Last month (May 2021 issue of Climate Control Middle East), I explained why the vast majority of chillers are operating above their design efficiency. I highlighted that the lack of rigorous start-up and commissioning was one of the causes. This month, I would like to look at another cause, the chiller heat transfer surfaces, and how they significantly influence a chiller’s efficiency.

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 evaporator and condenser tube surfaces have the lowest resistance to heat transfer possible. (Another term would be the 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):

1) Refrigerant film resistance

2) Tube wall metal resistance

3) Fouling deposits resistance

4) Fluid film resistance at 10FPS velocity

As this article is related to chillers operating in the field, the O&M and the Chiller Specialist Field Engineer influence three of the four components related to the overall heat transfer coefficient, and keeping it within design. These are: Fouling deposits resistance, refrigerant film resistance and fluid film resistance.

Fouling deposits resistance – tube fouling Waterside tube fouling can be classified into the following categories:

1) Particulate

2) Precipitation

3) Corrosion

4) Biofouling

5) Chemical-induced corrosion

CONDENSER TUBE FOULING

In HVAC applications, fouling of the chiller condenser tubes substantially impacts the power consumption of the centrifugal compressor. This is why the focal point of any water treatment programme should be the prevention of deposition. A layer of scale that is 1/64 of an inch in thickness on the condenser tubes can increase electrical usage in a centrifugal chiller by as much as 33%. Biofilms can decrease condenser heat exchange efficiency to an even greater degree.

Thus, it is of paramount importance to ensure that chiller heat exchange surfaces are maintained in a clean, deposit-free condition. Fouling occurs because cooling water contains minerals such as Calcium and Magnesium that precipitate to form deposits on heat transfer surfaces. Cooling water systems are also commonly plagued by biological growth that forms slime (biofilm) or algae on heat transfer areas. Additional foulants include mud, silt, sand, corrosion products and petroleum products, which are ingested into the cooling tower. A 1,000-ton cooling tower will ingest over 3,000 pounds of solids in one year. Some of these solids will inevitably make their way to the condenser tubes. All of the foulants, mentioned above, will reduce the heat transfer efficiency of even the best-designed heat exchangers.

EVAPORATOR TUBE FOULING

Evaporators are designed to foul at a much lower rate than condensers, as they are located in the closed chilled water loop, which is generally clean and tight. However, I have come across many chiller evaporators that have been the victim of severe fouling – and in some cases, they are as bad as the condensers. Why is this? Sodium Nitrite, mixed with Sodium Borate, is a standard offering among chemical sales companies in the GCC region. However, Nitrate treatment is a poor choice for closed systems for several reasons. Firstly, Nitrate is an environmental toxin. Secondly, it is aggressive to copper and brass. Thirdly, at levels above target concentrations, it hardens rubber gaskets and forms abrasive crystals at evaporation sites, wearing off seals and valves. At levels below target concentrations, it accelerates corrosion rates, making it worse than having no treatment at all. Fourthly, Nitrate is a ready source of food for the microbes that cause fouling. In addition, many chilled water loops are not flushed and treated to proper protection levels.

In the United States, I have seen Glycol added to stagnant loops for freeze protection in northern cold climates. When the cooling season was approaching, the loops were not properly drained, and the Glycol caused evaporator fouling issues. Additionally, many water treatment programmes on the condenser and chilled water side often do not consider the operational aspects of the treatment programme and the programme’s mechanical elements, leading to tube fouling. Evaporator tube fouling requires the chiller to consume excessive energy, just as condenser tube fouling does.

REFRIGERANT FILM FOULING – REFRIGERANT SIDE FOULING

Oil in the refrigerant causes evaporator tube resistance to heat transfer. Failure to control excessive oil buildup in a chiller’s refrigerant can badly impact capacity and efficiency. Oil has an excellent affinity for refrigerants, especially when the refrigerant is cold. Oil enters the chiller’s refrigerant charge, at the time of its circulation through the chiller’s compressor. The oil is used as a lubricant for the bearings, oil-pump, labyrinth seals, shaft seals, etc., and it eventually seeps through the compressor seals and becomes entrained in the refrigerant charge. On getting into the evaporator, the oil mixes with the refrigerant and degrades system efficiency and capacity. This occurs when the evaporator tubes become coated with oil, creating a thermal barrier. The heat transfer efficiency is retarded and drastically reduces the cooling effect.

