New to the Geo thing
Recently got a job with Geo thermal vertical ground loop system. It has 8 heat pumps in the 3 to 5 ton range and a 30 ton chiller all runnig off the cooling or heating affect of the ground loop. The 30 ton system is going through compressors and the place is a contols nightmare. However I think they may be running out of capacity on the loop and causing some of there problems. I would like to no how to test the tonnage of the ground loop but I am not smart enough. Seems like to many variables to figure out. Any ideas?
Hellow... anybody out there. I realise now that my original post was kind of vague. I have learned alot in the last few days so let me run this by you. My vertical ground loop has 28 holes at 215 feet each, thats a total bore of 6020feet. The tubing is 1 inch polyethylene on a 2 inch header the grout is non enhanced Bentonite. The guy who drilled the holes gave me a rule of thumb of 200'/ton. My loop pump is 120 Gpm. The caculated tons from 6020 feet of bore hole would be a little little over 30 tons. The system load is 60 tons in cooling and 54 tons in heating. An egineer came along and added a 30 chiller to the 30 ton load of water source heat pumps already in place. I really need to have my ducks in a row on this one. Tuff to argue with an engineer. What am I missing? If we add a bigger pump to the loop then I will be messing with the renolds number thing which I totaly dont get yet. At 3gpm/ton I would need 184 gpm for cooling and 162 for heat. Rather than put in a paralle boiler and cooling tower the best answer may be to dig more holes. I dont know but I am getting a head ache. The ground temperature is 55 degrees in ohio. My little chiller has 2@ 15 ton copeland scrolles and if my water drops below 50 I need to shut one off and then the hot deck has about 85 degree supply. Sorry for the rambling post, I am all wound up on this one. thanks for any insight.
I hope there is more than one 2" header on that many holes
Sorry, yea there are four 2 inch supply and 4 two inch return lines coming out of the ground. They tie into a 5 inch header pipe and then to a 3 inch building loop supply and return.
So just to get this right- the engineer added 30 ton to the loop field and didn't think you needed more holes? Are all units in the same mode all the time or are some heating a some cooling at the same time?
Its looking bad
Yep, both the engineer and the chiller manufacture are out of buisness. That should tell ya something. I have been doing a lot of math and going by the Copeland compressor information I could have 4 to 19, negative tons in the winter, and 19 to 29 possitive tons in the summer. Too and from the ground loop. Not much diversity in the building, there are times when they could all be running in heat or cooling at the same time. I am thinking for now all I can do is put high and low temperature limits on the water circuits to the chiller just to protect the thing. Dont know what the final solution is yet.
looks like I got me a one of a kind rig here. Nobody seems to have seen one of these. Water to water, reverse cycle, heat pump chiller. Just my luck.
Looks to me as though you have a loop field capacity of MAYBE 30 tons. Soil type, antifreeze, pipe installation and all that will cause deviations.
Check water flow rate. 2-3 gpm per ton in cooling
Check entering and leaving water temperature. 10-12 temp rise across the heat exchanger (water in and out)
You have a 30 ton chiller? Is this thing supplying tempered water to the rest of the units? Does it have two scroll compressors?
If so, make sure it has adequate flow.
My chiller is a monster
Two 15 ton scroll compressors on a water cooled reverce cycle chiller, that provides water heat to the A/H in winter, And cooling water to the air handler in summer. When the weather changes and the load drops off is when it has its problems. My math shows a 5 ton variance in load from heat to coolling. the flat plate heat exchangers have an internal volume of 11 pounds each. 200Lbs/min per ton devided by my RA gave me 30 to 35 pounds required. No way with such little coils was I going to go down that route. Could not get my head around how the HX manufatures Flow rate numbers came up with same mass flow numbers that i did, yet having a total volume of 22 lbs between them both. Turns out there is alot of time in one minute. They flow there volume and heat rejection in 3 cycles through the HX. The heat transfer coifishent is so good that 1/3 of a minute is enough to keep up with the load. The total system charge is 1/3 that of a normal system. My little unit should use 30 lbs per minute when you do the math, but the actual charge is closer to ten.the total volume of both coils together is 22 pounds. the charge is critical and they can fill up in a hurry if problems arise in the water flow or rapid air flow changes. Receivers are a must ,though you sacrifice your sub cooling,at least the compressors dont get slugged. Had to re learn all this stuff just to figuer out how mush gas to use, i will list it here incase some other poor bugger runs into the same issue.
