent heat of ammonia canalso result in liquid pools remaining in largeplant after refrigerant recovery and evacuation,creating a safety hazard for maintenancepersonnel.

Gas concentration in dispersing

plumes

Earlier wind tunnel experiments[4] have shownthat the highest gas concentration (C) in adispersing plume is given by

C= 9V/ux2 (1)

where Vis the gas discharge rate,uis the wind speed, andxis the distance along the plume fromits source.

This gives the highest likely concentration at agiven distance. In practice such exposure

would be intermittent as plume motion at short

ranges is usually unsteady. No account is takenof plume buoyancy or the influence of thesurroundings (including buildings) on wind

behaviour. Figure 1 shows curves for a range ofplume concentrations plotted as a function ofammonia release rate and distance from therelease, assuming a wind speed of 2 m/s. It is

clear that quite high gas concentrations (abovethe level for toxic effects) are possible atsignificant distances from the point ofdischarge. It should be emphasised thatFigure 1 shows worst-case short-termconcentrations. Practical experiments withammonia discharges are necessary todetermine more typical concentrations.

Dispersing plume behaviour

It is crucial to understand that the buoyancy ofa discharging gas is not sufficient alone todetermine whether the resulting plume rises orfalls.

The plume buoyancy is determined by thedimensionless buoyancy flux

F/u3L (2)

where Fis the flux of buoyancy defined as

F= (V/) g(/) (3)

where Vis the volume discharge of ammonia,Lis the scale of the release (in this case

the height of the building), is the density difference between

the discharged ammonia and ambientdensity , andgis gravitational acceleration.

High values of the dimensionless buoyancy flux(Equation 2) above about 0.10.3, mean that

the plume will have a positive buoyancy. A

positively buoyant discharge (for exampleammonia vapour) will rise, and a negativelybuoyant discharge (for example a heavier-than-air cold ammonia droplet and vapourcloud) will fall. Conversely low values for thedimensionless buoyancy flux, below about0.002, mean that the plume will have neutral

buoyancy and will neither rise nor fall,regardless of the discharge buoyancy. These

values have been determined from previousBRE work on fire plumes discharging from

warehouses (D J Hall et al, 1995, unpublished).

It is clear from these equations that thedimensionless buoyancy flux is proportional to

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Figure 1 Approximate distances to a given predicted ammonia

concentration in dispersing plumes (wind speed = 2 m/s)(Note: 1 mg/m3 1.43 ppm)

0.00

50.00

100.00

150.00

200.00

250.00

0 0.2 0.4 0.6 0.8 1

Ammonia release rate (kg/s)

Distance(m

)

Plumecon

centrat

ion=1

00(mg

/m3)

10

30

300

1000

3000


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the rate of gas discharge and its densitydifference from ambient as well as to the scaleof the dispersion pattern. (For discharges from

building roofs the dispersion scale is taken asthe height of the building.) The dimensionlessbuoyancy flux is also very strongly dependenton wind speed (as u3), so that the highest

levels of the dimensionless buoyancy flux occurat low wind speeds and reduce rapidly withincreasing wind speed.

From the range of possible ammonia releaserates estimated earlier, the range of values of

the dimensionless buoyancy flux that mayoccur in practice can be calculated. Figure 2shows plots of the values of ammonia releaserate and wind speed that give rise to specific

values of the dimensionless buoyancy flux, forpositively buoyant discharges (for exampleammonia vapour). The approximate divisions

between negligible, partial and pronouncedplume buoyancy effects are shown by dashedlines. Also shown are values of the 98thpercentile (20 m/s) and typical annual mean(3 m/s) wind speeds in open country at 10 mheight. In urban areas wind speeds aregenerally reduced significantly below these

values. The scale Lhas been taken as theheight of a typical building, 10 m.

Figure 2 shows that smaller release rates athigher wind speeds show negligible plume

buoyancy effects, while the larger release ratesat lower wind speeds will show verypronounced plume buoyancy effects. Althoughnot shown here, the equivalent effects occur forheavier-than-air releases: the larger releaserates at lower wind speeds will showpronounced slumping of the plume, while theplume will have a neutral buoyancy withsmaller release rates at higher wind speeds.

