Value Engineering Is No Bargain
Penny-wise and pound-foolish. It's an old maxim that sometimes can be used to describe management's attitude toward ammonia refrigeration systems, and it's one that often gets them into trouble.
Many refrigeration system maintenance problems are rooted in something called value engineering. That's where decisions are made to spend less money up front even though that decision may cost more money in the long run. Here are some examples.
Sometimes, a digital capacitance level indicator and controller is deleted from a chiller installation. This component does a couple of things: It indicates the refrigerant level in a chiller and functions as a high level alarm and shut down. Without it, an operator cannot see the liquid level during startup. Here's what can happen. If the liquid feed valve develops a slight leak, overcharging the chiller, the overcharge does not reach the high-level float switch. When an operator starts the screw compressor, liquid can carry over into the compressor, providing the owner with $50,000 problem to correct. The compressor must be replaced; a capacitance-type level indicator must be added; and the liquid infeed valve must be repaired. Now, the operator can see the refrigerant liquid level, avoiding the likelihood of the problem recurring.
Lubricants often are the victims of value engineering. There are many ammonia refrigeration compressor lubricants on the market, and most of them are pretty good. However, none of them can do any good if the lubricant escapes from the compressor and foul stage separators. Modern day screw compressors have excellent coalescing lubricant separators. However, they cannot separate a lubricant vapor from a refrigerant vapor.
Why be concerned with lubricant carryover? It fouls evaporator surfaces, decreasing system capacity and increasing energy consumption. Eventually, the lubricant collects in the lowest temperature portion of the system and must be removed. Operators can spend more than an hour draining 1 gal of lubricant from a recirculator oil pot.
Only a few lubricants sold for ammonia refrigeration systems have a low vapor pressure. This is key because low vapor pressure lubricants, when used in either screw or reciprocating compressors, reduce carryover at least 10 to 15 times. But, low vapor pressure lubricants cost about 20 to 25% more than other lubricants, so sometimes value engineering strikes them.
Here's another situation. Industrial evaporative condensers require some kind of water treatment or conditioning just about everywhere in the United States and around the world. Calcium and similar elements dissolve in most water supplies, and these elements plate out (scale) on the condenser's coils. This scaling occurs at different rates, depending on the compressor discharge temperature and the amount of dissolved solids presemt in the makeup water. Condenser fouling normally is less severe in screw compressor applications than reciprocating compressors due to the screw compressor's lower discharge temperature.
Three ways to minimize scale on evaporative condenser coils exist:
- Employ a blowdown process or a dumping process using a total dissolved solids (TDS) meter.
- Employ a water treatment system.
- Use both. This combined approach usually is recommended by qualified water treatment companies.
Well, probably it's either one or a combination of scenarios: The condenser may have a lot of scale. The system may contain noncondensibles. The design wet bulb temperature may have increased because of irrigation, additional heavy industry moving in or increased population. Or, simply the condenser may not have been properly maintained.
The source of the problem will determine the best solution. The scale problem can be corrected with proper water treatment. Noncondensibles can be removed by employing an automatic purge system. With a rising wet bulb, adding condensing surface is probably the only solution to keep energy costs down.
Proper condenser maintenance centers around keeping the coils fully wetted by checking the spray nozzles once a month. The water pump suction strainers may need attention more than once a month, depending on how many June bugs, chicken feathers, leaves or paper have gotten into the condenser fan. Keeping the fan belts properly adjusted is a must. (This also is true for evaporative condensers for other refrigerants.)
High discharge pressure in evaporative condensers or shell-and-tube condensers must be analyzed properly. Many times, fouling of the surfaces is considered to be the problem, when in fact it is the presence of noncondensibles.
I have found many large systems that operate at suction pressure above atmospheric that have considerable noncondensibles - sometimes 30 psi. Where does the noncondensible come from when there is no vacuum in the system?
- Air left in serviced parts of the system.
- Air entrained in the lubricant charging procedure.
- Air entrained in the refrigerant charging procedure.
- Chemical breakdown between ammonia, water and lubricant in the presence of certain metals at certain temperatures.
These foul gases can be removed manually or automatically. The manual process is a one-time situation and, when done properly, is time consuming. Several manufacturers offer automatic purgers that do an effective job of removing noncondensibles from the refrigeration system and keeping energy use to a minimum.