calculation of a 100% outside air cooling load 3

[pablo2410]

Hello, I´m new in this forum and have a question.
I need to calculate de cooling load of an area that demands 100% outside air, and to be honest, I don´t Know how to do it. There are some questions that comes to my mind: What should be the leaving air temperature? The loads produce by people, lights, walls, etc that we estimate in a recirculating system, should be calculated in this kind of system too?. I really apreciate some help. (I´m from Dominican Republic, so excuse my english).

Thanks.

 

[willard3]

For the outside air:

Sensible cooling
CU FT/MIN x 1.085 X outside AIR DELTA t = BTU/HR
SENSIBLE

Latent cooling:
fu ft/min x 0.66 x (delta moisture in grains/lb from
psychometric chart) = btu/hr latent

Sensible load + latent load = total cooling load for outside air

In general, cold air is delivered at 55F dry bulb and wet bulb (ie, saturated)

Building solar loads, lights, equipment and people loading should be included in your calculations.

You should probably hire a Mechanical Engineer to do this as he is familiar with it and you aren't.

 

[waross]

Hello willard3
I have met some intelligent dedicated engineers in the third world. Unfortunately, the training and educational systems there are not up to first world standards.
pablo2410 has recognized that he doesn't have the information that he needs to solve his problem and has wisely gone looking for good advice.
I thank you on behalf of pablo2410 for your kind help, but I wish to gently point out that your last suggestion may not be feasible.
pablo2410 is quite possibly the best mechanical engineer available.
You may find it rewarding, willard3 to mentor pablo2410 and help him to become a better engineer. From the tone of his post I believe he is intelligent and dedicated. He recognizes shortcomings in his education and is trying to improve himself.
You have already given pablo2410 the basics, go a little farther and help him through his calculations and you may find that you enjoy it.
respectfully

 

[MintJulep]

pablo,

Take a look at this site:

http://www.engineeringtoolbox.com/hvac-systems-t_23.html

 

[quark]

Pablo,

The room leaving air condition should always be the room condition.

Calculate heat load as per the general rules. Plot the room condition and ambient condition state points on the psychro chart. Draw a line with a slope of SHR from the room condition to the saturated curve. This is your dew point condition. Join the ambient condition state point to the dew point. The total load on the coil will be the mass flow rate of air times the enthalpy difference between the ambient condition and the dew point condition. In some cases, the SHR line becomes an asymptote to the saturation curve. In this case, assume some arbitrary dew point and add reheat into the process.

The link below gives you description and details of many psychrometric processes. Have a look into it.

http://www.coolit.co.za/psychart/

Waross,

Nice thinking and I feel that is the whole idea behind these forums.

 

[pablo2410]

Thank you all of you for your help

Here comes another question, how can i calculate mass flow, i mean, i know the volume of air in the room, but how do i know how many times do i have to recirculate it

 

[LSA]

correct me if I am wrong but I am assumming you meant Volume flow instead of mass flow.

Below is something I grabbed off a website descrbing what I think will be of help to you.

One air change occurs in a room when a quantity of air equal to the volume of the room is supplied and/or exhausted.
Air change rates are units of ventilation that compare the amount of air moving through a space to the volume of the space. Air change rates are calculated to determine how well a space is ventilated compared to published standards, codes, or recommendations.
Air changes per hour (ACH) is the most common unit used. This is the volume of air (usually expressed in cubic feet) exhausted or supplied every hour divided by the room
volume (also usually expressed in cubic feet).

Airflow is usually measured in cubic feet per minute (CFM). This is multiplied by 60 minutes
to determine the volume of air delivered per hour (in cubic feet).

ACH = (CFM X 60 minutes)/(room volume in cubic feet)

 

[pablo2410]

What i want to know is how can i determine the CFM needed for the area.

 

[willard3]

Pablo:
The sensible heat ratio and cooling load will dictate the air changes/hour and is the solution to the problem. Try not to confuse the cause for the effect.

In my climate, ie, 42 deg latitude and 73 deg longitude, in a frame-built, insulated building with approx 20% glass and 8' ceilings, cfm varies from 1 cfm/square foot to 3 cfm/square foot of floor depending upon internal loading, ie, number of people, lighting, equipment loads and ventilation loads.

BTU load varies from 30-50 btu/cu foot exclusive of internal loadings.

We have building codes in New York that make the range of these numbers pretty small.

Your latitude is 18 deg and longitude is 69 deg, so solar loading and air temperatures will be very different than New York. I am also sure that you construction materials and methods are different than ours so building mass and etc will be different

You should get a copy of ASHRAE Fundamentals Volume(American Society of Heating, Refrigeration and Air Conditioning Engineers); it contains answers to all the questions you are asking. You have access to the web, so "Google" ASHRAE.

You can also use Trane software to calculate heating/cooling loads, but you need first to understand the input. ASHRAE will be a big help with the input.

 

[willard3]

Wrong data:

BTU load varies from 30-50 btu/cu foot exclusive of internal loadings.

Right data:

BTU load varies from 3-5 btu/cu foot exclusive of internal loadings.