Wind tunnel visualisations of gas

discharges

The flow patterns of negatively and positivelybuoyant ammonia discharges around buildingshave been investigated using a small-scalemodel (representing a rectangular building of

height 20 m) inside BREs dispersion windtunnel at BRE Cardington.

Figure 3 shows dispersion of neutrallybuoyant discharges from the building roof andan upwind ground level source close to the

building. It can be seen that both dischargeplumes spread over the entire downwind faceof the building. Clearly both locations would beunsuitable for neutrally buoyant ammoniadischarges, as would be the case with smallerdischarges at high wind speeds.

Figure 4 shows flow visualisations ofpositively buoyant discharges from the samelocations. In both cases there was pronouncedplume rise, but when the discharge was atground level the rising plume was spread overall sides of the building, making such a locationhighly dangerous in the event of ammoniadischarges.

Safety guidelines

The requirements of BS 4434:1995[2], theInstitute of Refrigeration Safety Codes[3] andHSE PM81[1], as well as the generalrequirements of the Health and Safety at Worketc Act, should always be complied with. Evenso, the work described in this paper shows thatadditional consideration needs to be given to

the location of ammonia plant and possibleammonia discharges.

Minimise ammonia charge

The hazards of ammonia discharges can beminimised by restricting the amount of

ammonia charge in any single chiller. Carefulchiller design can lead to very low ammoniaspecific charges, and values of around 0.05 kgper kW nominal cooling capacity are possible

with current commercially available chillersusing plate heat exchangers. Note that it is notrecommended to use ammonia in directexpansion (dx) air cooler systems in which therefrigerant-containing parts of the system may

be located in occupied parts of the building (seealso BS 4434:1995[2]).

Location of ammonia chillersThe safest location for ammonia chillers is in aspecial plant room (complying with the safetyrequirements of BS 4434:1995[2] and theInstitute of Refrigeration Safety Codes[3]). The

4

Figure 2 Effect of wind speed and discharge rate on plume buoyancy for

positively buoyant ammonia releases

0.001

0.01

0.1

1

10

100

0.1 1 10 100

Wind speed (m/s)

Ammoniadischargerate(kg/s)

Buo

yanc

yflu

x=10

0

10

1

0.1

0.01

0.00

010.00

1

Typical UK meanwind speed

Typical 98%ile

UK wind speed

Discharge behavesas neutrally buoyantplume

Discharge behaves

as positively buoyantplume


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siting of ammonia chillers out of doors, eitherat ground level or on a building roof, wouldresult in any accidental release or leakage ofammonia dispersing in an unpredictablemanner, depending on the rate of dischargeand prevailing local wind conditions.

The advantages of locating ammonia chillersin special plant rooms is that liquid spills can

be contained and that external discharge ratesof ammonia can be controlled. Discharge could

be through a fan-assisted stack with highvelocity efflux. Specific guidance on stackdesign is given elsewhere (for exampleASHRAE HandbookFundamentals[5]).

An alternative to a special plant room for

smaller systems is a container or box with itsown ventilation system and ammonia vapourdetectors. If designed and built correctly thisshould offer safety comparable to aconventional special plant room but withsignificantly lower cost and weight.

Pressure relief valve discharge locations

It is fairly common for pressure relief valvedischarge pipes to terminate with a 90 elbow

to prevent ingress of water and dirt. However,this results in a horizontal discharge and

therefore destroys much of the verticalmomentum which assists the rise of thedischarging plume. An improved arrangementhas been suggested by Stoecker[6]where therelief valve discharges into a mixing box where

the ammonia is diluted with fresh air and thendischarged through a vertical fan-assistedstack. However, this makes little difference toconcentrations beyond a few metres from thedischarge point and it is better overall to retaina high velocity vertical discharge of ammoniafrom an elevated nozzle as this maximises theplume rise.

The design of the stack or elevated nozzlemust prevent water and dirt ingress. Guidanceon this is contained in the ASHRAEHandbookFundamentals[5].

