Another advantage (or actually a benefit of the bolumetric efficiency already mentioned) of a scroll is better performance a high condensing temperatures as a result of very little re-expansion due to the very small clearance volume.
Following on with less noise is less vibration.
A disadvantage is that scrolls are generally not repariable/rebuildable.
Many reciprocating compressors are happy to rotate in either direction. Scrolls must turn the right way.
quark:
you mentioned one of the advantages of scroll compressor is that "it Can handle liquid to some extent (atleast there won't be any damage)" could you pls. explain this?
MintJulep:
thank you for your interest,I am currently taking quotes to have air conditioning installed in new office building (2 floors). One of the contractors recommended two packaged units scroll comp type. Cooling capacity is about 20 TR for each.
Anything to look for or ask the contractor?
For a rooftop package unit with relatively undemanding conditions there is probably no great advantage to either scroll or reciprocating.
Compare the proposed systems on their merits and features. Scroll or recip should carry equal weighting unless noise and vibration are of significant concern.
Liquid entering into compressor(particularly at low loads and due to malfunctioning expansion devices) is a general problem and this is called as liquid stroke. This will damage the recips but scrolls are tolerant to this problem.
I would find out how many compressors in each unit. More compressors equates to better staging as the load varies, i.e., better temperature and humidity control.
Something I consider important is independent refrigeration circuits. This way, if a compressor fails, you still have some cooling capacity.
Hopefully they are quoting units with outside air economizers, to give you free cooling when the outside air temperature is low. Depending on your load situation, this feature could pay for itself in a couple of months.
A typical 20 ton package unit will have two compressors, but independent refrigerant circuits may be an expensive option at that size.
I am a little surprised at the lack of bias for scroll compressors. They are really superior in every way to a reciprocating compressor - fewer moving parts, lighter parts with less mass, progressive compression that doesn't depend on high-stress cylinder heads, connecting rods and piston-to-cylinder wall seal. Scrolls will run quieter, with less vibration, last longer, and have fewer maintenance issues.
On paper, the savings from outside air economizers are hard to ignore, but they are not without problems. The disadvantages are too complicated for a full discussion on this thread. I would opt for higher efficiencies, higher filtration capability, and the maintenance savings that will result from the scroll compressors.
I'm not familiar with relative costs from different manufacturers, but I just bought a York 7.5 ton packaged unit that had two circuits standard. I don't believe that Trane offers this in tonnages as low as 20.
As far as outside air economizers go, I'm curious to hear what kind of problems you've experienced that would lead you away from them even if the application was right. You can't get a better SEER/EER than with no compressors running.
KenRad, Independent circuits was the key you mentioned. There is a difference between multiple compressors and independent circuits. You are correct that Trane does not offer that in the sizes mentioned, and I didn't check York.
As far as outside air economizers - you are also correct that SEER/EER can't be better. I stated that. "On paper, the savings from outside air economizers are hard to ignore ..."
Yet, there are significant disadvantages, so much so that in our area of the country, water-side economizers are always preferred. Granted, you are assuming a water-cooled chilled water scenario, and that means a much larger system size than our discussion.
Nevertheless, my preference is still NO air-side economizer, even if that is not the case:
1. Filtration becomes more significant as outside air quantities are increased. Outside air may be polluted, and pollen is also unacceptable. With pollution, the use of air quality or CO2 sensors have become common, in order to limit the amount of outside air in cases where the quality is worse than exists indoors. Additional controls increase cost. Additional filtration increases pressure on the fan - a higher horsepower may be needed, and long-term energy costs increase.
2. Humidification (or the lack of it) is a huge issue. At the very same temperatures appropriate for economizer use, there exist medium or very low humidities. When heated by the load within the space (typically a 3/4 SHR or greater), Relative Humidity levels drop precipitously. These RH levels become a health issue beyond just simple comfort. Anything less than 30% is cause for liability from a health perspective. The humidification systems needed to compensate are more expensive in first cost, and negate much of the operating savings of the economizer.
It should be noted that in many parts of the country, a properly sized, typical SHR-rated unit and heat, with minimum acceptable outside air ventilation, may operate throughout the year with no need for additional humidification. Winter heating may dip below 30% on peak days, but the effect is tolerable for those short periods. Introduce anymore outside air, and that premise becomes moot.
3. Air balance - additional outside air presents a pressurization imbalance in the space. Manually balanced relief dampers do not work in practice - pressures to activate are much greater than typical non-economizer pressurization. Without digressing into a complicated discussion, the fact is that supply distribution occurs at an established pressure (determined with T&B). Yet, the switch between outside air intake with relief vs. return air presents pressure differences that must be absorbed by the fan on the return side. This must happen with no change in pressures at the supply side - or the T&B is violated. In theory, this is impossible.
In simple cases, this difference may go unnoticed. So, smaller units with built-in economizers often work "OK." On a larger scale, this difference cannot be ignored. In fact, the only way to make it work is to use a separate return fan in conjunction with programmed damper control.
By the time you have arrived at all of these disadvantages, an outside air economizer is no real savings.
Sorry - I mentioned it might be beyond the discussion in this thread.
