Energy savings from cooling load reduction

[cme]

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?

 

[lilliput1]

Is the air system constant volume reheat or VAV with reheat at minimum volume? VAV inlet vane of VFD? What is the floor area, no of floors, % windows?

 

[imok2]

CME, Here are some numbers I kicked around
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

 

[quark]

I got a bit lower value of 39.43TR from a quick estimate. Considering the data given by Imok2, your main savings come from avoiding cooling of 15000cfm ambient air to mixed condition. So it is 4.5x15000x(37.41-30.4) = 473175btu/hr or 39.43TR.

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,

 

[HVAC68]

With 100% OSA (20000 cfm), outside conditions of 92 deg F DBT & 74 deg F WBT, and an off-coil temp of 55 deg F DBT & 54 deg F WBT, the capacity would be 111.8TR.

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

 

[quark]

HVAC68,

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,

 

[HVAC68]

Well, my logic is like this.

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

 

[quark]

Yeah, I knew where I went wrong. Thanks.

Regards,

 

[HVAC68]

I used the McQuay Psychrometric analyser - didn't have to break my head in checking with a Psychrometric chart, as I used to do the pre-software era !!!

HVAC68

 

[quark]

Just a guess work based on my own pointers. It's fun and you can also try out.

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,

 

[SAK9]

I think before advising the client on energy savings you should investigate why a 100 % fresh air system was installed in the first place.Chemical or biological agents may be handled in a hospital lab and therfore a 100 % fresh air system may be more appropriate

 

[cme]

This is a dual duct unit for a hospital lab area. Currently 100% OSA and is using CHW off steam absorbers because the centrifugal chillers don't work so good if at all.

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.

 

[MintJulep]

All of the above calculations are for a single design condition. How frequently does that condition occur?

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.

 

[lilliput1]

For quick and simple energy usage calculations & comparison I suggest using Trane System Analyser. It cost about $1000 and is worth it. If you respond to my previous question I can run off comparison within about 5 minutes. Mint Julep is correct and the Trane System Analyser with its built in building types, equipment types, geographic location etc makes the work a snap. You can input actual OA CFM & operate system with or without dry bulb or enthalphy economizer. You can also use it for quick sizing of equipment but you should factor it up for safety during preliminary stage & to account for diversity being diffirent.

 

[lilliput1]

I calculate only about 5.9°F fan temperature rise.
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

 

[cme]

As far as fan heat, I took the bhp from the fan selection and used the value in the ASHRAE table for fan heat in the Fundamentals book. Then added a degree or so for safety.

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.

 

[HVAC68]

The Carrier HAP program also has an engineering cost analysis section. You may be able to get lot of data out of this.

HVAC68

 

[imok2]

With regards to Mint Julip's post What do members think of this: Outside summer temperatures and coincident air moisture content that will be exceeded only 2.5% of the hours from June to September. In other words, 2.5% design conditions are outdoor temperatures historically exceeded 73 out of the 2,928 hours in these summer months. Does this sound reasonable? i was reading this web site

http://www.proctoreng.com/articles/sidebar.html

 

[lilliput1]

Absorption chillers usually can not produce cchilled water colder than 44°F. Thus you may not be able to get 49°F off the coil and you would require more air. Also absorption chillers generally can not respond quick enough to varying loads. New absorption chiller from Trane may be an exception. For reliability it is recommended to have a hybrid plant using mix of electric centrifugal & absorption.
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.

 

[fastuser]

Hello!

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.