GaryP
Thu, 26th Oct 2006, 05:00 PM
OK, lets try a little physics for today...
HOW DOES A SKIMMER WORK?
Basically, a skimmer works on the principle of organic chemicals being "hydrophobic." That means that they are not water loving. In other words, its one reason why oil floats on water instead of being dissolved by it.
Actually its a little more complicated then that. Isn't it always? Most of the organic molecules we are trying to remove with a skimmer have both a hydrophobic and a hydrophilic (water loving) component. What this all comes come to is that the hydrophobic part of the molecule is attracted to the air bubbles and the hydrophilic part is attracted to the surrounding water. If you could look at the air bubble under a powerful enough microscope, you would see the hydroscopic ends sticking into the bubble and the rest of the molecule sticking out into the surrounding water.
As the bubbles rise through the water, they collect more and more of these organic molecules. Finally, the bubbles combine to form bigger and bigger bubbles and this results in the foam you see at the top of the skimmer. The techical term for this foam is floc. We call it skimmate.
What are the factors that effect skimmer performance?
1. Retention/Contact time - The longer the water is in contact with the bubbles, the more efficient skimming will be.
2. Bubble size - The smaller the bubble size, the greater the surface area available for organic adsorption. I know that sounds weird, but trust me its true. Which has more surface area, a boulder, or sand of the same volume. The sand does. So, the smaller the bubble size, the greater the efficiency. This is why many skimmers require to be tuned. If you are allowing in to much air, the bubbles can be to large. It's also why needle wheel skimmers are very effective. The needles on the impeller chops the bubbles into smaller sizes.
3. Volume - The more water passing through the skimmer, and the more bubbles, the greater the efficiency. Howver, there is a trade off here. If you simply use a larger pump to increase water throughput, you will also cut down on the contact time mentioned above. This is why you see a lot of high end skimmers that are either really tall or have a large diameter. Most skimmers are designed with a specific pump capacity in mind. Remember that a pump can become less efficient as it gets older or if it needs cleaning. I usually clean my skimmer pump every 3-4 months with vinegar and replace it every year or two. In old days when we were using air pump driven counter current skimmers, it wasn't unusual to see one that was 10-12 feet tall. I heard stories of guys that put them in their basement and had to run uptairs to clean the collection cup that was sticking through the floor. Most modern skimmers are designed so that water and bubbles move in a circular path and not up and down like in the older counter current skimmers. This gives more contact time.
4. Maintenance - Yes they do require maintenance. I recall going to an unnamed reefers house once and he said he was getting a new skimmer because his old one wasn't working very well. I suggested we take a look at it. I was somewhat shocked when I looked at it and saw that it was one of the best skimmers on the market. When I started looking at in closer I asked him when the last time he had cleaned it. He looked at me with a look that clearly said, "Huh, what do you mean clean it?" Apparently it had an overflow and he just emptied that whenever it was full. Later his wife complained that his fish room stank too. He was also getting a carbon filtered collection jug. The skimmer was at least a year old. Here are a few maintenance tips.
A. Chean and empty the skimmer once a week. I just use a paper towel to wip out the inside if there is any "gunk" deposited in the barrel below the collection cup. The collection cup and lid gets cleaned with a toothbrush and warm water.
B. As mentioned above - clean the pump every 3-4 months, and replaced every 1-2 years.
C. Every 6-12 months I take the skimmer out and run some watered down vinegar through the whole appratus. This will clean out any critters such as feather duster worms that may be growing in it as well as dissolve any scale that is deposited in the eductor. Calcium carbonate scaling is more likely to occur where there is a sudden pressure change, such as in the eductor (otherwise known as a venturi). The same process occurs in the pump impeller.
4. pH - organic molecules have a tendency to foam up more at higher pH. There isn't a huge difference in an aquarium because our pH does not fluctuate a whole lot. However, if you have a system where pH is going down into the 7's at night, then the skimmer isn'tgoing to be nearly as efficient as night as it is during the day.
OK, that's it for now. I'm sure one of the skimmer gurus like Matt can probably jump in and add a lot more info that I missed or got totally wrong.
