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Design Temperature Difference for Chillers
I reached out to Jeff Neiman, our resident HVAC School chiller tech, and he answered it. Here is the question:
Thanks for all the good material you provide. I mostly work on the commerical building side of HVAC where chilled water is used as cooling medium and cooling towers provide condenser water. We have chillers as well as heat pump and air cool splits throughout facilities. Most of your diagnostics and troubleshooting methods are for air cooled units. Can they be applied to water cooled evaporators and water cooled condensers? My thinking is yes and no, because with cooling tower 85 supply and return 95 is maintained and 45 supply and 55 return chilled water is provided. Since there is not much change in these temps as opposed to outdoor ambient temperature there won't be much pressure change in condenser. And as long water is regulated at proper flow to evaporators and condenser then all should hold steady. Do you have any input on this? I'm in NYC. Went to 2 year hvac school and worked almost 3 years in field starting out as a helper in service van as experience and learned as much then got into the building side for about 8 years now. I like listening to your podcast and reading your material as it keeps me refresh with field work as the building side is a little different but the basics and fundamentals are the same. Thanks!
The answer is yes.
Some of the measurements can be applied to chillers as well. Just some of the verbiage is different, and the values differ.
The numbers for chilled water (44 out 54 in) and condenser water (85 out 95 in) are industry-standard values at full load conditions. Most chillers, regardless of manufacturer, will have a 10°F delta T on the cond and evap. Machines that operate outside of those ranges are chillers that were ordered specifically to provide a lower temp or larger delta T.
Many people look at the compressor motor RLA% as the chiller capacity, which is not accurate. Chiller capacity is measured by the evap delta T. If the chiller is designed for 10°F (5.5K) delta, is currently providing 44°F (6.66°C) water, and the return water is at 49°F (9.44°C), the delta T is 5°F (2.75K). So, that chiller is currently running at 50% of its total capacity.
Subcooling is still measured the same, although the reading that you get will change as chiller capacity changes. At low loads, your subcooling will be lower and will increase as capacity increases.
Suction superheat is a value that I really don't look at because the reading on a flooded type of system will usually be very low or even 0. Rather, discharge superheat (discharge temp – cond sat temp) is a more accurate reading and will be a direct result of your suction superheat. When there is high suction SH, there will be high discharge SH and vice versa. Again, this value will change as chiller capacity changes.
However, if the chiller is a DX type, the suction superheat is just as valid as on a residential system.
One of the values described in the podcast was temperature difference (supply air temp – coil temp).
In regards to air handlers with chilled water coils, you can do the same thing. Measure your supply air temp minus the coil leaving water temp. This will tell you how well the heat transfers to the water from the air going across the coil.
In chiller lingo, this measurement is called the approach.
There are two different approach temps that I look at on a chiller:
- Condenser approach (cond sat temp – lvg cond water temp)
- Evaporator approach (lvg water temp – evap sat temp)
Approach values should range in 0 – 3°F (0K – 1.65K), given that your flows are correct.
Just like on air-cooled units where proper airflow is needed across the evaporator and condenser, you need to verify that you have proper water flows.
In air-to-air applications, you measure static to identify airflow issues. In water applications, I measure the pressure differential across each barrel. If I know my design pressure drop on the evap and cond, I can compare it to my actual to know if my flows are proper. However, keep in mind that most chiller manufacturers will give the design pressure drop in ft/hd. You will need to convert your real-time reading to ft/hd to accurately compare if you are using a gauge with a psi scale.
Even if your water temps stay pretty constant while in operation, your pressures will veer off as problems arise, and your approach values will increase.
The chiller will always try to maintain at 44°F (6.66°C) chilled water out (or whatever the setpoint is) as long as it can do so.
The refrigeration cycle doesn't change. Stick to the basics, and don't overthink it.
When running a building, try to get your condenser water as low as possible when running, but stay above 65°F (18.33°C).
Anytime you can provide condenser water lower than the design of 85°F (29.44°C), you will lower your condenser pressure and lower the lift (cond pressure – evap pressure). This will result in less work the compressor has to do and lower KW. This is a common method called condenser relief.