By Andrew Perks
Following the first part (published in RACA Journal September edition), we continue to look at what we can do about climate change.
Challenges still to be overcome
The biggest challenge facing ammonia is the large volumes of the refrigerant in some installations, such as Rosslyn Breweries where there are some 60 metric tonnes of ammonia on site. One of the drawbacks in the past has been the lack of smaller-capacity equipment, restricting industrial ammonia to the medium-to-large commercial sector of the market. Also, chasing after the most energy-efficient applications for major central plants has resulted in large volumes of ammonia being circulated around site.
The challenge in the industry is to reduce the refrigerant charge, which will most likely result in small modular packaged units dotted all over the site, which will drastically reduce the system charge.
Several methods are being investigated, including central plant systems, but with direct expansion (DX) evaporators, plate heat exchangers (PHE), or individual package systems spread throughout the site, using condensing units/evaporator assemblies at the service points. These can be flooded or DX. Based on this design concept, the ammonia charge can easily be reduced by 75% and, compared to a traditional pumped system, further reductions are feasible. There are three charge levels being investigated: ultra-low charge (< 50g/kW), low charge (< 0.5kg/kW) and regular charge (< 5kg/kW). These are very low charges by current standards in South Africa.
The PHE condenser requires a cooling tower and the PHE evaporator will require a secondary heat-transfer circuit. While the condensing side of the system will have very little impact on the overall efficiency of the system, there will be an impact on the evaporator side as the evaporation temperatures will need to be lower to get the required cascade heat transfer.
The intrinsic cost of an ammonia and CO2 installation will always be higher than the other refrigerants that use copper piping. More specialist installation techniques and equipment will be necessary for CO2 due to the higher operating pressures than that of ammonia. However, this must be considered when the superior thermal efficiency, absorbed powers and operational costs are considered. Initial project costs amount to only 20% of the project overall lifetime operating expenses.
What different stakeholders can learn from each other and how they can support each other
- The overall efficiency and plant operating costs really relate to the networking between the key contributors to the project:
- Involve refrigeration design at the design stage with the architect;
- Think about heat recovery for the buildings;
- Consider possible recycled water for condenser supply – reduce water usage;
- Consider possible chemical-free water treatment;
- Think about equipment locations to limit pipe runs thereby limiting the system charge;
- Think about equipment access early stage and
- Think about site limitations (power/water).
There are major advancements in technology taking place, as was recently noted at the 2019 Institute of Refrigeration (IOR) conference in Ohrid, specifically on NH3 and CO2 Refrigeration Techniques. The main drive is going to be training as the old principles are being redefined with the view on safety. This will involve safe handling, safety equipment and detection equipment.
With these smaller system charges, water becomes more important in effectively absorbing any ammonia being released via wet walls and dosing sprays, thereby negating the effect of the ammonia on the surrounding areas.
With the latest advancements in technology and electronics, the need for clients to get involved in the plant’s operation is reduced, and can be handled at arm’s length. After all, the client is only looking for cold air, not to become a refrigeration engineer. We need to involve the client at an early stage and look at their plants’ uses and needs. Waste-heat recovery is a big potential saving for the client. There are plants where the condensate draining from the evaporators is fed back to the condensers to supplement water make-up and heat is recovered from the oil-cooling system to give low-grade heat for use throughout the site.
The big question is to devise a water-wise cost-effective energy-efficiency solution, which really is where the new trends will lead us.
The solutions being proposed here are primarily based around ammonia, but other refrigerants should also be considered, especially CO2. Ammonia is not the only solution, but it is a well-accepted and widely known competitively priced refrigerant/medium. Due to ammonia’s and CO2’s vast possible applications throughout the world it is accepted that they will be around for some time to come. Currently in South Africa the technology around CO2 is still in its infancy, but that is changing.
Specific projects need to be identified and engineered considering the next generation of all refrigerants. New innovated component designs are coming onto the market specifically with a view of permitting ammonia to operate as a direct expansion system. This, coupled with evaporator and new condenser designs, indicates ammonia and CO2 will be entering new application fields. As always, training will be key.