Chillers: Repair or Replace
An HVAC chiller is not only the primary consumer of energy within a building's systems, but it is often the building's most costly piece of equipment. While chillers are typically reliable and can last for many years, that reliability and longevity require a rigorous maintenance program that includes regular inspections and scheduled maintenance activities. Even with the most diligent maintenance program, however, time will eventually take its toll. The question is, how do you know when it's time to replace that chiller or when continuing repairs and maintenance is the better answer?
The very idea of having to replace a chiller raises the blood pressure of many a building owner, engineer, and maintenance manager. It's easy to understand why:
Installing a new chiller can take a very long time: often 12 or more months from the time management makes the replacement decision to when the new chiller is installed and operational.
It can be disruptive to the entire building and its occupants: the piping system may require modifications, walls and ceilings may need to be opened or removed.
The building may have a critical need for uninterrupted cooling; if it does, management will have to arrange for the installation of temporary chilling systems.
These reasons alone are enough for many to opt for continued maintenance of an existing chiller, and that's even before they consider how much the purchase and installation of a new chiller will cost.
Of course, one should always proceed with caution when making decisions of this magnitude; however, replacing the chiller can be the more prudent and cost-effective approach. This is particularly true if the existing chiller could suffer a failure that leaves the building without cooling for any length of time. Even without a catastrophic failure, however, the arguments in favor of chiller replacement can be strong.
Technological advances made over the past 20 years have significantly improved chiller operating efficiencies, which can mean substantial savings in energy costs--savings that will certainly help offset the cost of a new chiller. Twenty years ago, chillers typically carried full-load efficiency ratings of 0.85 to 1.00 kW per ton; ten years ago, they ranged from 0.7 to 0.85 kW per ton. Today, a high-efficiency centrifugal chiller carries a rating of about 0.50 kW per ton, and some even reach 0.40 kW per ton. This means that a new chiller operating at full load may require only about 60% of the energy of the 20-year old chiller—and that does not even take into account the normal decrease in efficiency of the older chiller that results naturally over time.
Of course, in most applications, chillers operate at full load less than five percent of the time. Still, the full-load ratings are a good indication of the savings that might be realized through chiller replacement; even a moderate improvement in operating efficiency can produce significant savings, especially in buildings in which the chiller runs more than 2000 hours per year.
Over the typical chiller's operating life of around 20 years, a building can undergo any number of changes: it may have different occupants performing different activities during different operating hours. It may have new, energy efficient roofing, and old, leaky windows may have been replaced with modern, high R/low U factor windows. All of these changes affect the building's cooling load. If the load has increased, the existing chiller may now be undersized, and the building may need—or already be running—supplemental systems in some areas. If the load has decreased, the chiller may be oversized, which means it operates for more hours but at lesser loads, and this typically results in a significant decrease in its efficiency.
Installing a new chiller, or system of chillers, that better matches the current load profile can lower both energy and maintenance costs.
Reliability and Maintenance of the Existing Chiller
Over time, chiller components wear, scale accumulates, and chiller outages become more frequent. A good maintenance program can forestall these effects, but eventually chiller reliability—and efficiency—will suffer.
Perhaps the most important element of a good maintenance program is the chiller's operating log. The log tracks data from the chiller's operations, including the current and voltage of the drive motor, temperature of the condenser water supply and return, oil pressure and temperature, and evaporator and condenser pressures. The log is also the place to record all maintenance activities, including when oil is added and inspections performed. Regular reviews of log information eliminate the guesswork about how the chiller has been performing. They help you spot negative trends in system performance and reliability, such as more frequent breakdowns and the need for more unscheduled maintenance activities.
While you are evaluating the chiller's current and future maintenance costs and requirements, keep in mind that over time, replacement parts for older chillers can become harder to find and take longer to acquire. The longer it takes to get the necessary replacement parts, the longer the chiller system's downtime may be.
Operating Hours and Applications of the Chiller
Some cooling applications are more critical than others are; the impact of a chiller outage can range from the mild discomfort of the building's occupants to serious, heat-related equipment failures. Take an inventory of the activities performed in the building and their cooling requirements. Develop a simple scale to compare the impact of a chiller outage on each of the activities. Once you have rated each one, note how many there are that have a crucial need for cooling. A large percentage activities with critical cooling needs could make chiller replacement—or the installation of backup cooling systems—the better option.
Sometimes a decision to replace the chiller is perfectly clear. If your chiller is 20 or more years old, operates for at least 1000 hours per year, and services critical loads, it's probably a good candidate for replacement. On the other hand, it may be obvious that maintaining your current chiller is the better option: for example, if a review shows that the 10-year old system is well matched to the building's load and has a good maintenance record.
In some cases, however, the pros and cons may seem to balance and the answer to the replace-or-maintain question remains elusive. Perhaps the existing chiller is the right size and its reliability is not at issue, but it is far less efficient that a new one would be. Then again, perhaps the chiller's efficiency is not in question, but the system requires more maintenance and is suffering more frequent outages. These situations can put other options on the table, such as retrofitting or adding equipment to improve the system's efficiency at partial loads. In all of these cases, making the right decision will require more in-depth analysis—perhaps by an independent engineering firm—of the building's current and projected load profile and the chiller's maintenance log, life-cycle costs, and energy consumption.