EVENTS AND EXHIBITIONS
INCORPORATING COLD CHAIN could reject 49 kW of heat, significantly reducing the high-stage compressor ' s power demand. This was based on cooling from 97 ° C to 32 ° C, leveraging the 22 ° C wet bulb temperature. To implement this, specific design parameters were analysed:
• A typical evaporative condenser coil consists of 80 circuits. GEA ' s analysis showed that 10 circuits would be required for the desuperheating section.
• The internal heat transfer coefficient for the desuperheating coil was calculated at 750 W / m ² K, a positive indicator for effective heat removal.
While taking 10 circuits from the main condenser section for desuperheating might seem to reduce overall condensing capacity, the impact is mitigated. The condenser ' s initial capacity was 700 kW for a required 600 kW load( 15 % safety margin). By removing 10 circuits, the effective condensing capacity dropped to 611 kW. However, since the high-stage compressor now handles 49 kW less, its required condensing load also reduces( to 546 kW). This still leaves a healthy 12 % safety margin on the condenser, demonstrating that the desuperheating can be effectively integrated without compromising overall condensing performance.
A quick calculation also confirmed the feasibility of heat transfer: with a Log Mean Temperature Difference( LMTD) of 32 ° C and an assumed external heat transfer coefficient similar to the internal( though it would likely be better due to the wetted surface), the calculated overall heat transfer coefficient of 366 W / m ² K significantly exceeded the required 150 W / m ² K, further confirming the viability of the approach.
The implementation of this desuperheating strategy does introduce a small penalty on the booster compressor, as it has to work slightly harder due to increased discharge pressure. However, this is significantly outweighed by the substantial reduction in power consumption of the high-stage compressor, which drops from 107 kW to 96 kW.
This optimisation alone results in a 3 % overall power saving for the plant. While 3 % might seem modest, the return on investment is less than one year. The retrofitting involves relatively simple piping— a discharge pipe from the low-stage compressor to the condenser and back, with two isolating valves and uninsulated, smallbore piping,” says Steyn.
ON-SITE VERIFICATION: THEORY MEETS REALITY
On-site testing confirmed the accuracy of initial desktop studies:
• Mass flow and discharge temperature: Aligned with manufacturer ratings and robust operation, despite slightly lower overall conditions.
• Coil pressure drop: Calculated 10 kPa was remarkably close to the measured 10.96 kPa, demonstrating strong predictive accuracy.
• Desuperheating effectiveness: The actual approach temperature difference was less than 5 ° C, outperforming the assumed 10 ° C, confirming the evaporative condenser ' s excellent desuperheating capabilities. This was also consistent with initial U-value estimations.
" Building on this success, we explored further optimisation by using ambient ground source heat sinks to subcool the high-pressure liquid( HP liquid) exiting the receiver before it enters the evaporative condenser. This technique leverages the typically lower temperature( below 18 ° C) of the makeup water feeding the evaporative condenser.
“ By integrating a heat exchanger that facilitates this subcooling( as seen in the accompanying image of the plant, specifically the heat exchanger located after the receiver), we achieved approximately 8K( 8 ° C) of subcooling on the HP liquid. This subcooling significantly reduces the flash gas generated upon expansion, thereby lowering the volumetric flow requirement for the high-stage compressor. The result is that the highstage compressor can operate at a lower speed or even be downsized," said Steyn.
" The combined effect of desuperheating and HP liquid subcooling reduced total plant power consumption by 6.3 %( from 218 kW to under 205 kW), with the high-stage compressor power dropping to 92 kW. This also reduced the high-stage compressor ' s required swept volume by 14 %( from 578 m ³/ hour to 501 m ³/ hour), potentially allowing for a smaller, less expensive compressor and a lower initial capital expenditure for the customer."
The commencement of the presentation.
Steyn ' s key insights and recommendations for the ammonia refrigeration industry included:
• Evaporative condensers are highly effective for low-stage desuperheating and should be standard in two-stage ammonia plants
• Most existing plants have spare condenser capacity, allowing circuit splitting for desuperheating, which also reduces pressure drop and boosts efficiency
• These optimisations are simple, using existing, proven equipment in intelligent new configurations without complex hardware or controls
• Applicable to any two-stage ammonia plant, these strategies offer fast payback and easy retrofitting during upgrades
• Further optimisation could involve shorter serpentine coils within evaporative condensers for enhanced desuperheating performance CLA
16 www. coldlinkafrica. co. za COLD LINK AFRICA • September 2025