As Mr. Parsnow states, open-drive centrifugal and screw chillers represented over one-third of the large-tonnage chillers sold in the U.S. in the 1990s. That’s a 50% market-share increase from the 1980s. Chiller buyers and specifiers must have good reasons for switching to open-drive chillers in such large numbers, and they do.
- Reliability of heavy-duty ODP motors (open, drip-proof), with their simple cooling and lubrication systems.
- Accessibility and serviceability of open motors, without the need to disassemble the compressor.
- Less downtime for routine service, because the motor is not tied to the refrigerant circuit.
- Lower cost and less downtime in the event of motor burnout, which is considered a catastrophic failure in hermetic systems.
- Proven energy efficiency of open-drive designs.
- Inherent flexibility of open-drive systems for resizing and retrofit, parallel-drive and hybrid-drive applications.
So, a lot of owners, engineers and contractors are moving to open-drive chillers, including systems below 2,000 tons of refrigeration, and with good reason.
Now let’s examine five specific issues raised in the April 26 article:
1. Mr. Parsnow implies that open motors have an average life of only 10 to 15 years. On the contrary, with normal maintenance, all large-tonnage chillers should experience a useful service life of 20-25 years. ASHRAE pegs life expectancy at 23 years, with no differentiation between hermetic or open drive. There is, however, a substantial difference in motor operation, which we believe favors the open design.
At low-load operation, adequate motor cooling is an issue with hermetic systems because less refrigerant is being circulated to the motor windings. Thus, all hermetic chillers need and have high-temperature motor protection. Open, constant-speed chillers get the same amount of cooling at both full-load and part-load conditions.
2. Mr. Parsnow states that open motor-compressor alignment is a field-engineering challenge. This might be true on chillers with an external gear, such as shown in the picture accompanying the April 26 article. However, open motors on York factory-packaged chillers up to 2,200 tons use a flange arrangement whereby the motor casting is bolted directly to the compressor casting (see accompanying photograph). No field alignment is required.
The flange provides factory alignment of the compressor and motor shafts, eliminating concern about thermal growth from either component; the two work as one unit. If the motor ever needs to be removed from the chiller for repair, alignment is automatic when the motor is re-bolted to the compressor.
3. Mr. Parsnow states that open-drive chillers tend to have the highest leak rates in the industry. In fact, chillers from all the major manufacturers in the late 1990s are designed, built and tested to be leak-free. Today’s open-drive chillers are as leak-tight as both semi-hermetic and full-hermetic designs.
Leaks can occur in any chiller at pipe connections, screw-thread connections, and gasketed areas on compressor castings. York engineers have systematically reduced the number of potential leak points in all our chillers.
Specific measures include:
- Reducing length of piping and number of fittings;
- Replacing pipe-thread connections with face-seal fittings;
- Replacing compression-fit connections with brazed connections; and
- Utilizing “low-leak” valves.
Further, EPA’s acceptable chiller leak rate of 5% per year is well above field experience with all types of chillers. All new chillers have leak rates of less than 1%, with no difference between open and hermetic drives.
4. Contributing to this leak-tight performance are vastly improved open-drive shaft seals. Today’s shaft seals are made of advanced materials that are better lubricated throughout chiller operation. Positive-pressure chillers are equipped with double seals that also keep the seal faces moist during shutdown. The oil used to lubricate the seals has had the refrigerant previously removed. The bottom line is, refrigerant loss through open-drive chiller shaft seals is negligible.
5. Mr. Parsnow states that open-motor chillers generate more ambient heat than hermetic chillers, and implies that is a serious concern. All chiller motors generate heat. An open motor obviously adds heat to the equipment-room ambient, as does other equipment such as pumps and air-handling units.
Local codes already require sufficient equipment-room ventilation to ensure operator comfort. ASHRAE 15 Safety Code for Mechanical Refrigeration has a similar provision. The heat generated by the open-drive chiller is simply factored into the ventilation calculation.
What about the heat generated by a hermetic motor? It must be absorbed by the chiller and exhausted to the cooling tower. This lowers chiller capacity or increases the chiller-system power requirement.
In summary, we find Mr. Parsnow’s article lacking in both expertise and objectivity. It appears to be based primarily on obsolete Carrier chiller technology and outdated thinking about open-drive chiller operation and maintenance.
Robert W. Schmitt Marketing Manager