Compressor displacement/ constant volume = Mass flow f3/Hr x refrigeration affect of design condition. = BTU/hr convert to tons, Devide 200Btu/Lb/ton by RA = Lbs/min needed for 1 ton. Multiple by tonnage of unit get total pounds needed. Flat plate heat exchagers require 1/3 of that. Devide by 3.3, Boom its done. Consistant low or high head conditions will change the coifichent rating and may not be exactly a 1/3 number. It is a base line.
number. Had to re-learn all that stuff just to charge my little hightmare of a chiller. Later
I'm not sure i fully understand your problem, but the volume in the HX has not much to do with the flow rates.
Did you verify all flow rates (via balancing valves)? How is the main loop pump controlled, does it have VFD with static pressure sensor? Measure flow rate by adjusting for viscosity. the setter manufacturer has information how to do that.
It seems your piping is undersized. I assume yo have glycol and glycol at low temperatures has very high viscosity. Maybe you never get the design flow rate.
the capacity of a ground loop also depends on how it is layed out (distance and spread of holes), type of soil, if you have ground water etc. when your field is large enough, it also can drift over time. Let's say you always cool more than you heat, then the field gets warmer and warmer every year and loses cooling capacity. All not a problem if you had a good conservative designer. but since your engineers seem to just add 30 tons without any evaluation, I'm a bit worried about that :)
What are your loop temperatures in summer, winter? If they are very low/high this indicates too small field (and/or pipes). Some good fields stay at exactly 45-50°F all year, but some swing from 32-60°F etc.
We have a fire station with geothermal (horizontal)loop and the designers were idiots (to be most polite) and we spend a lot of money to get the system to work. those "designers" make the same mistakes on regular VAV systems, but those are more forgiving (they still work, just get inefficient)
Time will tell
There in lies the rub i have not seen the this system run in all possible conditions. Thanks for your interest though as this unit has been the most challanging thermodynamic system I have worked on in a while. The extreams the unit must run at vary from 1 extream to the other. The change from heat to cool mode really messes with the compressor tonnage. Bottom line is the ground loop temperature in winter, if it drops below 44 degree all bets are off. We are going to replace the ground loop pump to 180 GPM to get it up to the 3 GPM/ton requirment, but if the feild has the capacity is yet to be seen. We added receivers to the compresssor circuits to provide storage of liquid as the compressors can run at such varied tonnage capacities depending on the mode it is in. The storage area of the flat plate HX did not provide the space needed to store the 5lb differance in ref flow when running 130/35 condensing in winter to 40/80 condensing in summer. The compressor tonnage goes from 16 to 11 depending on the head. So far that worked. Anyway to much going on to explain and it seems to bore the massses with my rantings. But I feal for the next man that comes accross a system like this, it is a hand full.
On large tonnage water to water heat pumps you can end up with problems of you don't have enough water volume on the load side. You can get pretty severe short cycling problems. The trick is to add a buffer tank. Also it seems pretty bone headed to have added loads without well field expansion. However I have designed well fields for lower capacity than the sum of peak building equipment capacities due to oversizing of water to air equipment due to minimum box size. (i.e. a bedroom has a minimum 7.5 mbh console unit but a 4 mbh load.) Whenever you do a geo job you need to do standard load calculations, 8760 hour simulations of building operation and 40 year simulations of the well field to be totally safe. That all is on the engineer.