Why do we use chillers & cooling towers - 8th grade research question

[dglenn9000]

I am doing some research for a presentation I am doing for my class. I am in the 8th grade and don't know much about HVAC so this might be a dumb question to most of you.

In high rise buildings why do you use chillers & cooling towers to provide the cooling for the air handler machine instead of just use electricity to cool the coils in the air handler machines?

At first I thought it was because of cost but then I realized you have buy and to power all the chillers and cooling towers how much can you really be saving. On smaller commercial buildings i notice air handler machines and they are not connected to chillers but when I see diagrams for high-rise buildings, they always use a bunch of chillers and compressors and cooling towers.

If someone could explain, I would appreciate it. Thanks!

[yorktek37]

What other way would be as effective to get rid of the heat. It's energy and cannot be destroyed. Plus it is easier to manage chill water piping. Could you imagine how large the air handler would be for the sears tower. The ductwork wold be insane. And lastly, DX cooling is not as efficient when you get up to say,, 150 tons. The kw per ton is lower with a chiller. And water cooled condensors make then even more efficient because you are dealing with wet bulb temperature instead of dry bulb that DX is affected by.
Hope that helps and didn't throw you off too much.
I have noticed that chill water systems control humidity much better than dx. DX is like the system that you ma have on your house. If the indoor coil has refrigerant in it it's DX.
Great topic BTW. Not many people even care about this sort of thing until it quits working:grin2:

[dglenn9000]

Ok, everything you said makes sense to me and thanks for your help. The one thing I did not understand was
"Could you imagine how large the air handler would be for the sears tower. The ductwork wold be insane."

Are you saying the Sears tower doesn't use air handlers or did I misunderstand? I imagine it would have multiple air handlers with the chiller water going to them. So the ductwork would not be that huge if there were multiple air handlers right? I guess this is just my lack of knowledge. Oh yeah, I chose this topic because my dad is always telling me this country needs to focus more on engineering and manufacturing. We live in Seattle and people seem to be more impressed if you tell them you want to be a musician or artist then if you wanted to be a HVAC tech.

[jimmyhoffa]

Yorktek meant if the Sears tower only had one a/c the duct size and the amount required to cool all the areas of the building would be extensive. Many large building have a chiller plant with cooling towers. The chilled water is piped to each air handler many times on each floor of the building. Sometimes on smaller buildings the air handler can be very large and usally located in the penthouse/roof area. Dont forget Boilers are used for the heating portion and usally piped to variable air volume boxes (VAV) that are connected to the duct work of each air handler. Think of a VAV box as a big air valve that regulates airflow and sometimes heating for smaller areas the air handler serves. Ask as many questions as you would like!

[patrick]

Usually its for space. The chilled water lines are much smaller than if you had one hvac unit conditioning the entire building and the size of ductwork required for the ammount of air it needs to move. There isn't enough room on the roof if you had seperate condensing units for each of the air handlers plus you can have problems with referigerant linesets when you have long runs.

I don't work on any chiller systems but I do have a few buildings that use several smaller package watersource heatpumps. They have a cooling tower to cool the water if it gets too hot and a boiler to heat it if its too cold and a circulation pump. Most of the time the boiler or tower don't need to run. On larger buildings you could have some of the hvac units running in cooling and some running in heating at the same time. The ones running in cooling are transferring the heat from the conditioned space to the water loop, the ones running in heating are using the heat that is in the water to transfer it to the conditioned space. So basically you are transferring the heat (or energy) from one zone to the other, not wasting or creating it like in a typical hvac system so it should be more efficient.

Also some cooling towers have two stages, the first one turns on a pump that cycles water over the water loop to cool it. That pump uses only a few amps. The second stage is to turn on a fan with the pump to blow air over the water loop. The fan uses a lot more amps than the pump so its more economical to only run the fan when needed.

There are other cooling towers that use evaporation to cool the water. The circulation pump pushes the water through dozens of nozzles inside the tower which creates its own wind, the warm water evaproates and cools off, then the water is returned to the water loop. So the tower itself uses no electricity but the customers water bill will be higher.

[dandyme]

my concern is:




what is an 8th grade student doing on a roof top?

