|
|
|
The outcome of this simulation was that “here in Dubai we actually have a population of hours where the entering condenser water temperature is warmer than 85˚F, and even up to 95˚F. This is hot.” There are obviously also occasions where the load is reduced, meaning that cooling tower temperature could be reduced as well. “This is 6% of the time, according to our Trace700 simulation. For 6% of the time, the chiller load is 25% or less, and the cooling tower water is 65˚F or colder. IPLV states it is 12% of the time. Yes, this is a good thing, but IPLV has overestimated a good thing by a factor of two!
“What about 65˚F cooling tower water but 50% load? This is where drives really save energy. According to IPLV/NPLV, drives operate here almost half the time, at 45%; unless you are in Dubai, where it is only 2%. So for this condition, NPLV is overstated by 22.5 times.” What is the solution to this problem? Eppelheimer said that while the benefit of applying drives to fans and pumps was well known, “how do we really determine the benefit of drives on chillers?”
 |
He outlined three alternatives to illustrate this:
• Drives on a cooling tower, with a cold setpoint;
• The same system, but with a control change: instead of controlling the cooling tower for a cold temperature, we are going to control the cooling tower for a warmer temperature. This will increase the savings on the cooling tower; and
• A cold cooling tower, but taking advantage of variable speed drives on the chiller.
Eppelheimer said the immediate result from such comparative simulations was that “the variable speed drive centrifugal chiller always saves energy. We would expect it to be the most efficient chiller. But in order to take advantage of that efficiency of variable speed, we need to reduce the lift. And to reduce the lift, I need more cooling tower fan energy.”
“So when you look at the potential savings on the compressor by using variable speed, you also have to pay attention to the potential savings achievable on the cooling tower. It is not really possible to do both, however, a controller cannot follow two masters. This means a single strategy; and that strategy has to be focused on realizing savings on the chilled water system, even though the compressor is the largest motor.”
Looking more closely at the simulation, Eppelheimer said that, from January to March, “the variable speed chiller system consumed the least amount of energy. For the remainder of the year, the variable speed cooling tower saved more energy than the variable speed chiller. We might expect that because Dubai is a very hot climate. But this is not unique. We have found similar results even in the US, which is much cooler. It seems to be a recurring trend that a variable speed cooling tower saves more energy in the summer months, while the variable speed chiller saves more energy in the winter months.”
It is important to note that NPLV is used to show that the chiller you are considering purchasing complies with ASHRAE Standard 90. In order to be ASHRAE Standard 90 compliant, it has to exceed minimum full load efficiency, and it has to exceed minimum part load efficiency. That was the intent of APLV/NPLV. However, we do not think it is appropriate to use it to evaluate the performance of an entire chilled water plant.”
This is because the definition was compiled initially “for a single chiller, and now the industry is using it to make decisions or evaluations of a chiller plant that might have multiple chillers.” Another important point to be considered is the impact of climate; “Which is an excellent argument, in my opinion, for not using NPLV, as this regards the climate in Dubai as being exactly the same as in Chicago, for example. I do not think it is the number of chillers that is important; it is climate that is important.”
Don continued, “It is the leaving water temperature, not entering that affects compressor load. So, in the definition of NPLV, it is presumed that the condenser water flow is 3 gpm/t. Why does ARI rate chillers at 3 gpm/t and 44˚F? Well, as a chiller manufacturer we can claim its capacity at 44˚F is 1 100 TR. However, its capacity at 40˚F deg is only 950 TR. Now would a chiller manufacturer want to tell you this is a 950 TR chiller or an 1 100 TR chiller?
“At a condenser water flow of 3 gpm/t, the COP is 7.1. At a condenser flow of 2 gpm/t, the COP is 6.8. Again, would a chiller manufacturer, prefer to list its chiller as a COP of 6.8 or 7.1? Thus in terms of the standard ARI conditions, maybe we should strike the word ‘standard’ and replace it with ‘ideal’, as it overstates the applied capacity and COP of the chiller, based on warmer than applied water temperatures generally, and greater than applied condenser volumes.
“If the entering condenser water temperature is 85˚F, and the flow is 3 gpm/t, the leaving condenser water temperature is about 94˚F. At 3 gpm/t, we get a 9˚F deg condenser Delta T. At 2 gpm/t, the Delta T on the condenser is about 14˚F. So at 2 gpm/t, the leaving condenser water temp is 5˚F warmer – of course, the lift is greater and the COP is going to be reduced. ASHRAE Standard 90 actually assists with this scenario. If you select a chiller that is colder than ARI ideal conditions, or condenser flow that is less than ARI ideal conditions, the permitted COP you have to meet to comply with in terms of ASHRAE Standard 90 is actually reduced.”
This is because the fundamental understanding of what is ultimately required is not necessarily the most efficient chiller. “We want the most efficient building. If you can save pump energy by pumping less water through the condenser, if you can save pump energy by pumping colder water out of the chiller, ASHRAE Standard 90 will give you credit for that. So the minimum COP full load and part load that you have to meet, at applied conditions, is lower.”
| Articles | |
| Companies | |
 |
Site Map
 |
 |
 |
 |
|  |
\
FEATURED COMMENT
what is new thing Mr Eppelheimer told about chiller and advacncemnt in chiller technology.What is written here ,everyone