A/C Rule of Thumb? 2

[CuriouslyGeorge]

Everyone states (in a nasal tone) "An A/C unit is sized to drop the temp by 20 degrees". But, is that RA - SA = 20? Because in all the examples in HVAC books, Tmix - Tapparatus is greater than a delta of 20 degrees.(Persoanlly, I thought it was a delta of 20 dgrs across the evaporative coil...but that can't be right, can it?)

What's the scoop? Please set me straight!
- CuriouslyGeorge

 

[ChasBean1]

I dote state that at all. Id, fact, beddy differet sysebs cad be sized for beddy differet teberature drops (dats by adswer, id a dasal tode).

 

[pennpoint]

ChasBean1
Geezzz! thats so funny. Thanks! I really needed that right now at this time. Having a bad week.
Hope yours is better, you helped.

Best Regards
pennpoint

 

[quark]

Inhaling menthol vapors will cure the thing and bring life back to normalcy

George,

20(I presume it is deg.F) is generally considered during preliminary calculation stage as the difference between the room condition and cooling coil leaving temperature(including coil bypass). As already stated by CB, it is not a thumb rule. This basically depends upon SHR, coil BPF and the required humidity level inside the control space.

Regards,

 

[imok2]

George, Here's the scoop.
For a constant air entering dry bulb temperature, the DT across a coil increases with decreasing relative humidity. This is because the coil condenses a decreased moisture (latent) load. If a coil doesn’t have to condense much moisture from the air, it can perform more sensible cooling. Sensible cooling is what we are measuring. Humidity can account for several degrees of dry bulb deviation. Therefore, on a humid day, the sensible DT reading will be lower as more of the air conditioner’s capacity is used to dehumidify. Conversely, if the relative humidity of the return air is low, the sensible DT will be higher. On the average, about a quarter of an air conditioner’s capacity is used to dehumidify at 50% relative humidity. As an example, a 4 ton air conditioning system uses about 3 tons for sensible cooling, while one ton is used to dehumidify. This sensible heat factor (SHF) should be taken into consideration when taking the dry bulb DT over an evaporator to determine airflow. The dry bulb DT could vary from about 15oF, very humid return air, to 25oF, very dry return air. The 20oF DT is assuming about 50% relative humidity return air.

The air handler formula BTU/HR=CFM x DT x 1.08 won’t work with an air conditioning coil unless a SHF is considered.

BTU/HR x SHF = CFM x DT x 1.08

Examples: If we try to determine the CFM’s using the formula for a three ton air conditioner on which we’ve taken a 20oF DT without using a SHF, we will get an erroneous answer.

36,000 = CFM x 20 x 1.08

Solving for CFM we’d get 1666.67 CFM. This indicates too much air.

Inserting a SHF of .75

36,000 x .75 = CFM x 20 x 1.08

Solving for CFM we get 1250 CFM, about the right amount of air for a three-ton system.

 

[imok2]

George , the point of all of the above is if you have the instrument and time to take wet bulb readings and determine relative humidity, do it. Your judgement of proper airflow will be more accurate. If you do not, you’ll have to add to your judgement of airflow your judgement of the return air’s humidity. Remember high humidity = lower DT and low humidity = higher DT.

If the DT is greater than required across the evaporator, we are dealing with an airflow problem. Too low an airflow makes the air stay in contact with the coil too long.

Some causes of low airflow: Dirty filters, loose fan pulleys, slipping belts, slow running blower motors, dirty evaporator coil, anything that restricts or blocks the duct system.

 

[HVAC68]

For a typical comfort AC application,

Chilled water entering temperature - 45 deg F
ADP - 53 deg F
Canvass temperature - 55 deg F
Room temperature - 75 deg F

HVAC68

 

[CuriouslyGeorge]

Thanks everyone for their replies. Quark has the right idea for a preliminary design which is where I am. While I was taught good HVAC practice via a Carrier course, in reality, on the job short-cuts are taken to rough out some numbers (for an existing system with poor documentation on the HVAC system).

I assume the same is true for heating an area, as well. At least, that explains the guidance I've been given.
- CuriouslyGeorge

 

[lilliput1]

You need to study psychrometrics, sensible heat, latent heat, outdoor air and components of room load and system/cooling cooling coil loads. Sensible load is transfered to air by raising its temperature. Latent load is transfered to air by raising its humidity and moisture content. Following the air path on the psychrometric chart, invented by Dr. Willis Carrier, will give you insight on understanding what air conditioning is all about.