Natural cooling

With the use of refrigeration gases under strict guidance at both Federal and local level in order to reduce their environmental impact, the application of natural gases is increasing. Monika Witt outlines the current developments in ammonia refrigeration.

Natural Cooling
Natural Cooling

With the use of refrigeration gases under strict guidance at both Federal and local level in order to reduce their environmental impact, the application of natural gases is increasing. Monika Witt outlines the current developments in ammonia refrigeration.

Natural refrigerants such as ammonia, propane or carbon dioxide have no ozone depletion potential and zero or negligible impact on the greenhouse effect.

Ammonia has been used in refrigeration technology for nearly 130 years, making it the only refrigerant to have remained permanently on the market since it was first introduced.

This traditional refrigerant has to date been typically used in large-scale refrigeration plants. This has been due to of its excellent refrigeration effect and specific evaporation heat value of 1,262kJ/kg at 0°C, which is second only to water.

In recent years, ammonia has also been used to solve many technical challenges faced by medium and small-capacity systems; these include cascade systems, optimised operation and heat transfer and reduced refrigerant charges.

Reducing consumption

In the context of plant safety, the potential toxicity of ammonia is of major concern. Its development therefore focuses on reducing the plant charge, at the same time limiting the potential presence of ammonia in a cold store due to leaks.

Cascade systems are a possible solution, for example, using both carbon dioxide (R744), potassium acetate or potassium formiate solution as the secondary refrigerant in the second stage.

In each case, the reduction in ammonia charge has no major influence on the plant performance; at the same time, the ammonia is restricted to the refrigerant circuit in the plant room. This solution is also suitable for existing two-stage ammonia cascade systems as these can usually be easily modified.

Developments in the field of compact tubular and plate heat exchanger systems with high efficiency exchange surfaces aim to reduce the quantities of ammonia needed in the circuit through enhanced heat transfer.

For example, tubular heat exchangers can be baffled to better account for the different boiling behaviour across the heat exchanger. This means that the ammonia filling quantity can be reduced by around 80% compared to a conventional bare-tube heat exchanger without any loss in evaporator performance.

In addition, various heat exchanger parameters such as dimensions, plus the number of passes and tubes are far more favourable than in comparable machines, thus saving space and costs.

Further potential is offered by exchanger tubes with inner fins that are tailor-made for ammonia applications. The inner tube geometries and materials have until now been optimised for the use of fluorinated hydrocarbons and not for the needs of ammonia.

Blending options

New application areas and ranges for the use of ammonia also result from using refrigerant blends. New working substances that have low global warming potential (GWP) include non-azeotropic liquid gas blends of ammonia with propane (R290), octafluoropropane (R218), octafluorocyclobutane (RC318) or isobutane (R600a).

Experimental tests have shown that compared to pure ammonia, some of the blends tested have a lower discharge outlet temperature; a lower compression pressure ratio; 5-10% better refrigerating capacity; and better machine oil solubility.

In contrast to these dual-fluid systems tested under laboratory conditions, the refrigerant R723, which has been in practical use for several years, is an azeotropic blend. R723 consists of 60% ammonia and 40% dimethyl ether. Like ammonia, it is used as a working medium in refrigerating systems.

With an almost identical pressure level, R723 offers a series of refrigeration advantages over pure ammonia.

The discharge temperature can be reduced by about 15-20°C compared to ammonia, permitting the use of air-cooled condensers for example, instead of cylinder head cooling fans or water-cooled cylinder heads.

In addition, the lower temperature on the high pressure side avoids the thermal loads for the materials and refrigerating machine oils.

There are also decisive improvements in the oil solubility for mineral oils that can be extended into the low temperature range, while preserving miscibility with synthetic oils. Good problem free oil recovery is thus possible, so that separate oil recovery systems are not needed for such systems.