HOW DOES A SKIMMER WORK?
Basically, a skimmer works on the principle of organic chemicals being "hydrophobic." That means that they are not water loving. In other words, its one reason why oil floats on water instead of being dissolved by it.
Actually its a little more complicated then that. Isn't it always? Most of the organic molecules we are trying to remove with a skimmer have both a hydrophobic and a hydrophilic (water loving) component. What this all comes come to is that the hydrophobic part of the molecule is attracted to the air bubbles and the hydrophilic part is attracted to the surrounding water. If you could look at the air bubble under a powerful enough microscope, you would see the hydroscopic ends sticking into the bubble and the rest of the molecule sticking out into the surrounding water.
As the bubbles rise through the water, they collect more and more of these organic molecules. Finally, the bubbles combine to form bigger and bigger bubbles and this results in the foam you see at the top of the skimmer. The techical term for this foam is floc. We call it skimmate.
What are the factors that effect skimmer performance?
1. Retention/Contact time - The longer the water is in contact with the bubbles, the more efficient skimming will be.
2. Bubble size - The smaller the bubble size, the greater the surface area available for organic adsorption. I know that sounds weird, but trust me its true. Which has more surface area, a boulder, or sand of the same volume. The sand does. So, the smaller the bubble size, the greater the efficiency. This is why many skimmers require to be tuned. If you are allowing in to much air, the bubbles can be to large. It's also why needle wheel skimmers are very effective. The needles on the impeller chops the bubbles into smaller sizes.
3. Volume - The more water passing through the skimmer, and the more bubbles, the greater the efficiency. Howver, there is a trade off here. If you simply use a larger pump to increase water throughput, you will also cut down on the contact time mentioned above. This is why you see a lot of high end skimmers that are either really tall or have a large diameter. Most skimmers are designed with a specific pump capacity in mind. Remember that a pump can become less efficient as it gets older or if it needs cleaning. I usually clean my skimmer pump every 3-4 months with vinegar and replace it every year or two. In old days when we were using air pump driven counter current skimmers, it wasn't unusual to see one that was 10-12 feet tall. I heard stories of guys that put them in their basement and had to run uptairs to clean the collection cup that was sticking through the floor. Most modern skimmers are designed so that water and bubbles move in a circular path and not up and down like in the older counter current skimmers. This gives more contact time.
4. Maintenance - Yes they do require maintenance. I recall going to an unnamed reefers house once and he said he was getting a new skimmer because his old one wasn't working very well. I suggested we take a look at it. I was somewhat shocked when I looked at it and saw that it was one of the best skimmers on the market. When I started looking at in closer I asked him when the last time he had cleaned it. He looked at me with a look that clearly said, "Huh, what do you mean clean it?" Apparently it had an overflow and he just emptied that whenever it was full. Later his wife complained that his fish room stank too. He was also getting a carbon filtered collection jug. The skimmer was at least a year old. Here are a few maintenance tips.
A. Chean and empty the skimmer once a week. I just use a paper towel to wip out the inside if there is any "gunk" deposited in the barrel below the collection cup. The collection cup and lid gets cleaned with a toothbrush and warm water.
B. As mentioned above - clean the pump every 3-4 months, and replaced every 1-2 years.
C. Every 6-12 months I take the skimmer out and run some watered down vinegar through the whole appratus. This will clean out any critters such as feather duster worms that may be growing in it as well as dissolve any scale that is deposited in the eductor. Calcium carbonate scaling is more likely to occur where there is a sudden pressure change, such as in the eductor (otherwise known as a venturi). The same process occurs in the pump impeller.
4. pH - organic molecules have a tendency to foam up more at higher pH. There isn't a huge difference in an aquarium because our pH does not fluctuate a whole lot. However, if you have a system where pH is going down into the 7's at night, then the skimmer isn'tgoing to be nearly as efficient as night as it is during the day.
OK, that's it for now. I'm sure one of the skimmer gurus like Matt can probably jump in and add a lot more info that I missed or got totally wrong.