Energy savings from cooling load reduction
Energy savings from cooling load reduction
(OP)
I am trying to calculate the cooling energy savings freehand via psych chart and a formula. I have a formula for heating and am looking for one for cooling. My psych chart tells me I'm reducing the load by 52 tons by going to a return air system. The system will then have 95.5 tons for a hospital lab area ..... 20,000 cfm. The client wants to see what the payback is by going return air instead of 100% OSA which it is now and so the calculated reduction in cooling load. The project is close to Newark, NJ.
Anybody have something in archive?
Anybody have something in archive?





RE: Energy savings from cooling load reduction
RE: Energy savings from cooling load reduction
Assuming an OSA DB of 92*f and WB 0f 74*f with 20.000 CFM
If you have a Return of 15,000CFM and OSA of 5,000CFM I Get
the following:
Mixed air enthalpy = 30.4btu/lb with 13.8 cuft/lb for a total of 1470 lbs air
Total OSA enthalpy = 37.41btu/lb with 14.21 cuft/lb for a total of 1407 lbs air
Supply air at 55*f DB and 54*f WB enthalpy = 22.51btu/lb
If you go with the 15,000 CFM mixed air You will reduce your total heat load by 57.25 tons or about 48% The point is that the savings is quite sugnificant. So at 1.2KW/TON for air cooled and other associated energy costs such as environmental the pay back should be quite reasonable
RE: Energy savings from cooling load reduction
I am thankful if Imok2 or somebody points out the error, if any, or I will do that later leisurely.
I just can't overcome friday syndrome
Regards,
RE: Energy savings from cooling load reduction
With 5000 cfm as OSA and 15000 cfm as Return air, the capacity would be 58.9TR.
With Zero OSA, the capacity would be 42TR
Correct me if I am wrong.
HVAC68
RE: Energy savings from cooling load reduction
Once the air is mixed, the flowrate will be 20000cfm. I think you considered 15000cfm only. Your calculation should go this way,
Total tonnage = 4.5x5000x(37.41-30.4)+4.5x20000x(30.4-22.51)
(or) = 4.5x5000x(37.41-22.51)+4.5x15000x(30.4-22.51)
which will give you a total load of 72TR.
Regards,
RE: Energy savings from cooling load reduction
Total tonnage = 4.5x5000x(37.41-22.51)+4.5x15000x(28.1-22.51) = 59TR
The only difference between your equation and mine is in the condition of the return air. I have considered an enthalpy of 28.1 as against 30.4 considered by you.
30.4 represents the enthalpy of the mixed condition which will happen only after the mixing of the outside air and the return air.
28.1 represents the enthalpy of the room condition of 75 deg F DBT and 50% RH assumed by me.
Is this right ?
HVAC68
RE: Energy savings from cooling load reduction
Regards,
RE: Energy savings from cooling load reduction
HVAC68
RE: Energy savings from cooling load reduction
1. We generally don't participate in threads where there is discussion about codes.
2. We are submissive(atleast in forums)
3. We seldom use our original names(not sure for good or bad).
4. We generally tend to explain the hints given by another poster(humble again)
5. If anybody mentions Furnace Oil, I am damn sure he/she is Indian.
6. We generally don't participate in threads which talk about heating up of control spaces.
7. We generally say TR rather than Ton
8. We generally say blowers rather than fans
Infact, I contacted another guy 2 years back(who is not very active in these forums now and he stays in Mumbai) based on some of these pointers.
It seems you also have some free time today.
Have a good day,
Regards,
RE: Energy savings from cooling load reduction
RE: Energy savings from cooling load reduction
Room conditions are 72DB, 50%RH. Supply air is 49/48 in blow thru arrangement and there is about 8.5 deg in fan heat pickup.
There are 2 fans, 1 per deck, each on a VFD. 10.5" TSP. I am forcing more air down the system and dropping the temp in order to cool/dehumidify the space properly. They currently have 60 deg supply air.
The load reduction takes into account return fan heat and heat gain from the return duct since it is proposed to be run on the roof back to the AHU. So it is mixed air condition vs OSA condition for the cooling load reduction; all else is the same.
So, I'm searching for a formula given hand calcs.
RE: Energy savings from cooling load reduction
To calculate energy savings - as opposed to peak power reduction - you need to consider internal load patterns and outdoor ambient conditions as they vary with time.
RE: Energy savings from cooling load reduction
RE: Energy savings from cooling load reduction
Also if blow thru the cooling coil is downstream of the fan so the leaving air temperature is 49°F. Where did 60°F come from? What is the hot deck leaving air temperature?
Fan bhp = CFM x in wg TP /(6344 x Eff)
Btu/hr Q = bhp x 2545
°F TD = Q/(1.1 x CFM)
I used Eff = 0.65
RE: Energy savings from cooling load reduction
I broke down and modeled the OSA only with the Carrier HAP program and it appears that the cost for conditioning the osa to 49 deg F is $2.50 per cfm, based upon no unoccupied periods. This is also not taking into account humidification.
RE: Energy savings from cooling load reduction
HVAC68
RE: Energy savings from cooling load reduction
http://www.proctoreng.com/articles/sidebar.html
RE: Energy savings from cooling load reduction
You may not be able to reduce OA because labs may not want recirculation of air from other labs.
But anyway I used Trane System Analyser to come up with the following. i made assumptions since you have not given information.
For 22,050 sq ft square (2) story hospital in Newark NJ with 100% OA 24 hour operation, dual duct VFD centrifugal airfoil fans at 10" wg (max allowable input) 49°F cold deck, 125°F hot deck, two single stage steam absorption chillers: OA & SA CFM = 21554, Max building cooling load = 139 tons; max building heating load = 1961 mbh; building cooling coil load = 217,783 ton-hrs/yr; building heating coil load = 2,755,169 kBtu/yr; building energy usage = 649,261 Btu/(SF-yr); annual building electric consumption = 1,026,670 kwh; annual gas consumption = 108,122 therms.
For case using 20% OA & same as above:
Max building cooling load = 80 tons; max building heating load = 848 mbh; building cooling coil load = 175,623 ton-hrs/yr; building heating coil load = 2,428,294 kBtu/yr; building energy usage = 547,884 Btu/(SF-yr); annual building electric consumption = 928,843 kwh; annual gas consumption = 89,107 therms.
RE: Energy savings from cooling load reduction
I am new here and actually, in computers but hoped that you may help me out with HVAC question. In my program, I need to somehow determine an approximate cooling energy savings (Btu/Hr) when raising the thermostat setting by “n” degrees F, for an average single-family house.
This is a sales feature for the client of ours, and I hoped there might be a simple ‘rule of thumb’ for that in HVAC? I realize that I’m probably, being too unspecific.
Any help would be highly appreciated. Thank you very much.
RE: Energy savings from cooling load reduction
I'd recommend, if you already haven't, starting a new thread with a descriptive title. When a new idea happens in a large thread it usually gets missed.
Secondly, you would have to model the house to get an idea of the proposed energy savings - too many factors to consider.
You could model a few dozen 'standard' houses and then develop an average for marketing, but no rule of thumb exists.
RE: Energy savings from cooling load reduction
All that being said, I am more interested in SAK9's question: Who said the OA could be reduced from 100% anyway? Obviously, the premise was vetted long ago when the system was first installed. If a laboratory area is completely converted for non-laboratory use, return air may be justified. Yet, a system designed to condition 100% OA may be a poor choice to apply to a converted area that uses return air. The coil ratios, air change rates, etc., are all wrong.
Besides, what about the exhaust fans?