One of the recommendations we frequently make to our clients is to move from air-cooled to water-cooled compressors. Air-cooled is the most common type of cooling equipment out there, serving spaces from individual residences, apartment buildings, and large commercial buildings.
In the buildings we work on, we see it most commonly in the form of a Rooftop Unit (RTU). We seem to see RTUs everywhere, but, notably, not on more efficient buildings. Those will almost always have a chiller and cooling tower – the bones of the centralized water-cooled system.
We’ll explain the details of “how” below, but the basic reason that water-cooled equipment is so much more efficient (up to 215% more!) is that it takes advantage of evaporating water, just like the human body does through sweat. If it is so much more efficient, why would anyone install an air-cooled system?
First Cost: RTUs often win (or at least they used to…)
Too often, the decision on which type of cooling system to install is based on the first cost of the unit – but not the lifetime cost of owning and operating the equipment. RTUs are selected because they “cost less” than installing a water-cooled system. RTUs, or “packaged units,” contain all the needed equipment in one module. Thus, they require little engineering to install. Additionally, they have basic controls (thermostats) and do not require skilled in-house staff for operation. Since they are modular, they can be added piecemeal as spaces are built out without needing to consider system-wide impacts. However, “first cost” only takes into consideration the upfront cost of a unit and does not factor in the higher ongoing maintenance costs and increased energy consumption. Additionally, most RTUs only last about 15 years before they need to be replaced. Water-cooled systems typically last 25 – 30 years or more with good maintenance. This often means that one water cooled system can last two lifetimes of air-cooled condensing units.
Full Cost of Ownership
If the total life cycle costs of the equipment options are considered, then the water-cooled system has a significantly lower cost of ownership. The figure here shows the installed, annual electrical costs, and life cycle replacement costs for both types of systems designed for a 200-ton load on a 120,000 SF building. The first costs are: the air-cooled is $250,000; and the water-cooled system is $340,000. With a price tag that is roughly 1/3 more expensive than the air-cooled system, it is easy to imagine why an owner without good information on life-cycle costs might choose air-cooled systems. However, the water-cooled system will quickly pay for itself. In the St. Louis climate, for example, a typical water-cooled system will have more than made up for the additional cost by the middle of year four. Based on a summer electrical rate of $0.10/kWh, the annual operating budget is reduced by $20,000. The $90,000 premium first-cost thus has a 23% ROI. Over a period of 30 years, that initial premium is only 4.8% of the total cost of ownership of the air-cooled system, and the life-cycle cost of the water-cooled system is only 58% of the air-cooled system. Every dollar invested in energy efficiency (in first costs) is repaid more than eight times over the life of the equipment. Depending on the run-time of your cooling systems, and the climate conditions, the savings can be fifteen times the investment.
These days, with energy efficiency financing available through PACE or other third-party lenders, that extra price tag for the more efficient system should not be an obstacle. The lending costs are overwhelmed by the savings generated, and you’ll have lower costs from day one.
System Differences, the Maintenance Perspective
All types of conventional HVAC equipment will have parts that will require regular inspection and maintenance. In the two-hundred-ton case study being used, it is likely that the air-cooled system is comprised of five, forty-ton RTUs, each serving 24,000 SF. This means five compressors, five furnaces or heating coils, five sets of condenser fans, five condensing coils, five evaporator coils, etc. The central plant will have one chiller, one cooling tower, two chilled water pumps, and two condensing water pumps that provide the chilled water to five, forty-ton air handling units that deliver the cooling to 24,000 SF each. As a general rule, with more pieces of equipment, more maintenance will need to be performed, so the RTU system is likely to have significantly higher annual maintenance requirements. If maintenance is not conducted frequently enough, then the performance of the units may be significantly impacted.
There are also specific cooling related maintenance requirements: air-cooled condensing units have coils that, ideally, require cleaning multiple times throughout the cooling season. Water cooled compressors typically reject their heat to cooling towers that evaporate water in this process. This evaporation process requires makeup water that will require on-going water treatment. Biocides are typically added to prevent fouling; this is typically done through automatic feeders and is usually handled by an outside vender. The other water quality issue is scaling from hard water. As water evaporates the total dissolved solids (TDS) build up in the water loop and require periodic “blow down,” which is the dumping the water with a higher TDS concentration and replacing it with “clean” water.
These two processes need oversight but are often automated and only require a few hours monthly. Water cooled condensers should have their tubes cleaned annually. This work is often scheduled during slow times in the off season. If the building owner does not want to employ maintenance staff for this work, it can also be contracted out. Some cooling towers have basin heaters to prevent the water from freezing during the winter months. However, for most buildings the chillers can be shut down and the towers are drained during the winter months. Any required cooling during the winter months is better provided through outdoor air economizers – free cooling using cold outdoor air.