Containment of accidental spillage and

leakage of liquid ammonia

Liquid ammonia releases tend to remain in theliquid phase for a long time, owing to the highlatent heat of ammonia and insufficient heatnormally being available in the air or through

the ground surface to cause rapid evaporation.This results in low rates of evaporation ofammonia, which is advantageous in avoidinghigh concentrations around the building. Thefire brigade, who tend to be summoned to deal

with large ammonia releases, are trained todrench the area of the spillage with water. Thisis actually potentially dangerous as the water

provides sufficient additional heat to causeliquid ammonia to boil violently and createhazardous droplet and vapour clouds with a farhigher effective rate of discharge. All plant, butespecially when it is located out of doors,

5

Figure 4 Flow visualisations of positively buoyant

discharges from the roof and upwind ground level

sources

Figure 3 Flow visualisations of neutrally buoyant

discharges from the roof and upwind ground level

sources


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should be surrounded by an insulated bund tocontain any liquid spills and reduceevaporation rates . Liquid ammonia should also

be prevented from entering roof-top drains orventilation inlets. Clearly it is safer for the plantto be contained in a special plant room, or boxfor smaller systems.

Conclusion

The phasing out of CFC and HCFCrefrigerants and environmental concerns aboutHFC-based alternatives mean that ammoniarefrigeration plant will be increasingly specifiedfor building air conditioning applications.However, ammonia is highly toxic and issuessuch as location of the plant and safetymeasures require careful consideration. Inparticular it has been shown that the dispersalof accidental releases of ammonia is notproperly understood by the industry, andcurrent practice could lead to hazardousconcentrations of ammonia around the

building which could enter any buildingopening. The safest location for ammonia plantis in purpose-built plant rooms, or boxes forsmaller plant, complying with the relevantstandards and codes (for example BS 4434:1995[2] and the Institute of Refrigeration SafetyCodes[3]). Any accidental release should be

vented vertically above the roof in a controlledmanner, so that it cannot be entrained back to

the building. This may require the use of fan-assisted stacks.

Despite its toxicity ammonia is an excellentrefrigerant, and industry experience has shown

that accidents are rare if plant is designed,installed and maintained strictly in accordance

with existing codes and standards. BREsresearch has shown that it is also necessary toconsider the dispersion of ammonia tominimise the risk of it reaching building

occupants or people on the ground. Furtherwork is needed to determine actual rates ofammonia discharge so that the degree ofhazard from ammonia leaks and discharges can

be more accurately assessed.

Acknowledgements

This work was carried out by BRE withfunding from the DETR ConstructionSponsorship Directorate. The views expressedin this paper are those of the authors and notnecessarily those of the Secretary of State for

the Environment, Transport and the Regions.

References

[1] Health and Safety Executive. Safe management of

ammonia refrigeration systems. Guidance Note PM81.

London, HSE, 1995.

[2] British Standards Institution. Safety and

environmental aspects in the design, construction and

installation of refrigerating appliances and systems. British

StandardBS 4434:1995. London, BSI, 1995.

[3] Institute of Refrigeration.Safety code for

compression systems utilising ammonia. Part 1: Design

and construction. Part 2: Commissioning, inspection and

maintenance. Carshalton, Surrey, Institute of

Refrigeration, 1990.

[4] Wilson D J. Flow patterns over flat roofed buildings

and application to exhaust stack design. ASHRAE

Transactions, 1979, 85 284295.

[5] American Society of Heating, Refrigerating and

Air-Conditioning Engineers (ASHRAE). ASHRAE

Handbook 1997. Fundamentals. Chapter 15. Airflow

around buildings. Atlanta, ASHRAE, 1997.

[6] Stoecker W F.Expanded applications for ammonia:

coping with releases to the atmosphere. CFCs: Todays

Options Tomorrows Solutions. CFC Technology

Conferences, US National Institute of Standards and

Technology, Gaithersburg, Maryland, 2728 September

1989. ASHRAE, 1989.

Further reading

BRE Digest358 CFCs in buildings

BRE Information PapersIP1/94 Minimising refrigerant emissions from air

conditioning systems in buildings

IP16/95 Safety and environmental requirements of new

refrigerants

IP6/98 Performance of air-conditioning systems with

alternative refrigerants

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