Professional sports teams perform many hours of analysis prior to drafting new talent. They study film and assess the skills of potential draftees to make decisions that will fit their team needs.
The evaluation process for choosing equipment mimics the athletic world. Selecting the proper chiller for an application presents many variables. The most obvious considerations are temperature range and cooling capacity, or sizing the chiller appropriately. Less obvious but equally important are:
• Materials of construction.
• Pump type.
• Flow rate.
• Flow pressure.
• Internal bypass.
• Communication options.
• Power-failure mode.
• Warranty/service support.
Making the correct choices based upon these options can provide the proper fit for years of trouble-free operation.
Avoid Chiller 'Sticker Shock’
Many times, end users select a product by perusing the chiller specifications from manufacturer literature and think the equipment is suitable as long as it matches the cooling capacity and temperature range needed for the application. But, buyer beware! That is not always the case.
Chiller cooling-capacity specifications are determined under ideal testing conditions. Manufacturers apply a known external heat load in a closed loop and measure the chiller cooling capacity at various temperatures. The external closed loop typically has a small external fluid volume — not at all like a full-scale application installation. These cooling-capacity tests do not mimic real-world situations. To be safe, it is best to size up the chiller by adding a safety factor of 25 percent. This will account for environmental losses in the real-world application and ensure adequate cooling capacity at the point of use.
Always use insulated tubing or piping for the installation to minimize any environmental cooling losses. Consult with the chiller manufacturer to outline the application and setup. Be prepared to provide the following information:
• Cooling capacity needed.
• Temperature range required.
• Heat load of the application.
• Piping type, inner diameter and length, including elevation changes.
Completing the sizing exercise with the manufacturer will help ensure selection of the correct chiller model and eliminate the sticker shock of purchasing an incorrectly sized chiller based only upon published specifications.
The Right Stuff: Materials of Construction
The automotive industry has replaced steel body and door panels with plastic for years to save weight and increase durability. However, the use of plastics in chillers should be eyed with caution. This really does not matter from an external standpoint; for chillers, it is what is on the inside that counts.
Some chillers utilize plastic internal bath tanks and synthetic tubing. Two factors can affect longevity of plastic tank durability: temperature fluctuation and volume. The plastic can become brittle if the application temperature fluctuates regularly from -4 to 104°F (-20 to 40°C) or more over time. Additionally, internal tank volumes of 7.92 gal (30 liters) and higher necessitate plastic tanks with thicker walls. The combination of temperature fluctuations and the mass of internal bath fluid can stress the plastic tank, which can lead to cracking. Internal synthetic tubing can stress at fitting junctions with the potential to split or loosen on barbed fittings. Choosing a unit with an internal stainless steel bath tank and tubing will avoid any such issues.
Pump It Up. The two most common chiller pump types are positive-displacement and centrifugal. Positive-displacement pumps include gear and turbine pumps, which typically are installed inline. This eases serviceability because the pump is installed between the heat exchanger and chiller outlet. Gear pumps are excellent for high-pressure applications to 100 psi or more. However, care must be taken when using these pumps as they are susceptible to particulates, so an inline filter might be required. Chillers with gear or turbine pumps should have a pressure-relief bypass to eliminate damage or potential tube bursting if the flow path becomes blocked.
Centrifugal pumps typically are installed inside the chiller bath tank. The pump provides external flow and circulates bath fluid in the internal chiller tank. Centrifugal pumps can provide greater than 40-psi pressure and are not as sensitive to particulate matter. Discuss the application flow rate and pressure needs with the chiller manufacturer.
But Not Too Much. Multiple pump options and pump capacities look great on paper. However, many large chillers only have large-capacity pumps. What happens if a 20 kW chiller with a pressure of 30 psi is needed but the single-speed pump is rated to greater than 80 psi? Can the pressure be dialed back?
Look for a chiller with an internal pump-pressure adjustment. This feature enables the operator to dial down the external supply pressure to a level that is acceptable for the application. Because the remaining flow diverts internally into the chiller bath tank, no damage will result to the chiller pump or the external application.
In many processes, a chiller is not the most expensive piece of equipment. If the chiller breaks down, will your process stop? Often, the situation becomes critical to get the application running as quickly as possible. Chiller warranty and service support from the manufacturer should be factored in with any purchase decision. Planned maintenance programs, on-site service options and loaner programs can save a lot of time and money when implemented proactively. Confer with the manufacturer during the chiller purchasing process to discuss all the options and capabilities.
In conclusion, careful consideration and consultation of the points discussed in this article with a chiller manufacturer should bring success when selecting a chiller. And, unlike draft-day busts, properly specified chillers will produce positive results from the day of installation. With proper maintenance and service, the chiller will function at a high level for many years.