Reply to Thread

Originally Posted by kaleun

there are many things that play a role. but in the end you increase the lift (either dT or dP) of the heatpump (=chiller) and COP depends on the absolute temperatures. Let's say you try to maintain 45°F chilled water temp and outside is 90°F, you at least have 45°F lift (actually add 5-10°F on each side since you need a temperature difference to transfer heat). So you refrigerant is 40°F on one since, and 95°F on the other. Makes 55°F lift. Now you have 100°F outside, so your refrigerant is 105°F, making it 65°F lift. thsi means the compressor needs to compress the refrigerant more to reach that temperature, the motor will draw more power. At some point the motor can't keep up compressing and refrigerant flow is reduced, hence less cooling capacity in the evaporator.
I'm not sure I'm explaining this well, but google for heatpump COP and absolute temperatures and you'll find it.

it is kind of ironic, when you need most cooling, you get the least. Same with heatpump heating, when you need most heat, you get the least heating performance. that's life....

I understand what you are saying..mostly. I studied COP and abs temp in thermo. But is ref. flow reduced because the compressor is compressing a lessor volume of ref. to a higher pressure and bypassing the rest of it, therefore less ref. can be used to remove heat?

Originally Posted by spinning wheel

The building has a set of engineered drawings that state how much tonnage the building needs at max capacity, as in a hot start-up. The unit very seldom runs at this peak load, but you do need it. This is a perfect application for a series piped set of units. Both units for max load, and one for the rest of the time. You can get as fancy as you want to as well. Make it a VPF loop, with VFD centrifugals. Now we are saving money. As for load readings, you need to measure water flow, and temperature difference. Then it becomes a simple math problem to calculate tonnage. Load (Btu/hr.) = Flow (US gpm) x (°F in – °F out) x 500. Do this at set times in the day, and you will see that your utility company is sticking it to you on kw per ton.

Is hot start-up when the empty building starts heating up since the temp fell below a specific set point, before the building is occupied? Also when you wrote "series piped set of units" would that mean that the pump are in series so the head is doubled but flow is the same?
I believe that they are going with a primary-secondary system for their chiller plant. I would think that a VPF loop with VFD centrifugals would make is more cost effective since it removes equipment cost for 2-3 pumps and cuts power consumption down.

The building has a set of engineered drawings that state how much tonnage the building needs at max capacity, as in a hot start-up. The unit very seldom runs at this peak load, but you do need it. This is a perfect application for a series piped set of units. Both units for max load, and one for the rest of the time. You can get as fancy as you want to as well. Make it a VPF loop, with VFD centrifugals. Now we are saving money. As for load readings, you need to measure water flow, and temperature difference. Then it becomes a simple math problem to calculate tonnage. Load (Btu/hr.) = Flow (US gpm) x (°F in – °F out) x 500. Do this at set times in the day, and you will see that your utility company is sticking it to you on kw per ton.

Originally Posted by Saman1366

Is the loss of chiller/condenser efficiency related to the higher delta T? I am thinking that a higher outside temp would mean a higher return water temp and a higher delta T for the chiller.

there are many things that play a role. but in the end you increase the lift (either dT or dP) of the heatpump (=chiller) and COP depends on the absolute temperatures. Let's say you try to maintain 45°F chilled water temp and outside is 90°F, you at least have 45°F lift (actually add 5-10°F on each side since you need a temperature difference to transfer heat). So you refrigerant is 40°F on one since, and 95°F on the other. Makes 55°F lift. Now you have 100°F outside, so your refrigerant is 105°F, making it 65°F lift. thsi means the compressor needs to compress the refrigerant more to reach that temperature, the motor will draw more power. At some point the motor can't keep up compressing and refrigerant flow is reduced, hence less cooling capacity in the evaporator.
I'm not sure I'm explaining this well, but google for heatpump COP and absolute temperatures and you'll find it.

it is kind of ironic, when you need most cooling, you get the least. Same with heatpump heating, when you need most heat, you get the least heating performance. that's life....

