Dan Mizesko

Most District Cooling plants and buildings in this region and, in fact, worldwide, suffer from some sort of low Delta-T in one form or another. Often, end-users pay penalties to District Cooling companies for low Delta-T syndrome, when the problem may not even be their fault. Also, District Cooling companies are forced to operate at a higher cost than necessary due to low Delta-T, which is not their fault, either. So what is the solution?

Everyone in the HVAC industry needs to know that design conditions occur during one per cent of the operational hours in a year. Therefore, chilled water plants are going to have to operate as efficiently as possible under off-design conditions 99% of their operational life. However, now with the latest technology, it is, in fact, possible to operate more efficiently at light-load conditions versus full-load conditions.

Get it right the first time
The solution to the problem is properly designed, selected, installed, operated and maintained air side equipment and properly designed, selected, installed, operated and maintained chilled water plants and pumping systems. It really is as simple as that. But this seems to be ever so elusive a solution, though easy to achieve. The responsibility lies with the air side people and the chilled water side people. It starts at the conception phase of the project, and it carries on, and is handed over to each phase of the project, and it is everyone’s responsibility from design to construction to commissioning and, finally, to the O&M and service people to maintain the conceptual vision.

Going to the root of the problem
First, let’s look at some common causes of low Delta-T syndrome:
1. Improper coil and control valve selection – Oversized or undersized coils impact occupant comfort and contribute to mismatched chilled water design temperatures.

2. Dirty coils (air and water side) – The dirty nature reduces the overall heat transfer capacity of the coil, which results in the control system opening two-way control valves. This leads to increased flow and decreased return water temperature.

3. Dirty air filters – The dirtiness reduces the overall heat transfer capacity of the coil.

4. Loose belts on AHUs/FAUs – The condition reduces the overall heat transfer capacity of the coil.

5. Laminar coil flow – It reduces overall heat transfer capacity of the coil.

6. Mismatched design conditions – System components designed for different chilled water ΔT can result in lower than plant design return water to the plant.

7. Use of three-way control valves – This allows bypass of chilled water around the coil at part load, resulting in lower return water temperatures for all conditions except design.

8. Low supply air temperature set-point – Arbitrarily lowering the supply air temperature set-point below design can lead to uncontrollable operation and result in lower return water temperature.

9. System differential pressure above valve shut-off – This forces control valves open, leading to unwanted flow through coils, increasing system flow and resulting in lower return water temperature.

10. Coil piping configuration – Coils must be piped such that water is counter-flow to air. If they are piped in reverse, the heat transfer efficiency of the coil will decrease, resulting in lower chilled water return temperatures.

11. Plant chilled water mixing – Flow from chilled water supply to chilled water return through the plant de-coupler occurs at part-load conditions when the primary loop is pumping more than the secondary loop (primary/secondary).

12. Building de-coupler mixing: Flow from chilled water supply to chilled water return through the building de-coupler occurs in similar manner as with the plant.

13. Building air side and chilled water plant controls out of calibration

The good news
The causes of low Delta-T syndrome are primarily related to building operation and, in many cases, cannot be controlled by the chilled water plant operators. However, the chilled water pumping system used and the design of the configuration, as well as chiller selection can significantly magnify the effects of the problem. The good news is, there are measures to mitigate the effects on plant operation and performance, and even improve the situation.

Before I get into the pumping system that would be best for a chilled water plant to mitigate low Delta-T syndrome and improve efficiency, I would like to primarily highlight the improvements in chiller technology over the past 10-15 years. We now have electric-driven centrifugal chillers, which are oil-less and have magnetic bearing. I have covered these chillers in past issues of this column*. However, in summary, these chillers offer the best efficiency in the industry, and with their VFD technology allow superior off-peak performance.

Off-peak efficiency is key to overcoming the ill-effects of low Delta-T syndrome. We can now operate chillers under off-peak design loads and light loads and capitalise on condenser relief energy savings. We can operate chillers at superior energy performance at off-peak loads and take advantage of energy savings due to the compressor VFD. An oil-less magnetic chiller will allow up to six compressors in one circuit, and provides the ability to use the entire heat transfer surface even when using few compressors, thereby ensuring very close approach temperatures and improved efficiency.

The following is a brief list of benefits of oil-less magnetic bearing centrifugal chillers:
• No oil in the refrigerant system
• No oil pump
• No oil heater
• No oil sump
• No disposal issues of waste oil disposable (considered a hazardous material)
• No oil storage requirements
• No oil filters
• No oil and filter cost
• No oil analysis required
• No bearing wear/bearing replacements
• No vibration
• No vibration testing required
• Compressors have VFD technology installed
• Greatly reduced sound
• Exponentially improved efficiency
• Reduced maintenance costs
• Smaller and lighter chiller
• Compressor redundancy
• Water-cooled centrifugal chiller with a range of 85-1,200 tonnes, so it is suitable for any chilled water plant from the smallest to the largest • Chillers can be custom-designed and built to meet any flow requirement (series counter-flow, primary/secondary, variable primary)

Having pointed out the vast improvement in today’s chillers and underscoring that this improvement can be used to directly overcome the ill-effects of low Delta-T syndrome, there is one more important point: Today’s centrifugal chillers can be designed for variable flow in the heat exchangers (evaporator and condenser). This will be highlighted, along with system-flow configurations in the next issue.

* Oil in chilled waters

Dan Mizesko is Managing Partner, U.S. Chiller Services Int, HVAC & Energy Services International. He can be contacted at dan@uschillerservices.com.

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