 

[pablo2410]

Ok, (willard3) the first formula you gave, me was a variation of the first law of thermodinamics, another way to put it is using the mass flow times the enthalpy difference between ambient condition and dew point condition, but in both cases i need a flow of air in terms of mass or volume, and there is my confusion. The other question is : do i need a 55ºF leaving air temperature in a system using 100% outside air?. I make this question because i assume that in this kind of system, there is no mix between the air in the room and the air from outside to achive a medium temperature, because eventually the total volume of air in the room will be exhauted and replace for a new volume from the outside, so i was thinking that the leaving air temperature should be the temperature of the conditions i want in the room, let´s say 73º.

 

[LSA]

I am going to take a stab at this.

Design condition:
Outside air temp (DB/WB) = 92F/68F (0.5% mean summer temp Los Angeles)
Supply air temp = 55F

Assumptions:
Dimension of space 60'(L)x60'(W)x15'(H)
20 people x 250 btu/hr = 5000 btu/hr
2.5 watts/sf lighting = 9000 btu/hr
Assume 5,000 BTU/hr exterior loads
-------------------------------------
ACH (Air change per hour) = 10
therefore CFM = ACH x Volume / 60 = (10 x 54,000 cu ft)/60 = 9,000 cfm

So 9,000 cfm is required to exhaust the room 10 times per hour.

Now to determine the load on the cooling coil:
Cooling coil load = 1.1 x CFM x (T1 - T2) = 1.1 x 9,000 x (92-55) = 366,300 BTU/hr

Summing all the loads:
366,300 btu/hr + 5000 btu/hr + 9000 btu/hr + 5,000 btu/hr = 385,300 btu/hr or 32 tons.

So to select a unit to suit all the above loads, I will select a 35 ton unit.

This is just a generic example. There are many other loads that I am not incorporating. Hopefully this can get you in the right direction.

 

[pablo2410]

Ok, so far, all the things that had been told here, i already knew it.I think i,m not asking the right question, so here i go, how can i determine the total cfm that is required to pass trough a cooling coil in a system that requires 100% outside air, or how do i know how many changes of air do i need.

 

[LSA]

The CFM required is calculated by the engineer. Typically for a laboratory the ACH is 6-15 ACH. A restroom is 4-8 ACH. (these are recommended numbers we use in our office. These are not a standard.)

Does that help?

 

[pablo2410]

ok, here is the problem

a locker room
Area 3550 sqft
height 10ft
35 persons in the room
what will be the diference in my load calculation if the system is 100% recirculate air or 100% outside air.

 

[LSA]

Air change method
Assumptions:
Summer design condition 92F/63F
Indoor design condition 72F
Interior location therefore no external load
Locker Room ACH = 15 (again, this is our office standard)
Volume: 3550 sf x 10' = 35,500
People load sensible = 520 btuh/person (18,200 btuh) (just got done working out)
People load latent/person = 105 btuh (3575 btuh)
Lighting load = 1.5 watts/sf (5325 btuh)


CFM = 15 x 35,500 / 60 = 8,875 cfm (lets say 9000 for simplicity)

1. For 100% outside air:
Cooling coil load = 366,300 btuh
Total sensible load = 366,300 + 5325 + 18,200 = 389,825 btuh or 32.5 tons.

2. The difference will be substantial but Locker rooms are never 100% recirculated. There will always be some amount fresh air brought into the space.

The air change method is a simple method of getting an approximate idea of what size unit is required. In locker room applications many times they are heated and ventilated only. And Make-up air is usually always required to achieve proper air balance.

Hope that clears up some questions you may have. GL

 

[cme]

a rule of thumb ..... 150 cfm/ton for a preliminary sizing load

final psych chart analysis on DB/meanWB per ashrae geographic data

don't forget fan heat gain

 

[62hog]

One other thing must be taken into account. If you are planning on using a packaged system (ie., rooftop unit), you must consult with the manufacturer. As a general rule, (at least according to my local reps) a packaged unit is only capable of accepting approximately 25-30% of the total airflow in the form of unconditioned outside air.

Any system you use should be discussed with you local rep. You should also require them to also provide you with any documentation for information that may differ from you "sound engineering judgement."

FYI, ASHRAE list the design condtions for Dominican Republic cooling, in the 0.4% range, as 91/80 dF db/wb.

 

[lilliput1]

62hog is right. Chilled water systems are recommended for 100% OA applications. However it is also possible to use multistage package DX AC units by recirculating a portion of the supply direct to the return and supplying to the room only the CFM equivalent of the OA handled. For example, if the load for say 4000 CFM 100% OA is 20 tons. Use a 20 ton unit say nominal 8000 CFM, but use 4000 CFM OA, 8000 CFM total SA, 4000 CFM recirc direct back to AHU and 4000 CFM supply to the room. Solve for the mixed air conditions of the OA and recirc air to get cooling coil entering air condition. Interpolate performance from tabulated date. Solve for cooling coil leaving air temperature. Determine fan and duct heat gains and deduct from tabulated capacity to get net capacity.

 

[thootz]

First you need to compute the total heat load .like number of window, size of doors what materials used in the building Wall. You may find the value of each material in Ashrae handbook or Mechanical handbook.You may also use Manual J method, a software for load computation.
This wil guide you step by step method.