[local 832s]

[QUOTE
In high rise buildings why do you use chillers & cooling towers to provide the cooling for the air handler machine instead of just use electricity to cool the coils in the air handler machines?


[/QUOTE]

How do you use electricity to cool coils?

Chiller plants are mostly used for large capacity buildings/process. It would not be cost effective to cool a million sqft facility with DX.

[local 832s]

[QUOTE
I have noticed that chill water systems control humidity much better than dx. DX is like the system that you ma have on your house. If the indoor coil has refrigerant in it it's DX.
[/QUOTE]

Really?? I find just the opposite as the DX coil is colder thus removing more RH. Most chiller plants send out 41-42* water and it might be say 45* at the coil. Not sure how this DH better than a DX. Can you explain please?

[Pneuma]

I am doing some research for a presentation I am doing for my class. I am in the 8th grade and don't know much about HVAC so this might be a dumb question to most of you.

In high rise buildings why do you use chillers & cooling towers to provide the cooling for the air handler machine instead of just use electricity to cool the coils in the air handler machines?

At first I thought it was because of cost but then I realized you have buy and to power all the chillers and cooling towers how much can you really be saving. On smaller commercial buildings i notice air handler machines and they are not connected to chillers but when I see diagrams for high-rise buildings, they always use a bunch of chillers and compressors and cooling towers.

If someone could explain, I would appreciate it. Thanks!

The considerations for what type of system you put in are as follows.

1. energy consumption. Do you want to track and bill each tenant/user seperately.
2. distribution of energy. A high rise building cannot use refrigerant piping because you cannot run piping vertically for long distances and maintain any sort of efficiency. There is also a total line length limit on refrigerant piping that is easily exceeded in a high rise. Water can be pumped an unlimited distance. So you have a central plant with a chiller, you can pump the chilled water underground to the next block and cool a building, it is really limitless and so for a high rise building, you generally use chilled water.
3. the cooling tower rejects heat to the outside. Because of the before mentioned limtis to pumping refrigerant, the condenser of a chiller is typically mounted right to the chiller with very short refrigerant pipes. So the chiller can be mounted outside where it is air cooled through coils, or it can have a water cooled condenser mounted to the frame of the chiller. Putting it outside as an air cooled requires a lot of space, wich you may not have and it is also noisy. Using a cooling tower, it is smaller per BTU of heat transfer and it is relatively quiet. You can also put the water cooled chiller anywhere because you can pump the condenser water as far as you need to.

All HVAC system are really energy distribution systems, or energy moving systems. So you select what is practical in that sense. If you look at systems that way, it makes sense why you use one system over another.

[JRINJAX]

On multi-story buildings the cost to build one is very expensive for each square foot of room inside of it. Do to this very high cost they want to maximize the amount of useable/rentable space inside the building. The central chillwater systems use the least amount of space in order to air condition the building. There are also few limitations on high or low you can send the chillwater unlike conventional air conditioning systems that send refrigerant [Freon] directly to the air handlers. Large rooftops or rooftop AHUs use of lots of [expensive] space in the building to provide room for large ducts traveling up/down from the roof.
Clear as mud?

[flange]

wow, some interesting answers, many opinions, etc. there is more to it than space, although that it is a consideration. when looking at buildings and conditioned spaces, it comes down to "total cost of ownership". this can mean many things, but for example, I am aware of a building at 1.3 million square feet that is fully conditioned with dx aircooled units. some say it cannot be done. i am also aware of many other building that are of that size or so, and use chillers, and yet another that uses condenser water and heat pumps, water source. We jsut finished starting some air handlers that were central station, four totalling 450,000 cfm and serving a high rise. some say it cant be done, but forget duct work can be run in the space that a mechanical room would use. so, think about this: what does the initial system cost to purchase and install?, what does the system cost to maintain?, how much energy does the system use?, hiw much service is required, and finally, who is paying for utilities and repairs. this is the real answer. an owwner will look at lifecycle cost or total cost of ownership and then make decisions based upon those numbers. the reality is that people build these building to rent and make money. many chilledwater systems offer flexibility in terms of changing needs, occupancy times and loads etc. dx can offer differnet flexibility.

