What Is Maximum Chiller Efficiency?

Contrary to popular belief, running the chiller at full load design and achieving the design kW/ton does not necessarily mean the chiller is running at maximum efficiency. Maximum chiller efficiency is producing the greatest tonnage at the lowest kilowatt usage.

Maximum efficiency occurs with most chillers running at approximately 70 percent to 75 percent load and the lowest entering condenser water temperature (ECWT), based on design. Knowing a chiller's efficiency and the effects of load and ECWT will help the facility determine the most efficient chiller configurations, saving the maximum on energy costs.

Document Chiller Data

The first step in maximizing chiller efficiency is to establish a method for recording chiller operational data in a daily log (Figure 1). It's common for facilities to maintain chiller logs, but unfortunately they rarely get reviewed, which is critical.

The daily logs can be entered into a chiller efficiency program, such as Efficiency Technologies Inc.'s Web-based EffHVACâ„¢ chiller efficiency tool (Figure 2). EffHVAC is designed to accurately measure chiller performance at full and partial loads, calculate efficiency, and diagnose the causes of inefficiency. If past chiller logs exist, the data can be entered and a baseline can be immediately established. Once the chiller status (baseline) has been determined, operational changes can be made to increase efficiency and measure the results.

Ensure Accurate Data

Ensuring accurate data can be difficult. One of the most common assumptions made by a facility is that the flow to the chiller is constant and always at design. Unfortunately, this may not be the case and there are several reasons why.

Chiller systems are dynamic, ever-changing models, which must adapt to the environment around them. They expand and contract from the original design. They are subject to wear, tear, and age. The best advice is don't assume anything until proven by accurate, continuous verification.

The best way to provide precise data, obtain concrete results, and minimize problems is to verify flow rates to the chiller for tonnage measurements and other calculations to determine efficiency. Four methods for determining flow are inline flow meter, external flow meter, delta pressure, and delta temperature.

Flow meters can be a high-quality turbine type, magmeter (inline) or ultrasonic (external), and give the most accurate gallons per minute (gpm) flow readings. The gpm can be determined by delta pressure using a gauge or annubar. Delta temperature cannot actually measure the flow rate in gpm, but it can identify proper flow and problems associated with flow.

It can also be affected by other conditions not directly related to flow, such as a scaled or fouled chiller barrel, non-condensable gases, and refrigerant level, making interpretation more difficult.

However, the use of delta temperature along with a flow meter or delta pressure gauge creates a powerful diagnostic tool that can detect problems affecting efficiency in the chiller system.

Along with proper flow, check and calibrate temperature sensors/gauges, pressure sensors/gauges, electrical meters, etc., periodically or when a problem is detected.

Increase Chill Water Temp, Lower ECWT

For constant speed chillers, every 1 degree F increase in chill water temperature can increase chiller energy efficiency 1 to 2 percent. For variable speed chillers, every 1 degree F increase in chill water temperature can result in a 2 to 4 percent efficiency increase.

However, it may not be possible to increase the chill water temperature to save money due to design constraints, occupant comfort levels, or real-time energy pricing (sacrificing efficiency at one time to improve the efficiency at another time).

Take advantage of wet-bulb conditions in the cooling tower system to lower the chiller's entering condenser water temperature. This can result in a 1 to 1.5 percent efficiency improvement for every 1 degree F below the chiller full load design.

It is important to note that part loads associated with chiller type (high or low pressure) and compressor motor style (constant or variable speed) will affect the chiller's performance. Consult the chiller manufacturer to establish appropriate guidelines for entering condenser water temperature.

Aggressive Water Treatment Programs

A good water treatment program is a necessity for efficiency. Maintaining the proper water treatment will prevent costly problems. If a problem already exists, take the necessary steps to correct it immediately. The results can provide significant energy savings with greater chiller efficiency, maximized equipment life, and reduced overall maintenance costs. Remember, always wear appropriate personal protective equipment (PPE) when using chemicals or cleaning equipment.

Biocide and scale/corrosion protection: A water treatment program provides a biocide program that minimizes microbiological growth along with excellent scale/corrosion protection.

Microbes, if not properly controlled, can cause numerous problems, such as forming sticky slime deposits in the tube bundle of a chiller, possibly reducing heat transfer efficiency 15 percent or more. The situation can be compounded by the formation of permanent scale or iron deposits on the sticky site.

If this occurs, an additional 10 to 20 percent loss in heat transfer efficiency may result. To fix the problem and restore lost efficiency, an unscheduled shutdown and physical cleaning of the chiller may be required. Furthermore, if no action is taken to improve the water treatment, underdeposit corrosion may occur throughout the condenser system, which may create leaks in the transfer piping.

Cooling tower cleaning and lay-up: Cooling tower system cleaning is essential for peak efficiency. A good time to consider cleaning is fall and spring, just before and after winter lay-up.

This usually means part or all of the condenser system may be dormant for several months. Dead legs (no circulation/stagnant water) (Figure 3) in the condenser system are potential areas for producing many types of microbes.

One type of anaerobic bacteria of particular importance is sulfate reducing bacteria (SRB). SRB can cause significant localized pitting corrosion and severe damage in a relativity short period of time. Treating these areas of a condenser system with biocides and biodispersants prior to lay-up can help minimize microbial problems. Lay-up treatments also ensure an easier startup in the spring, minimizing maintenance problems.

