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Design Temperature Difference for Chillers
I get emails from time to time with questions that stem from the articles or the podcast. This was a great question, but I was not the best person to answer it.
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.
I`ll divert ” a bit” the discussion from strictly refrigerant and refrigeration towards the hydraulic side of the chiller if permitted. The refrigerant-refrigeration cycle of the chiller is already answered by that guru.
Most manufacturers have a precise chw ( chilled water) and cdw ( condenser water) pressure drops across chiller`s evaporators and condensers for a maixum efficiency ( heat exchange ( chw temp givin off to refrigerant and heat being picked up by condenser water). These dp ( delta pressure) has to be exact with manufacturer cut sheets. An evaporator usually has to achieve a pressure drop between 3-5psi for a proper heat rejection ( from chw to refrigerant) and a pressure drop between 9-11 psi for a proper pressure drop across condenser for a proper heat rejection ( from refrigerant to condenser water. In both cases, chiller is equipped with an electronic flow switch device which range will be set at initial startup of equipment. Not to mention the each system ( evaporator and condenser water systems) has to achieve the design flow conditions measured at CBV.
Some of older systems don`t have a balancing valve installed and flow through evaporator or condenser cannot be measured. Then the ultrasonic measurement come in place to allow for an approximate flow reading.
Most of older systems has been upgraded with new pumps which incorporate adrive ( VFD-variable speed/frequency drive) which will allow pumps to respond to variable demand-variable flow instead of constant flow systems.
Evaporator is less likely to become clogged and is less prone to plug-up conditions, even though pumps strainer needs to be cleaned on a regular PM ( preventative maintenance). The most common problem which will lower chiller efficiency and will deliver chw setpoint is the opened condenser water cooling tower system which due to way cooling tower fan`s blade draw the air, will draw dust particle and entrain into cdw flow which will plug some of condenser tubes eventually, lowering condenser efficiency, thus chw setpoint! A proper maintenance of cooling tower, powerwash, sand balst systems or centrifugal screening pumps installed wiil prolong time between cooling tower maintenance.
Here I`d like to mention a bit about differential pressure sensors installed on chw system and on closed loop condenser system. These r the one speeding up and down a pump depending on differential pressure drop settings! As more zones (terminal units as fan coils, ahu`s chw coils, in-row coolers for IT racks, you name it!) opens, dp sensors will ramps the speed of the pump up to maintain pressure setpoint and reach the design GPM. As soon as zone satisfied, zone valve closes and system pressure starts to increase, dp senses and will sense that increase and will open the by-pass valve between supply and return or will ramp the pump down to maintain the dp sensor. What a beauty!! Same as an harmonic!
There are soo many subjects could be talked about a chw and cdw systems, pumps, sensors, drives etc!