Heat transfer during evaporation is also improved. There is no negative impact on the efficiency of the refrigeration process from impurities in the gas phase of the dual-fluid blend, such as vapour density, specific heat capacity and evaporation enthalpy. The precautions taken when working with ammonia must be observed.

Using waste heat

Given the steady increase in energy prices, operators are finding it more important to reduce the operating costs of their refrigeration systems by lowering their energy needs.

Cold store operators and food processing firms, for example, will find considerable potential for savings in recovering waste heat to generate hot service water.

Depending on how the systems are designed, operators of ammonia plants that have medium and large refrigerating capacities can more than half their annual heating costs.

Natural refrigerants also provide attractive solutions for heat pump applications. New developments in recent years, such as hermetic scroll compressors, two-stage and 40 bar compressors for ammonia or two-stage centrifugal flow compressors for carbon dioxide, help to improve the energy efficiency.

In addition, new developments in directly evaporating systems that have soluble oil and plate heat exchangers enable reductions of up to 50% in the specific ammonia filling quantity.

Considerable progress has also been achieved for use in domestic applications: initial prototypes of heat pumps with an output of 6kW work reliably with a filling quantity of less than 100g of ammonia.

To date wide-scale market use of such systems has been hindered by a lack of components suitable for ammonia, such as hermetic or semi-hermetic compressors.

Future focus

In the future ammonia will continue to maintain its established position in large-scale refrigeration, especially at evaporation temperatures of more than -33°C. But things are also moving in the middle and lower capacity range, with an increasing number of refrigerating firms being involved in this sector.

This is particularly true for systems that have small charges, where the latest results confirm the high energy efficiency obtained as above that which was previously possible with ammonia as refrigerant.

The progress that has been made clearly indicates a trend towards systems with smaller capacities using ammonia, either as a working substance in its own right or in the form of blends.

Monika Witt is chair of Eurammon, the European initiative for natural refrigerants.

Refrigeration issues: the terms explained

Ozone Depletion Potential (ODP)

The ozone layer is damaged by the catalytic action of chlorine and bromine in compounds, which reduce ozone to oxygen when exposed to ultraviolet (uv) light at low temperatures. The Ozone Depletion Potential (ODP) of a compound is shown as an R11 equivalent (ODP of R11 = 1).

Global Warming Potential (GWP)

The greenhouse effect arises from the capacity of materials in the atmosphere to reflect the heat emitted by the earth back onto its surface. The direct Global Warming Potential (GWP) of a compound is shown as a CO2 equivalent (GWP of a CO2 molecule = 1).

Ammonia (NH3)

Ammonia has been successfully used as a refrigerant in industrial refrigeration plants for over 130 years. It is a colourless gas, liquefies under pressure and has a pungent odour.

Ammonia has a zero ozone depletion potential (ODP = 0) and no direct global warming potential (GWP = 0). Thanks to its high energy efficiency, its contribution to the indirect global warming potential is also low.

Ammonia is flammable and toxic to skin and human mucous membranes. However, its ignition energy is 50 times higher than that of natural gas and it will not burn without a supporting flame. Due to the high affinity of ammonia for atmospheric humidity it is rated as 'hardly flammable'.

Ammonia is toxic, but has a characteristic, sharp smell that provides a notable warning below concentrations of 3mg/m³ ammonia in air. This means that ammonia is evident at levels far below those that will endanger human health. It is also lighter than air, therefore it rises quickly.

 

Most popular

Awards

CW Oman Awards 2020: Meet the winners
A round of the thirteen winning names at the Construction Week Oman Awards 2020 that

Conferences

Leaders UAE 2020: Building a sustainable, 'resilient' infra
AESG’s Phillipa Grant, Burohappold’s Farah Naz, and Samana's Imran Farooq on a sustainable built environment
CW In Focus | Inside the Leaders in KSA Awards 2019 in Riyadh
Meet the winners in all 10 categories and learn more about Vision 2030 in this

Latest Issue

Construction Week - Issue 767
Sep 01, 2020