There are also different space requirements. Often chillers are placed indoors, either in basement or penthouse mechanical rooms. RTUs are placed closest to their loads, which means scattered across rooftops. In multistory applications, this requires allocating chases to provide for supply and return ductwork to reach lower floors from the roof. On balance, there is less maintenance for cooling towers and chillers than for RTUs, though the maintenance of chilled water systems requires a broader familiarity with mechanical systems.
How is water cooling more efficient?
There are two primary reasons why water-cooled systems are more efficient than air-cooled systems. The first reason comes down to the basic physics of the two heat transfer media: air and water. The specific heat of water is 1.0 BTU/lb.-°F, and the specific heat of air is 0.24 BTU/lb.-°F. Therefore, the system needs to move four pounds of air compared to one pound of water to transfer the same amount of heat that is being rejected. As a result, this means more fan power to move the air than pump power is needed to move water. The second reason is that compressors on air-cooled systems must create higher head pressure. Air-cooled condensing units and RTUs are commonly placed on rooftops. When units are located on the roof of a building they are in direct sunlight. This raises the sensible temperature around the unit by 10 – 15°F. On the other hand, a cooling tower, using water, depends on the wet-bulb temperature, which is often about 10° cooler than the dry-bulb (sensible) air temperature. This means that the effective air temperature between these two systems can differ by more than 20° under the exact same outdoor weather conditions. At the higher temperature, the compressor on the air-cooled system will have to create higher head pressure to effectively reject heat.
For example, consider equipment using R134a refrigerant in St. Louis. The average high temperature for July is 90° with 70% relative humidity. Under these conditions, the wet bulb temperature will be about 82° whereas the ambient temperature for the air-cooled unit may be 105°. This means that the compressor on the water-cooled system will need to produce about 90 PSIG and the air-cooled compressor will need to produce 135 PSIG to reject the heat in the system. A typical evaporator pressure is around 37 PSIG. This means the compressor on the water-cooled system needs to produce a 53 PSI pressure differential and the air-cooled system needs to create a 98 PSI pressure differential. Thus, the air-cooled system needs to produce nearly twice the pressure differential which requires nearly twice as much energy.
Energy efficiency for a cooling system is often measured in kilowatts per ton (kW/ton). That is, how many kW of electrical power are needed to produce the equivalent cooling of one ton (2000 pounds) of ice (the measure of the ton goes back to the days when cooling was accomplished through ice delivery). Air-cooled condensing units typically have an efficiency rating ranging from 1.13 – 1.25 kW/ton. A conventional water-cooled unit with a cooling tower has an energy efficiency rating ranging from 0.58 – 0.79 kW/ton. Therefore, water-cooled systems can produce the same amount of cooling as an air-cooled system while only consuming about half the energy.
Do Air-Cooled Chillers and RTUs have a place?
Yes, they do! First, air-cooled systems are a good option when hydronic cooling is needed through the winter. This happens in certain primarily industrial uses. Ideally, they are put in as winter-only use when they can even be slightly more efficient than water-cooled systems. Air-cooled systems can also be the best choice when they are put in as a backup of more efficient systems. In this case they have few run-time hours and can provide system resilience at lower cost.
They can also make sense when your system is not very large. It is difficult to find a water-cooled system smaller than 80 tons. This means that chilled water systems are not often found on buildings less than 50,000 square feet (while small systems can use the ground for heat exchange, this can be impossible given site conditions, and is a subject for another day). This is a function of the way building complexity and maintenance increase with scale. If the system is small, it takes longer for the energy savings to justify the initial first costs of the additional engineering and the number of systems. The real maintenance benefits of chilled water systems also grow as the number of air-cooled systems being replaced increases.
RTUs may also make sense if the system is temporary, or where modularity or mobility is more important. However, installing a temporary system can be a slippery slope; once one RTU is installed they tend to stick around longer than intended and multiply. We recommend that if an RTU is temporarily installed that a plan also be put in place for when and how it will be integrated into the centralized cooling system. In such circumstances, RTUs can readily be replaced with air handling units with chilled and hot water coils (or gas fired furnace modules) that “plug” into the existing piping infrastructure.
It should also be mentioned that the air-cooled vs water-cooled argument varies by region. Factors such as water cost, full-load cooling hours, humidity, and energy costs will impact the efficiency and economical operation of a cooling system.
