cairo design conditions

From ASHRAE

0.4% cooling is 100 F with MWB of 69

1% is 97/69

2% is 95/69

Thanks for the response. I'm not an HVAC person, so what do the %'s you mention represent? Also, is MWB mean wet bulb?

Hi DJV,

Just in case MintJulep is done for the day --

The % numbers represent the percent of annual hours during which the design condition (statistically) will be exceeded.

MWB is mean wetbulb coincident with that design dry bulb.

Good on ya,

Old Dave

Egyptian summers are hot and dry in most of the country and humid in the Delta and along the
Mediterranean Coast. In recent years the humidity has spread to Cairo and the city swelters in
August. Winters are mild with some rain, but usually there are bright, sunny days and cold
nights.

There is a short spring and autumn and during the 50 days (khamseen) between the end of
March and mid-May, dust storms can occur sporadically.
for more go here:

I couldn't understand something. As the total load is constant for all the three conditions, why 100DB and 69MWB is 0.4% only?

The only problem I see is the selection of coil ADP if the above data is correct. I would settle with 1% conditions. I will also have a look at 1% values of WB and MDB for extra latent loads.

Regards,

If I understand correctly, the percentage refers only to the DB temp, so that accounts for the differenc in percentages.

Do they give %data for WB as well? My application is cooling air for a plastic blown film line. I'd like to account for those hot and humid days, but I don't want a giant, expensive cooling coil. One supplier used 110DB/ 91 WB, another used 128DB/ 95 WB, another used 102 DB/ 76 WB. The latter seems most appropriate.

DJV,

We run into this often when it's process cooling. The MWB is an average of wet bulb temperatures that occur coincident with design dry bulb. That doesn't mean it won't hit the design dry bulb one day and rain at the same time, in which case you'll have less than the capacity you need to control humidity. If that's acceptable on occasion, then the 0.4% numbers or even the 1% numbers are OK. However, if loss of temperature control would be extremely expensive (say, cause the need for a process shutdown), you might size the unit up. It's a judgement call....

Let us know how it works out for you!

Old Dave

DRWeig,

Thanks. I understand that the MWB is the mean and that sometimes WB will be higher and I will have more latent heat to account for. If it is a small amount of the time, then ok, no big deal we just have to run a little slower. The percentages don't tell you how often that will be, just how often the DB will be above, correct?

What is one size up (ref my numbers above)? As I said, 102/76 seems reasonable. I will be pulling air from inside a factory, which is not air-conditioned, but is at least out of the sun. It can get warm though because of the processing equipment, so I figure that is a wash (i.e. inside temp same as outside temp).

Are you designing HVAC equipment for the space that process line will be in, or are you designing cooling for the process equipment?

MintJulep,

The cooling is for the process. It is a blown film line. We want to supply 3500 CFM, 54F air to the outside of the bubble and 2000 CFM, 54F air to the inside of the bubble.

DJV,

The design condition for cooling can be based on either maximum dry bulb temperature with mean coincidental wet bulb temperature or maximum wet bulb temperature with mean coincindental dry bulb temperature. Your local weather data book can provide you these conditions. For tropical areas where humidity is a big problem, 1% values of WB and MDB can be a safe bet, when you see the total enthalpy that is to be removed by the cooling coil.

Total enthalpy for the above 3 design temperatures given by MintJulep is same but there is a significant difference in latent load. If WB is nearly constant over a span of one year(it looks like, from the above data), you will have high humidity problems if you choose 0.4% values. This can be as high as 99.6% of time(because for 99.6% days DB is less than 100F theoretically)

The latent load difference between the 1% and 2% values is not significant. So you can safely choose 1% values. If your area is highly humid then check for WB and MDB values to know the increase in total enthalpy. You need not worry if the total enthalpy at this design condition is nearer to the total enthalpy at 1% DB and MWB conditions.

Moisture problem will be predominant if you are pulling air from inside.

Plot all the design coil conditions on a psychrometric chart and also plot the actual design values given by MintJulep. Select those coil conditions which can take care of higher hmidity ratio than the actual design values.

Regards,

Thanks quark. Here's the RH and h for 1% condition from MintJulep and design conditions used by suppliers...

1% conditions: 23.77%RH, 33.23 BTU/lb
102/76: 30.76%RH, 39.6 BTU/lb
110/91: 48.86%RH, 57.58 BTU/lb
128/95: 30.71%RH, 63.41 BTU/lb

What do you think? It seems a big difference. Someone mentioned sizing the unit up from the 1% numbers. Well, would that be 1 ton, 5 tons, etc...? Any rule of thumb there?

I understand Cairo to have hot and dry summers, but you suggest pulling inside air may mean I pull more humid air, correct? The factory has very large doors each end, and open windows along the sides, so I don't know how much more humid it might be that the outside.

102/76 seems to be a better option. The other coil conditions are too redundant and put much load on control valves(approximately 50% valve is closed at maximum load condition).

Is it a once through system? I may be of little help as I am totally unaware of the production process your are speaking about. If it is for space cooling then I will consider around 1 ton for 400 cfm as a rough estimate.

Regards,

Yes it is a once through system. It is not space cooling - it is blowing air on a plastic melt extruded from a die in order to cool the melt. The melt comes out of an annular die to form a tube and we blow air both inside the tube (bubble) and outside the tube.

Can you elaborate on the control valve? How do you determine the 50% value?

I appreciate the help. As I said I'm not an HVAC guy and haven't had too many air cooling projects yet in this job.

Where does the air come from? Are you taking outside air? Air from inside the factory?

What will the air condidtions be when it is done in the process? It may be more efficient to recycle the process air than go 100% OA.

Air is from inside the factory, which is not air conditioned. There are large doors open each end and windows along the side.

The air is around 180F after the process. Nobody recycles to my knowledge.

Alright, then you need to use the interior conditions as the design conditions for the cooling equipment.

Since the factory is not air conditioned, and if full of heat-producing equipment it will be hotter than the outside design conditions noted by ASHRAE on a design day.

You might actually be better off using outside air than air from inside.

MintJulep,

Unfortunately I don't know inside conditions and don't have time to wait 'till summer to collect data. I will use the 102/76 condition and start by taking inside air, and change to outside air if needed later. As long as the air inlet is not too close to the machinery, then I don't think the air will be hotter than outside air as the factory is somewhat open and ceiling is up to 60' tall in some areas, so there is plenty of place for the heat to go.

quark (or anyone),

Still curious about the comment that some of the coil conditions I mentioned are redundant and would put too much load on the control valves. In what way do are they redundant? In what way do they put too much load on the valve and how can this be known at this time without more detailed info on the valve and the system details?

Actually I underestimated the cooling load. The enthalpy corresponding to 97/69 is 33.4btu/lb and for 54F saturated air it is 22.8. The total load will be 4.5x(3500+2000)x(33.4-22.8) = 262350 btu or 21.86 TR.

Just calculate total heat removed from the extruded parts by mCpdT where m is mass rate of components, Cp is specific heat and dT is temperature difference of initial and final conditions. I imagine potential savings by heat recovery from exhaust air.

No rocket science in the control valve thing. Fro ex. if you select the coil with 110/91, there is a heat flow of (57.58-22.8=) 34.78 btu/lb from air, where as you require to remove only (33.4-22.8=)10.6btu/lb. So you coil capacity is high by 300%. So the maximum flowrate should be not more than 30% of the maximum coil capacity.

Regards,