ANYONE who has read my column, License to Chill over the past many years knows I have been a huge proponent of “chiller retro- commissioning/re-commissioning and non chemical water treatment. To be frank I have been a proponent of non- chemical water treatment since 1992, when I was an engineer for the Kuwait Oil Company and saw just how scarce potable water was, and how destructive and damaging chemical water treatment was. So, I was always looking for a non- chemical alternative to chemical water treatment that prevented corrosion, scale and biological contamination.

It was not until 1999, when working in California for Siemens Building Technologies as Director of Mechanical Services (Chiller Services) that I came across Pulse Power Non-Chemical water treatment at an industry conference and discovered, without question, its positive impact on any chilled water plant that uses the technology and its huge environmental and water savings impact. (A side-note: My team and I specified and installed the Pulse Power technology at the ISF District Cooling plant in Qatar; since then, it has been recognised not only by Kahramaa but internationally, as well, as a means to saving substantial water and power, and as a major contributor to reducing hazardous discharge into the Qatar wastewater systems. The Qatar ISF plant operates below 0.7 kW per ton (total plant load, including chillers, towers, all pumps, lighting, elevators and plug loads. This is due to two major contributors.)

Okay, having stated the above, I would like to highlight the following before I get into the condenser water flow topic.

We at US Chiller Services (USCS; now called Dalkia US Chillers) pioneered, and have developed and implemented centrifugal chiller retro- and re- commissioning services since 2003 and have seen firsthand the savings from retro-commissioning recently commissioned chillers and much older chillers. No matter what the age of the chiller, retro-commissioning has delivered from between 15% and 50% savings, with 30% being the norm. I also introduced Non-Chemical Pulse Power water treatment to the region, and we
at USCS (now called Dalkia US Chillers) have successfully installed over 150,000 tons of non-chemical water treatment. I wish it was more, and hopefully, the industry will further embrace the technology, as it would be a massive benefit to not only chilled water plant owners but also to the entire region.

All right, now let’s get to the topic at hand. In previous License to Chill articles, I had explained why most chillers operate above their design efficiency. One major issue is the chiller heat transfer surfaces and how they significantly influence a chiller’s efficiency. Please allow me to summarise, as follows…

A chiller’s efficiency is affected most by its resistance to heat transfer, the Leaving Temperature Difference (LTD) of its heat exchanger tube surfaces, more than anything else. So, it is imperative that the chiller’s condenser tube surfaces have the lowest resistance to heat transfer possible. Another term would be Approach Temperature.

LTD is the difference in temperature between the saturated refrigerant temperature and the leaving water temperature.

The temperature difference between the refrigerant and the fluid is the driving force to overcome heat transfer resistance. Resistance to heat transfer consists of four components related to the overall heat transfer coefficient (U):

■ Refrigerant film resistance
■ Tube wall metal resistance
■ Fouling deposits resistance
■ Fluid Film resistance at 10FPS velocity

The effect of Water Flows on tube’s resistance to heat transfer – Fluid flowing through a tube forms a static film or boundary layer, which has zero velocity at the tube wall. This film acts as an insulator and hinders heat transfer.

The lower the velocity, the thicker the boundary layer becomes, which increases the resistance to heat transfer.

Fluid tube velocities should be kept between 3 FPS and 12 FPS. Velocities less than 3 FPS result in laminar flow with thick boundary layers, dramatically increasing the fluid film resistance. Fluid tube velocities more than 12 FPS increase tube erosion and should be avoided. For erosion to occur, an agent must penetrate the fluid layer. These agents may be chemical, mechanical or a combination of both. Chemical agents attack the tube, and mechanical agents cause damage by impingement of entrained gas bubbles or suspended materials.

I still am often asked by consultants and plant owners to investigate implementing Variable Condenser Water Flow for them at their chiller plants, and more often than not I try to dissuade this from being implemented. The efficiency of a centrifugal chiller depends on design flow rates, and any deviation from these design flows can lead to inefficiencies, including increased energy consumption and reduced tonnage production.

Low condenser Flow/Variable condenser water flow
If the condenser flow rate is below design, the saturated refrigerant will not be cooled, leading to higher condenser temperatures and pressures. This will lead to increased compressor power, thus increasing chiller energy consumption. Variable flow in the condenser is not recommended by some chiller OEMs, as it generally raises the energy consumption of the system by keeping the condenser pressure high in the chiller. Although reducing the condenser flow will Improve the cooling tower LMTD – and a smaller tower can be used – the savings from this strategy will not offset the increased cost of the chiller’s increased power consumption. In addition, if implementing this on an existing plant that has been designed for 3 GPM operation, cooling towers typically have narrow ranges of operation with respect to flow rates and will be more effective with full design flow versus the lowered flow.

Another major consideration not generally mentioned by proponents of reduced condenser water flow is that the rate of fouling in the condenser will increase at lower water velocities associated with variable flow. This will not only increase maintenance costs but also increase power consumption tremendously.

My recommendation is if you want to employ variable condenser water flow, you ought to reconsider this, as the potential drawbacks – such as reduced heat transfer efficiency, control complexity, system compatibility as well as chiller performance issues – are a major concern. Not to mention the increased condenser and tower maintenance as well as chiller and tower reliability concerns.

My advice is that you stick to the NPLV/IPLV design condenser flows, and you will have the most efficient chiller you can have – that is, after retro-/re- commissioning those chillers!

Dan Mizesko is President, Dalkia US Chillers, USA. He may be contacted at dan.mizesko@dalkiasolutions.com