Design Outdoor temps Vs Peak Load Temps

[ub313]

Hi can anyone guide me on this?
All Cooling Load programs use Design Outdoor conditions of DB and WB (usually from ASHRAE 1% as an example say 89F/73F). But the actual DB/WB conditions (lets say 94/75) when peak load occurence are always different from Design outdoor conditions entered.
While sizing Equipment are rated to have capacities are as per peak design load but rated at design Outdoor conditions. Is there an anomaly?

For testing indoor design conditions, which set of outdoor design conditions are more relevant, between the Design values entered and the Peak load occurence?

 

[tombmech]

If I interpret you correctly, I think you're asking why the calculated maximum load occurs at different temperatures than the listed outdoor peaks.

That has to do with solar load and other load factors that don't depend strictly on temperature. Depending on the partial loads, or an unusual construction with a single load, the total peak may not occur at the peak time for outside air temperatures during the year, which almost always occur at 3 or 4 o'clock during July or August. The solar load is more dependent on exposed glass surface or large dark roof areas. Of course, it is possible to have high enough internal loads that outside loads don't matter much. Or, you may have a situation where de-humidifying is most important with large amounts of outside air.

As for sizing equipment, I believe you're referring to the ambient conditions specified for air-cooled equipment capacity. The peak outside temperatures put the highest strain on the system, so it is important to know that the equipment can perform as rated at the worst conditions - even if that may not be your case.

Testing can occur at anytime of the year for most systems. This is because individual components are measured for their individual response, and a confidence may be built that ensures they will also perform at peak conditions when combined with all the other components. In some cases, this is not sufficient, and simulated loads are used. Even so, some systems may, in fact, require specific seasons for complete testing.

 

[quark]

The question is not clearly put. Understanding in the same lines of tombmech, the ambient load is one part of the total load and the peak ambient load may not be a part of peak total load.

Further, the actual load exceeds for 1% of annual hours than the design condition (i.e 87.6 hours or 3.65 days), going for higher capacities may not be beneficial. In your case, considering a total flowrate of 10000 cfm, for example, you are less by 6.87 tons for 3.65 days in a year.

With all the presumed redundant factors of safety, this may not pose a great problem.

I hope I understood your question.

 

[HVAC68]

There are 2 different things.

1. Design ambient load (1% as per ASHRAE) which is generally used for cooling load calculations. This means that 99% of the times in a year the ambient temperature may not exceed the design value. It doesn't make economical sense to design a system for 3.65 days in a year, and probably that's the reason why this has been taken as a design standard by ASHRAE.

2. Peak ambient temperature - This is the maximum temperature that may be experienced in the location - it could even be for 1 hour. However, the equipment has to be designed to ensure that it delivers, albeit, a lesser capacity.

My experience in the Middle-East country - Design temperatures used to be 46 deg C, we used to select equipment which were to deliver the rated capacity at 46 deg C. However, the equipment were to continue working upto 52 deg C !!!

HVAC68

 

[lilliput1]

For offices you can use the 89°F db/73°F wb. But for critical spaces like hospitals, use the ASHRAE 1% (0.4% in later editions). If 100% OA use the 1% °F wb because it has the higher enthalphy value than peak db w/ coincident wb.