• Reliability and safety
• Energy efficiency

The reliability and safety aspects cover many areas in a district cooling plant. Among them, the most critical one from my perspective is proper water treatment. I have chosen water treatment for discussion, because contaminated water was wrought the maximum damage so far and will continue to pose a serious challenge in the realm of cooling towers.

Improper treatment of the water used in cooling towers can lead to the following serious causes:

• Outbreak of legionellosis (the disease produced by Legionella species) that will first affect the health of the district cooling plant operator and, then, the whole surrounding community. It might lead to death of young children and elderly people with weak immunity system and will cause health complications similar to pneumonia in healthy adults.
• Fouling of condenser tubes, which will lead to repetitive surges or shutdown of chiller compressor and, in some cases, the complete loss of the compressor due to repetitive pitting of the compressor impellers.
• Fouling and the growth of algae on the walls of the cooling tower, and precipitation of dissolved solids, dust and silt in the cooling tower basin, condenser water boxes and tubes in all the piping low points, which will lead to rust acceleration and reduced efficiency of the whole district cooling plant. Disinfection of the whole cooling lower and water basin is required when there is algae growth or a deterioration of water quality, beyond the control of the operator.

To address water treatment issues, designers must rely on proven technologies, such as chemical treatment; specify reliable and environmentally friendly products that have short half life; and allow for a full monitoring programmed will full integration with plant industrial control SCADA system.

New and alternative technologies related to water treatment need to be tested in the field on small trial basis prior to their implementation. Some methods, such as ozone treatment, have failed due to various reasons in the field that have, in turn, led to the failure of some centrifugal chillers. The failure of ozone treatment could probably be attributed to the soft water prevalent in the UAE, the lack of technical expertise needed to maintain the equipment, and the lack of a proper maintenance programmed. Indeed, the technology might work well under different circumstances.

Likewise, the use of a basic control package supplied along with dosing equipment from the current water treatment service company is not sufficient for a large district cooling plant. The make up as well as down water measurement using reliable industrial grade water meters is critical for both chemical dosing as well as monitoring of the concentration of dissolved solids in the water. The same is also critical for plant performance and production cost monitoring. The technology involved in the sensing pH value and conductivity and other new instruments that are surfacing in the market allow for proper measurement and monitoring of critical aspects of water.
The monitoring helps, but it needs to be supplemented with a scheduled manual testing and monitoring of water. Daily dip sticks and weekly tests with certified laboratory reports are other key methods of keeping the situation under control. The plant operator needs to have the ability to interpret results and not rely entirely on an external water treatment company.

The other aspect is proper design that allows silt to be accumulated in easily accessible areas that can be cleaned by means of a wet vacuum system. Other technologies, such as side stream filtration or basin sweeper system, can also be implemented, but after a thorough study that will ensure proper implementation.

The proper new or alternative water treatment technologies, such as chemical free solutions, can be implemented after field trials for a sufficient period on a small scale to monitor their effectiveness and the human support behind.

It is important that the people involved in district cooling get down to respecting and prioritizing water treatment, as it is often neglected or given a less of a priority. Water problems perhaps go unnoticed because water is colorless and odorless. The truth is, oftentimes, problems are often discovered after it is too late.

WHAT ABOUT DELTA T?
Another district cooling issue that is relevant from an operations perspective is energy efficiency.  And in this, the most serious problem facing the district cooling industry is low delta T syndrome.  The contractual obligations of district cooling operators are to maintain the supply chilled water temperature on year-round basis, as stipulated in their service agreement.  The clients that are connected to district cooling services, control the return chilled water temperature.

The various district cooling service providers have adopted different approaches in the UAE.  My approach is to serve the client beyond the limit of BTU or energy meter.  This is to control the customers’ comfort while achieving the required delta T.  Others have passed the responsibility to the customers and imposed on them penalty clauses that can increase the tonne-hour charge by as much as 50%.  Yet others have ignored the situation, as if it does not exist.

The implication of low delta T syndrome is that chillers have to operate at part load in order to maintain the supply chilled water temperature.  The chiller auxiliaries – such as primary chilled water pump, condenser pumps and cooling tower – have to run fully, irrespective of how loaded is the chiller.  For example, these auxiliaries represent 0.15 kW/ton-hour, or around 15% of the total plant room power, which is around 1.0 kW/tonne-hour for 100% loaded chiller.  At 50% load, this increases to 0.3kW/ton-hour, and besides the chiller compressor penalty that goes as high as 0.1 kW/tonne-hour for chillers loaded around 50% or less, the resultant plant total efficiency will climb to 1.25 kW/ton-hour or higher.

The other aspect of low delta T syndrome is its implication on the distribution network, hence depleting the ability to deliver the full plant room capacity.  The distribution pumps and chilled water distribution piping are designed for a certain flow linked to the design delta T.  For example, each refrigeration tonne (3.52 kW) would require 1.5 US GPM (0.095 L/s) for 16OF (8.89 OC) delta T.  If customers typically achieve 10OF (5.56 OC) delta T, they would require 2.4 GPM (0.151 L/s) or 50% more flow.  That means if a plant is designed to produce 30,000 TR it can now only deliver 20,000 TR to its customers due to the low delta T.  Perhaps that problem did not surface yet in the UAE because most often the plant room capacity is oversized compared to the actual connected cooling load.

This is a serious technical and commercial problem for any district cooling service provider.  The solution is in the district cooling service provider adopting an integrated approach by seriously looking at the FCUs and AHUs, the air side  control system, control valves and balancing valves, the whole hydraulic system (primary, secondary & tertiary), heat exchangers and their controls, customers’ pumps and hydraulic balancing.