Although it is common knowledge that oil buildup occurs, the impact on the system’s capacity and energy costs are not fully understood. I have highlighted a few studies on the importance of this, so that chiller owners and service contractors can recognise and address the problem. ASHRAE conducted a study, titled ‘Effects of Oil on Boiling of Replacement Refrigerants Flowing Normal to a Tube Bundle, Part I: R-123 and Part II: R-134a’. The study concluded: “Flow boiling results have been obtained for the low-pressure refrigerant, enhanced boiling tube in the presence of R-123. This enhanced tube shows a marked decrease in heat transfer with the addition of even a small amount of oil throughout various heat loadings. Even at 1% to 2 % oil, the heat transfer coefficient is reduced by one-third from its no-oil baseline. At substantial oil content (5% to 15 %), a 40% to 50% reduction (in heat transfer) is noted.”

Part 2 of the ASHRAE study arrived at a similar conclusion: “Flow boiling results have been obtained for a newer enhanced boiling tube with R-134a. This enhanced tube shows a decrease in heat transfer with the addition of even a small amount of oil throughout various heat loadings. Even at 1 percent (by weight) oil, the heat transfer coefficient is reduced by 25 percent from its no-oil baseline. At higher oil content, a 30 percent reduction has been typically measured.”

A major OEM has also studied the effects of oil on chiller efficiency. According to the manufacturer, the oil necessary to lubricate a chiller has the potential to contaminate the refrigerant, degrading energy efficiency. CFC chiller designs typically allow 3-7% oil absorption, increasing operating costs by up to 15 per cent. Oil, as a contaminant, significantly impacts chiller efficiency. The more oil contaminates the refrigerant, the more is the efficiency that is lost and, consequently, more is the money that is spent on energy. If the refrigerant charge in a chiller contains even 3.5 per cent oil, it could mean up to an eight per cent loss in efficiency, which will impact operating costs.

ASHRAE conducted a study, titled ‘ASHRAE Research Project 601-TRP’. In the study, refrigerant samples were taken from 10 operating chillers and analysed for oil content. All contained excess oil in varying amounts, from three per cent (enough to degrade performance) to 23%. This condition increases energy consumption drastically, thus increasing a chiller owner’s electric bill. Also, the system loses a significant amount of capacity, and a harder working system increases its potential for breakdown.

Evaporator tubes and sheet fouled with corrosion, Iron Oxide and particulate matter

FLUID FILM RESISTANCE: WATER FLOW’S EFFECT ON TUBE’S RESISTANCE TO HEAT TRANSFER

Fluid flowing through a tube forms a static film or boundary layer, which has a 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 in excess of 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.

Condenser tube and sheet fouled with biofilm, scale, corrosion, mud and sand

OTHER PROBLEMS THAT CAN CAUSE HEAT TRANSFER ISSUES

Low refrigerant charge

When the chiller’s charge is low due to improper commissioning procedures, improper service procedures, refrigerant leaks or excessive purge operation, the refrigerant level will drop, uncovering evaporator tubes. When the tubes are not covered in the refrigerant, they lose the refrigerant’s heat-absorbing effect and do not transfer the evaporator water BTUs to the refrigerant. This will cause the evaporator LTD to increase and the chiller to lose efficiency.

AIR AND NON-CONDENSABLES IN THE CHILLER

Air and non-condensables can enter the system with improper service procedures, including improper evacuation and dehydration, as well as a low side leak on a low-pressure chiller. The air and non-condensables can blanket the tubes, insulating and eliminating them from performing any heat transfer work. Air and non-condensables lower the efficiency of the chiller by as much as 8-14%.

Next month, I will cover other factors that reduce chiller efficiency.

Dan Mizesko is Managing Partner/ President, U.S. Chiller Services International. He may be contacted at dmizesko@uscsny.com.