Effiecency Ratings

Dnkldorf

Hi guys.....

I am looking for some info on what to do with roof top units on commercial buildings. Anyone have any info on how to measure effiecency on these.

I am looking for ways to save customers on energy consumption, but the Hvac guys that come out to fix them or maintain them have no way to measure how effective they are working. All they can tell me is the compressor is working, the fans are working, so the unit must be working fine.

Is there a way to figure out that a 30 ton unit is actually providing 30 ton of cooling?

Me and my customer did some BTU calculations of poeple and equipment and the roof top units (RTU) should keep up but they don't. It is like they only are providing half of what they should. We checked all the ducting and registers, had the units serviced, but still had to add 3 more units on the roof.

Anyone have any experience in this?


Dnk.....

mdshunk

A rule of thumb for air conditioning compressor motors is that there is one horsepower per ton. This rule will not work for applications other than comfort cooling air conditioning applications. The number of horsepower required to provide each ton of cooling varies with the suction pressure and head pressure. The higher the head pressure or the lower the suction pressures, the more motor horsepower required to achieve a ton of cooling. Another way of stating it is that as the pressure difference between the suction and head pressures increase, the system tonnage decreases. This is why it is so important to keep condensers clean, evaporator air filters replaced and airflow on both condensers and evaporators up to normal. Anything, which increases head pressure or decreases suction pressure will decrease system capacity and at the same time increase operating costs.

Some things, in my experience, that have caused reduced capacity in otherwise nicely installed systems are:

• Refrigerant charges more than 5% over or under spec
• Economizers with broken, frozen or missing drive system components, outside air or mixed air sensor failure, low changeover temperature setpoint, or the use of a single-stage cooling thermostat
• Low airflow (less than 300 CFM/ton, normal is 400/ton)
• failed sensors
• misadjusted or failing expansion valves

Almost every rooftop unit that I came up against had a wrong refrigerant charge and a bad or misadjusted economizer. Ensuring that all stages of cooling are energized when the refrigerant charge is checked and upgrading to solid state economizer will almost always allow a previously underperforming RTU to work more near it's rated capacity.

The actual equipment to measure the actual performance of a particular unit, in tons of cooling, is super expensive. It involves setting up a variety of sensors on the unit and connecting them all to a box that hooks to a laptop. You're in the 20 grand range here. The CheckMe! tool is used by some people, but it won't display actual tonnage performance.

mdshunk

I just re-read your original post, and I got the feeling that the installed equipment never really performed well from the beginning. Sounds like perhaps you have some load that you didn't account for. Solar gain from big glass walls? These can heat a building like a microwave in 15 minutes. Radiant floor heat or hot water/steam duct coils that didn't get turned off for the cooling season?
Just some thoughts.

747

In chicago they use the big industrial water cooled airconditioners if that makes any sense. I don't no what there called. AS a matter of fact the old bank in town had one of those my plumber told me because he had to service it. He said that one was natural gas. He said he was glad when they tore down that bank and built a new one because he hated working on it. I found that interesting because it was a natural gas airconditioner.

locofoco

The big question is did they ever work properly? If they did you must find out what changed to cause the problem. Discharge air temperature and airflow readings(cfm) are the truest gauge to performance. Ask a local A/C company to send a certified air balance tech out to evaluate unit performance. Same company should be able to determine the building load. Normal well insulated commercial buildings should run about 400sq. ft. per ton of cooling, however diversity factors such as ceiling hts., south facing windowalls, excess computers/elect.equip., location ( Fl. MN. etc.) could drop sq.ft. per ton significantly.
Loco

locofoco

If you have a problem finding an air balance company. Call your local Sheet Metal Workers hall, they should be able to direct you to a shop that has SMACNA
Certified techs.