Originally Posted by kaleun

You would size the chiller for the 99% percentile of your load. Meaning on 99% of days you meet load, only on 1% you are slightly below. Some people use 99.6% or so. But you also put some more safety in and a load calculation is an educated estimate. in Trane Trace or some other load software you can enter what percentile you want to design for. It also should take into account performance degradation at higher OA temp. Your chiller/condenser combo has a nominal capacity at specific ambient conditions (i.e. 90F). when it is warmer, it loses some capacity. It requires some judgment.

sizing only for 40% of load would make you pretty unpopular among occupants

It would be more important to have equipment that can modulate (i.e. has many stages, or some variable speed compressor etc.), this increases efficiency and reduces cycling and improves comfort (better de-humidification)

Is the loss of chiller/condenser efficiency related to the higher delta T? I am thinking that a higher outside temp would mean a higher return water temp and a higher delta T for the chiller.

Originally Posted by Saman1366

kaleun,

Your are correct. My cooling load on average is 40% of max load. The building is high rise (13floors) office building located in los angeles and the hours of operation are between 8am-6pm. I am only considering fluctuating loads.
I am trying to put together a chiller and pump set up that is most efficient but if i design my equip for 100% load and run them at 40% capacity, that would be oversizing but it ensures i have max capacity in case the max load is reached. I recently acquired some info from the owner that states their building history shows a pick of 850 tons of cooling duing june and july.

You would size the chiller for the 99% percentile of your load. Meaning on 99% of days you meet load, only on 1% you are slightly below. Some people use 99.6% or so. But you also put some more safety in and a load calculation is an educated estimate. in Trane Trace or some other load software you can enter what percentile you want to design for. It also should take into account performance degradation at higher OA temp. Your chiller/condenser combo has a nominal capacity at specific ambient conditions (i.e. 90F). when it is warmer, it loses some capacity. It requires some judgment.

sizing only for 40% of load would make you pretty unpopular among occupants

It would be more important to have equipment that can modulate (i.e. has many stages, or some variable speed compressor etc.), this increases efficiency and reduces cycling and improves comfort (better de-humidification)

kaleun,

Your are correct. My cooling load on average is 40% of max load. The building is high rise (13floors) office building located in los angeles and the hours of operation are between 8am-6pm. I am only considering fluctuating loads.
I am trying to put together a chiller and pump set up that is most efficient but if i design my equip for 100% load and run them at 40% capacity, that would be oversizing but it ensures i have max capacity in case the max load is reached. I recently acquired some info from the owner that states their building history shows a pick of 850 tons of cooling duing june and july.

Originally Posted by Saman1366

I calculated the max cooling load (tons) and total monthly load (ton-hrs) using trace. it comes out to be about 40% of the utility company charged the building for. i wish i had a way to monitor the building to see if my calcs are corrects.

It sounds to me your cooling load on average is at 40% of maximum if your load calculation is correct. I don't know if you have normal building load (fluctuating), process load (always 100% when on). you could run at 100% at 40% of the time, or 40% at 100% for 40% of the time or anything in between.

If you have a building automation system it should be easy to trend. It sounds like your application is large (considering your are hooked up to chilled water). Maybe tell us more about what type of application, also if you have large summer/winter changes etc.

I calculated the max cooling load (tons) and total monthly load (ton-hrs) using trace. it comes out to be about 40% of the utility company charged the building for. i wish i had a way to monitor the building to see if my calcs are corrects.

Is it correct to assume that the ton-hrs represents the total cooling energy used for the specified months?

Yes, if the record is from a months periond.

Also is there a way to figure out what the max cooling load for the building is?

No

You could do 1000tonhrs with a 1000ton machine running one hour or with a 1ton machine running 1000hrs. But you could monitor 1 hr at peak load, then you know the max cooling load.

Energy Analysis Help

Friends,

I have acquired information regarding the chilled water load of a building. The info is in ton-hrs and is broken down based on the monthly consumption. I also have the utility cost in $ from the chilled water provider. Is it correct to assume that the ton-hrs represents the total cooling energy used for the specified months? Also is there a way to figure out what the max cooling load for the building is?