Chiller Efficency

Chiller Maintenance for Efficiency

Chiller plants represent up to 53% of the total energy used in a building and consume large amounts of electricity.  Chillers represent a substantial capital investment for our building and are a major contributor to operating costs in commercial buildings. For many buildings, chillers are the largest single energy users, and comprehensive maintenance is critical to ensure their reliability and efficient operation.

Building Facility Manager’s work with their contractors to operate at high standards of preventative, predictive and proactive maintenance in solving their chiller plant problems.  In addition to the daily, weekly, monthly and annual service requirements needed for their chillers, managers often include predictive maintenance in their maintenance plan to maintain the central plant. These predictive maintenance steps include vibration analysis, infrared thermography, rotor bar testing and eddy current testing. Each test helps to diagnose problems in advance. All preventive maintenance (PM) is geared to ensure the high performance and reliability of the chiller plant for everyday use.

Here are five simple things you can do to ensure a sustainable chiller plant.

1. Daily logs – keeping a daily log lets you trend data to see when things are going bad. What is the correct lube oil temperature for your chiller when it is not running? A typical Trane centrifugal chiller would have an oil temperature of 120 -140 degrees when it was idle to keep refrigerant from migrating to the coldest area of the oil sump. When it is running, oil temperature would be from 90 to 100 degrees. In addition, the reading would tell you if you had a failure in the heater. In addition, oil readings can tell you when you have non condensibles in your condenser or dirty tubes. How, you ask? Take the refrigerant condensing temperature and the saturation temperature of the refrigerant and subtract them to figure out your approach temperature. Consult your manufacturer and see what the readings should be. Then you know when to clean your condenser tubes. If machine operators notice a gradual increase in condensing pressure during a month’s time, they can consult the daily operating log and systematically check and correct the possible cause of this condition, such as fouled condenser tubes or non-condensables. Non condensibles are gases that will not condense (turn back into liquid such as nitrogen oxygen and inert gases that are trapped in your condenser from air entering from leaks). Your purge system takes these gases out if you don’t log your purge time it may need to be serviced or your condenser is building up with these gases causing improper operation.

2. Condenser tube cleaning - is the biggest factor in energy savings.  Dirty condenser tubes in a chiller impede heat transfer and as a result, decrease efficiency and consume large amounts of electricity. Centrifugal chillers operate with a large amount of tubes that carry water through them. When they get dirty, the heat transfer becomes less and the machines experience high head pressures in their condensers and it is detrimental to the compressors and lessens their life cycle.

It is essential that they be cleaned at least annually, if not more, depending on the operating conditions of the cooling side (cooling towers). Air cooled chillers require that their condensers be cleaned as well to prevent high head pressure on their condenser sides.

The way the condenser tubes should be cleaned is through removing the condenser heads and tube brushing the tubes to remove dirt, mud, algae, sludge, scale or contaminates that accumulate in the condenser tubes. Each manufacturer recommends a procedure for cleaning and it should be followed closely. In some cases, scaling will have occurred from high heat transfer and a high conductivity in the cooling water. Conductivity meters are installed on cooling towers to keep the level of conductivity below a micromoho per centimeter level that is acceptable to protect your chiller. Many times, no one checks to see if it is working properly and as a result chillers are scaled up badly. Badly scaled chillers require, in some cases, chemical cleaning to remove the scale. Consult with a reliable chemical treatment company to discuss your needs and hourly and daily check your conductivity meter for proper operation. The price of re-tubing a new chiller is costly and can be avoidable.

3. Leak free Chillers – Low pressure chillers operate below atmospheric pressure and can suck air into their condensers causing high operating conditions. We log our purge units daily and can see when a machine is starting to leak. This is accomplished by the purge counts that are counted on our purge equipment. The purge machine located on top of the condenser routinely pumps out air and non- condensibles gases out of the condensers. Air trapped in a condenser will become trapped otherwise and cause high pressures in our machines, increasing compressor power requirements and reducing overall efficiency and cooling. Low pressure chillers operate below atmospheric conditions and have dedicated purge units installed on them to maintain designed condenser pressure limits. Alarms are also installed to help identify problems. It is estimated that a one pound increase in pounds per square inch (PSI) equated to a three percent loss in chiller efficiency. 

On top of that, moisture coming into the chiller through the entrance of air causes the formation of rust and acid inside the machine and causes deterioration of motor windings and the corrosion of condenser shells.

4. Stop Scale and dirty tubes – hire a water treatment company to help you with your plant. Some systems require more frequent treatment and monitoring of the chemical injection system. Over treatment and under treatment can cause severe problems for you. If scale is present, the treatment company may suggest chemical removal of the scale. Be very cautious when de-scaling utilizing chemicals. In most causes acid will be used and if not monitored correctly will cause thinning of the copper tubes and cause potentials for rapid tube failure.

Proper maintenance of cooling towers is needed to prevent harmful scale, dirt, sludge and other containments from entering your water system. In most cases it requires a three day weekend to shut down the plant and thoroughly clean the towers. It is not uncommon to find ferns or trees growing in the tower. A tree was once found in a cooling tower that had grown there for five years.  It was 9 feet tall and no one thought it was a problem.

            

Clean your towers and check their operation daily, weekly, monthly, quarterly, semi annually, and annually. Conduct your preventative maintenance and keep your towers clean. Clean all strainers and check filter pumps to ensure a long and trouble free system.

5. Oil and refrigerant analysis – should be conducted annually when you do the annuals on your condenser for cleaning. Oil and refrigerant analysis can help you to detect problems with your chiller. Testing is done by spectrometric chemical analysis to determine contaminants, moisture, acids, and metals in your systems.

Spectro-chemical Analysis  identifies the type and concentration of wear metals such as iron, aluminum, and lead, copper, tin, zinc, and other metals in the system. When concentrations of a certain type of metal are found it can help detect  where in the machine the problem is and it can be resolved. For example, a high concentration of iron could be bad bearings in the compressor.

Refrigerant analysis determines also the amount of oil present in the system. Refrigerant likes to migrate to the coldest part of the machine and most times that is in the oil sump. Heaters keep the oil from becoming contaminated and should be checked daily when you log the oil temperature of your machine. One chiller manufacturer estimates that a 2 percent loss in chiller efficiency for every 1 percent oil found in the system. It is not uncommon to see as high as 10 percent oil in the older chiller refrigerant systems.

Water analysis of the oil helps identify when water is present and the Total Acid Number (TAN) measures the acidity of the oil. A high TAN value indicates a moisture concentration or abnormally high operating temperatures in the refrigeration system. Viscosity of the oil verifies that the correct oil is in the machine and that it has the proper lubricating values to maintain a proper operating machine.