Protect Your Cooling Coils!

Engineered Systems, March, 2001 by Arthur Hallstrom

Protecting chilled water coils in colder weather isn't as easy as it used to be. Fortunately, several alternative strategies remain, from air blenders to glycol to preheat coils and beyond. Survey the possibilities and consider what might work best for your system.

Chilled water coils are widely used in air-handling systems because of their flexibility, exceptional capacity, and system efficiency. But if left unprotected, they can freeze.

Before ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 62 was adopted, one of the most popular chilled water coil freeze protection solutions was simply closing off the outside air. However, ASHRAE Standard 62 significantly increased required outside air ventilation rates. Now, flow-measuring outside air dampers are used to verify compliance with ASHRAE Standard 62. If the outside air dampers are closed during occupied operation, the building automation system (bas) alerts building operators that adequate outside ventilation air is not coming in and that indoor air quality will suffer.

What's the solution? Keep the fluid in the cooling coil from freezing so that the right amount of outside ventilation air can be brought in at any outside air temperature.

This article presents the latest tried and tested alternatives available to hvac engineers to provide outside ventilation air and avoid freezing the chilled water coils. These solutions have varying costs and effectiveness. Choose the one that is best for your application.

USING ENHANCED MIXING DAMPERS

The standard mixing box generally offers minimal mixing (see Figure 1). Air handler manufacturers can slightly improve the mixing box effectiveness by increasing the air velocity through the dampers. Damper velocities around 1,500 ft/min mix somewhat better than lower velocity dampers. Another option is to reposition the two dampers closer together or to interlace the outside and return air streams.

Advantages: Inexpensive. Low pressure drop.

Disadvantages: Mixing dampers have very limited effectiveness when compared with other alternatives. If we assume that a standard mixing box stratifies, or creates freezing air at the cooling coil at an outside air temperature of 26[degrees]F, the enhanced mixing box alternative could work with outside air in the 12[degrees] to 23[degrees] range. The exact performance is dependent on the enhanced mixing box design.

USING AIR BLENDERS

Fixed angle "swirl" blenders can be placed just after the mixing box to improve mixing of the outside and return air streams (see Figure 2). The blender uses about 0.2 in. of static pressure to swirl the air and helps mix the outside and return air streams. There are many types of blenders with varying effectiveness. The better ones require the space of one-half the blender's diameter upstream and of one blender diameter downstream. Typically, blenders allow the outside air to be 25[degrees] to 40[degrees] degrees colder before the preheat coils need to be turned on. This option is generally used with outside air designs between 10[degrees] and 25[degrees].

Advantages: Does not require maintenance or moving parts. Works effectively and consistently.

Disadvantages: Air blenders need to be designed based on outside and return air damper positions. They increase unit length and have an operating cost penalty but are considered "middle of the road" among the seven options. First cost is typically higher than preheat or enhanced dampers, but less than energy recovery.

ADDING GLYCOL TO THE CHILLED WATER

Glycol can be added to the cooling system water to lower the water freezing point. The coil freezestat can be reset downward to reflect the new fluid freezing point.

Advantages: Effective, fairly easy to maintain, and always "active." Offers internal pipe corrosion protection. Works well with variable-air volume systems.

Disadvantages: Glycol is expensive to purchase, resulting in high first cost. Glycol also reduces the density of the water, increasing pumping energy and chiller operating costs and it increases the potential for leaks at pipe joints. If glycol leaks inside the air handler, it can remove the zinc coating from a galvanized drain pan allowing it to rust. (This can be prevented by using stainless steel coil casings, supports, and drain pans.) The glycol type and the concentration level determine the effective temperature of this option.

DRAINING THE COOLING COIL

Provide drain and vent connections on every coil as well as shutoff valves to isolate the coils from the chillers. In the fall, drain the coils and then use compressed air to remove as much water as possible. Add glycol to prevent any remaining water from freezing. Finally, disconnect the freezestat to prevent nuisance trips.

Advantages: Low first cost, no increased energy use, works in any climate. Works well with variable-air volume systems.

Disadvantages: There is a high maintenance cost because the coils must be drained in a timely fashion. In climates with wide swings in outside air temperatures below freezing and into the cooling mode, the coils would have to be drained several times in intermediate seasons. It could even require coils to be drained on a daily basis. This is an added labor expense and there is the risk that it may not get done in a timely manner.