A lay-up treatment is designed to protect the equipment and piping by reducing pipe chip scale (flash corrosion). This chip scale or flash corrosion can have a serious impact on startup, causing blockage of distribution holes on the tower hot deck, plugged strainers, and in extreme cases, blockage in the chiller. Any of these problems will reduce flow and heat transfer efficiency in the condenser system.

When cleaning the tower basin (Figure 4), all debris should be removed (i.e., sand, silt, trash, and, most importantly, biofilm). Biofilms are home to many living organisms. Some of the more common organisms include Pseusdomonas slime, which can reduce heat transfer efficiency, and SRB.

Tower cleaning should also include inspection of the drift eliminators, fill, and louvers to minimize airflow restriction across the cooling tower system. Make sure the tower fans are working properly to produce the desired airflow for heat transfer removal.

Visually inspect the wood and metal construction, looking for signs of deterioration (Figure 5). Wood deterioration may be a sign of microbiological problems (mold, yeast, or fungi) or overfeeding the oxidizing biocide, causing wood delignification/deterioration.

Look for white rust on the metal construction caused by either a tower that was never properly pretreated and passivated or a chemical program that may not fit the water chemistry. A thorough spring cleaning can assist in maintaining maximum efficiency throughout the summer months.

Pretreatment: Pretreatment is recommended for a new system (condenser, evaporator, and tower system), or when there is a new add-on to an existing system, to ensure heat transfer efficiency and prolong equipment life. The purpose of pretreatment is to remove oil and grease from new piping and chillers.

If pretreatment is not performed, the oil and grease may adhere to the heat exchanger, reducing heat transfer. Oil and grease can also provide food for microbes to bloom, requiring additional costly biocide treatments. Pretreatment should passivate the new metals and minimize white rust and flash corrosion.

Galvanic corrosion: Galvanic corrosion is associated with dissimilar metal coupling and can exist in all areas of the HVAC system (though it primarily occurs on the condenser side of a chiller), and if severe enough, can affect the life of the chiller.

Metal passivating chemicals commonly used in the evaporator minimize galvanic corrosion. Most chillers have copper tubes with carbon steel tube sheets and end bells, in which a galvanic reaction can occur between the copper and carbon steel. Installing sacrificial anodes and painting the inside of the chiller end bells and tube sheet with an epoxy coating can also minimize this corrosion.

Preserve Design Flow Rates

Maintain condenser and evaporator design flow rates, checking them annually. A rule of thumb is to always maintain flow greater than 90 percent of design because lower flow will reduce chiller efficiency. When the flow is reduced or restricted, it can create undesirable laminar flow (less than 3 feet per second) through the chiller, which can also cause a water treatment program to fail.

Above design flow (greater than 12 feet per second) through the chiller may cause vibration wear and erosion/corrosion of the tubes, reducing reliability and life. Cracks and pitting holes can develop, causing leaks in the tube bundle.

Noncondensables

Noncondensable gases (air) are associated with low-pressure chillers with evaporators designed to use refrigerants that operate in a vacuum. When a leak develops in the evaporator, air and moisture are pulled in, which affects the compressor and reduces heat transfer efficiency.

The compressor is working to move the noncondensable but getting no refrigerant effect. In fact, noncondensables can blanket tubes in the condenser, lowering the overall efficiency up to 6 to 8 percent at 60 percent load and 8 to 14 percent at full load. To help minimize the effect of noncondensables, purge units are required.

Refrigerant Levels

The ability of a chiller to efficiently remove heat directly correlates to the compressor's ability to move the refrigerant per unit of time. It is important to maintain proper refrigerant levels because low levels cause the compressor to work harder and less efficiently. Check for leaks regularly, especially when a chiller shows signs of low refrigerant level. Trending refrigerant levels will help determine if the chiller has a leak(s), a bad purge unit, or refrigerant carryover.

Refrigerant analysis: Regular refrigerant analysis is an important part of determining chiller inefficiencies. If oil content in the refrigerant is above the chiller manufacturer's guidelines, it may be reducing heat transfer. Keeping good maintenance records on oil usage in a chiller will help to avoid this condition.

Preventive Maintenance

Compressor oil analysis should be performed annually. Low-pressure chillers may require more frequent analysis, based on purge run hours. This test should include a spectrometric chemical analysis containing information on metals, moisture content, acids, and other contaminants that can affect chiller performance. Replace oil filters on an as-needed basis on high pressure drop or when the compressor oil is changed. Consult your chiller manufacturer, lubricant supplier, and/or oil analysis laboratory for oil and filter change intervals.

Monitor Refrigerant Approach Temperature

One of the earliest signs of chiller inefficiency is an increase in refrigerant approach temperature (RAT). The RAT is determined by calculating the difference between the leaving fluid (water) and the saturated temperature of the refrigerant being heated (evaporator) or cooled (condenser).

Newer chillers or chillers that have been retrofitted perform this function. Older chillers may require taking the suction pressure (evaporator) and head pressure (condenser), then converting these pressures to temperature from a refrigerant temperature/pressure table.

Every chiller has a manufacturer design RAT. When it is exceeded, a problem with heat exchange in the chiller exists. Problems associated with high RAT include low refrigerant level, noncondensable gases, low/high flow rates, part loads at low ECWTs, and finally, a scaled or fouled chiller.

Don Clark is with Efficiency Technologies Inc., a Tulsa, Okla.-based company that develops energy efficiency programs for commercial/industrial HVAC systems. Clark has over 24 years of experience in the fluid dynamics, water treatment, and chiller industries. He can be reached at 866-333-8321. For more information, visit www.efftec.com.

Publication date: 01/31/2005