[kdocsr05]

The newest technology in commercial heating and cooling design is the cogeneration power plant.

Growing ecological awareness, coupled with the knowledge that fossil sources of primary energy are limited, compels us to make more economical use of our existing sources of energy. Combined heat and power (CHP) plants generate electricity and heat locally where they are needed.

Efficient on–site power systems that produce electric power and thermal energy for heat, steam or air conditioning while reducing greenhouse gases.

Combined heat and power (CHP), is the production of two kinds of energy – usually electricity and heat from a single fuel. Systems can be powered by either natural gas or diesel. They typically involve a reciprocating–engine generator that produces electricity and a heat recovery system to capture the waste heat from the engine’s exhaust and cooling system.

By capturing and using the waste heat, these systems consume only 50 percent of the fuel burned by a central power station to provide an equivalent amount of energy. Since greenhouse gas emissions are directly related to the amount of fuel burned, CO2 production is also cut in half.

Economical cogeneration systems based on reciprocating–engine generators area available from as small as 30 kW to more than 100 MW. By making continuous use of both electricity and thermal energy, customers can save up to 35 percent on overall energy costs.

All this being said, combined heat power is the system design of now and no longer the future. Most new high rise combined buildings in the New York area have the elements of CHP in the system application.


Good luck with your report

[dglenn9000]

WOW, a lot of usefull info. A lot of this info will be used in my report, I appreciate the response.

One last question. I am having trouble understanding one thing. The Air handler heats or cools the air and the air is then sent thru the ducts to a vav unit. the vav unit acts like a valve controlled by a thermostat and controls how much hot air is added to the room.

But how does the air handler know what temperature to make supply the air? Let say one person sets the thermostat to 78 degrees in one of the zone areas, does the air handler raise its temperature to 78 degrees to match the requested heat request at that time?

[triggerhappy]

60862

Check this out.
If you put your email in your profile bigger things can be sent.

[triggerhappy]

60872

Or try this you said that you are in the 8th grade??????????/
Maybe the first was a little to simple for you.

[Joe Harper]

Another consideration to look at is on a dx system requires the transport of refrigerant from the condensor to a remotely located air handler. The longer the distance between the two, the more capacity you lose. A larger compressor, and lineset has to be factored in to push/pull the refrigerant across the longer distances.
Also factored in is the cost of the refrigerant.

[dglenn9000]

I guess I am making this to complicated for myself. All I really need to do for the assignment is explain some sort of engineering process. I chose HVAC. All I wanted to do is explain the process of a easy system that involves a chiller, cooling tower, air handler, & vav box. It seems the deeper I investigate the more I get confused. I thought I understood it but I don't. I guess maybe I should have just ask someone to explain the process instead of asking multiple questions. I just want to explain the process of the type of system that might be in our school or in a large building.

So does any know of any simplified diagrams or pictures of a simple commercial system involving an air handle and vav box? I just want to show them the process similar to this

http://www.betterbricks.com/graphics/assets/images/Building_Ops/BOpEqSysChillers_1w.png

[kdocsr05]

VAV systems were originally introduced as a more efficient alternative to constant-volume reheats systems. The VAV concept offers two major efficiency improvements

1 it reduces or eliminates reheat
2 it minimizes fan power

Unfortunately, the full efficiency potential of many VAV systems has not been achieved in practice. In many systems, the terminal units contain reheating coils. These are used to provide space heating, or to reheat the chilled air to allow a minimum air flow to be maintained in the spaces, or for both purposes.

As VAV systems became widespread during the 1980’s, it became apparent that they incur a number of conflicts with comfort and air quality. Indeed, VAV has become notorious for its comfort problems. Attempts to resolve the comfort problems and to reduce the installation cost of the systems often resulted in systems that squandered much of the efficiency potential of the VAV concept.

By far the largest opportunity for energy conservation in VAV reheat systems is minimizing the operation of the reheat coils. Also, you can save both reheat energy and fan energy by using accurate fan modulation to match cooling or heating load changes.

It is typical in today’s design applications to have a combination of VAV cooling only and VAV reheat terminal units connected to common fans air distribution ducts. The designer will consider heat loss in the perimeter zones applying terminal reheat to the VAV outlets. When addressing the interior zones the designer relies on interior heat gains and will closed the VAV to a ventilation minimum position typically 10%- 15% of the cooling maximum.

Full efficiency potential cannot be realized without effective systems commissioning. The designer will schedule the CFM minimum and maximum values for the zones. The fan speed is controlled by a static pressure sensor located down stream in the main supply duct. As the VAV’S begin to close down the duct static pressure will increases, the fan speed is decreased to a commissioning static pressure set point typically ( 1.”-1.5” of water column ) . The air flow out of the VAV’S can now be calibrated using instrumentation by a air balancer.

[Joe Harper]

I guess I am making this to complicated for myself. All I really need to do for the assignment is explain some sort of engineering process. I chose HVAC. All I wanted to do is explain the process of a easy system that involves a chiller, cooling tower, air handler, & vav box. It seems the deeper I investigate the more I get confused. I thought I understood it but I don't. I guess maybe I should have just ask someone to explain the process instead of asking multiple questions. I just want to explain the process of the type of system that might be in our school or in a large building.

So does any know of any simplified diagrams or pictures of a simple commercial system involving an air handle and vav box? I just want to show them the process similar to this

http://www.betterbricks.com/graphics/assets/images/Building_Ops/BOpEqSysChillers_1w.png


You are trying to simplify a process that is not very simple. Might I suggest using a convention a/c system. Diagrams and pictures will be much easier to get. The process will be easier to explain.

[amickracing]

I'd say you're farther along than some of the guys I've worked with even if you are confused.

Like Joe said though, you've picked probably one of the top 5 most complex systems that a good deal of HVAC guys have problems understanding and making it simple enough to make a report on it (and hopefully make some people understand it who you are presenting it to).

I commend your enthusiasm, but perhaps talking about a residential system would be a lil easier and something more people could relate too.
Either way, be sure to let us know how it turns out!

[r404a]

[QUOTE
I have noticed that chill water systems control humidity much better than dx. DX is like the system that you ma have on your house. If the indoor coil has refrigerant in it it's DX.


Really?? I find just the opposite as the DX coil is colder thus removing more RH. Most chiller plants send out 41-42* water and it might be say 45* at the coil. Not sure how this DH better than a DX. Can you explain please?[/QUOTE]

think it is because (1) water is a better heat tranfer component and (2) entire coil can be used ie no need for superheat to ensure dry gas back to a motor compressor

imho

404

[local 832s]

Really?? I find just the opposite as the DX coil is colder thus removing more RH. Most chiller plants send out 41-42* water and it might be say 45* at the coil. Not sure how this DH better than a DX. Can you explain please?

think it is because (1) water is a better heat tranfer component and (2) entire coil can be used ie no need for superheat to ensure dry gas back to a motor compressor

imho

404[/QUOTE]

Ok but the temp of the water is still much warmer than refrigerant thus it won't remove the humidity as well.

Not trying to fight, just have always seen that 45* water doesn't dehumidify very well or as well as a colder DX coil.

[JRINJAX]

The best dehumidification I have observed is with a 40* chillwater coil with a face-bypass damper. The entire coil remains cold and the supply air temperature is tempered by the air bypassing of the coil to mix downstream. The total grains of moisture seem lower than mixing/diverting flow coils.

This seem to work better than the mixing/diverting valve systems at very low flows through the coil. There seems to be distribution issues when the valves are just cracked open that you don't seem to have on a face bypass system.

This is just my opinion and could vary from system to system do to engineering differences.

[Pneuma]

WOW, a lot of usefull info. A lot of this info will be used in my report, I appreciate the response.

One last question. I am having trouble understanding one thing. The Air handler heats or cools the air and the air is then sent thru the ducts to a vav unit. the vav unit acts like a valve controlled by a thermostat and controls how much hot air is added to the room.

But how does the air handler know what temperature to make supply the air? Let say one person sets the thermostat to 78 degrees in one of the zone areas, does the air handler raise its temperature to 78 degrees to match the requested heat request at that time?

the VAV air handler will discharge 55 degree air all the time. Temperature is maintained by the volume of air delivered.