Chiller Unloading Pueblo

Issue:

How low can this chiller unload?

Resolution:

RTAE

The RTAE has one AFD screw compressor per circuit. The minimum unloading is 20% of chiller's maximum capacity.

RTAF

The RTAF minimum unloading is 15% of the chillers maximum.

RTAC and RTWD/RTUD

As a rule of thumb, the RTAC and RTWD can unload as low as 15% of the total capacity of the machine. The individual compressors can unload as low as 30% of their nominal capacity. The RTAC and RTWD chillers use the same GP2 compressors. The RTWD chiller will always have 2 compressors. The RTAC chiller will have anywhere from 2 to 4 compressors, depending on the chiller nominal tonnage.

Each compressor has only four moving parts; two rotors that provide compression and male and female load-control valves. Capacity control is accomplished by means of a female step load control valve and a male control valve. The female step valve is the first stage of loading after the compressor starts and the last stage of unloading before the compressor shuts down. The male control valve is positioned by a cylinder along the length of the male rotor. Compressor capacity is dictated by the position of the loading valve relative to the rotors. When the valve slides toward the discharge end of the rotors compressor capacity is reduced.

Let's take a look at an example with a nominal 200 ton chiller configured with two nominal 100 ton compressors. When there is a call for cooling the first compressor will start fully unloaded and run a minimum of 45 seconds with the female step valve off, this is considered 30% of compressor capacity. If there is enough load the female step valve will be activated and the compressor will now be ~ 60% loaded. If the load cannot be met the male control valve will begin operation from the ~ 60% to 100%load. The male control valve will infinitely modulate between 60 - 100% load based on the actual load.

If the load still cannot be satisfied with only one compressor running, the first compressor will begin to unload via the male control valve to ~ 60 % load before bringing on the second compressor. The second compressor will now be brought on and follow the same loading method as the first compressor at start up. At this point the compressors will continually modulate based on the load and the algorithms running in the unit microprocessor (CH530). When the chilled water set point has been met and the chilled water temperature is approaching the differential to stop the chiller will now begin to unload the compressor with the greatest number of run hours. Unloading of a compressor is the exact opposite of the load sequence; hence, the female step valve will be the last stage of unloading before the compressor shuts down.

A 100 nominal ton compressor at 30% load has a capacity of 30 tons; therefore, 30 tons is 15% of 200 nominal tons.

Unloading Visual for RTAC and RTWD compressors

RTHD

The RTHD chiller uses a single semi-hermetic, direct-drive helical rotary type compressor designed specifically for the RTHD chiller and can unload to around 20 to 25% of the maximum capacity of the chiller. The AFD allows the chiller to unload to 20%.

Unlike the the GP2 compressor that is used on the RTAC and RTWD chillers, the RTHD compressor does not have a female step-loading valve. Capacity is controlled by the "load" and "unload" solenoid valves. These solenoid valves receive signals from the CH530. based on system cooling requirements. When the slide valve fully covers the rotors, the compressor is fully loaded. Because the minimum load of the RTHD compressor is 20 to 25% of the chiller's maximum capacity, it is important to not oversize the chiller if it will be operating at low load conditions to avoid excessive compressor cycling.

In June 2014, Pueblo introduced the RTHD (Optimus^TM) chiller with an optional AFD and UC800 controller.  See RTHD with Adaptive Frequency Drive (AFD) for additional information.

In August 2014, the CH530 controller was obsoleted and all RTHD (Optimus^TM) chillers, both y-delta and AFD, are now built with UC800 controller.

CGAM

CGAM models 20 to 35 have two compressors per circuit. Sizes 20, 26 and 30 can unload to about 50%. Model 35 can unload to about 43%

CGAM models 40 to 120 have 4 compressors, with 2 compressors per circuit. Most of the models have same size compressors, so the minimum unloading capability is about 25% of maximum capacity.

CGAM models 70, 90 and 110 have different sized compressors per circuit, so the minimum capacity will depend on which compressor is running at that time. 

The CGAM 130 has 3 compressors manifolded on each circuit - (2) 20T compressors and (1) 25T compressor.  Depending on which compressor is running at that time, the chiller will unload to either 15% or 19%. 

Older models

RTHA, RTHB, RTHC (actually the same compressor as RTHD), and RTAA (130-400) unload similar to RTHD compressors with a single slide valve for compressor unloading. RTAA (70-125) and RTWA/RTUA compressors unload similar to the RTAC and RTWD compressors do.

Important notes

The actual minimum unloading capability will depend on the operating conditions and the type of unloading that the chiller will encounter.

To see the minimum load at your specific conditions, use the part-load graph/chart in TOPSS. Change from the default of 10% unloading increments to 1%. Typically, the unit will unload to a lower capacity with constant condenser unloading. With AHRI relief the lift is lower, so the capacity of the compressors minimum load would be higher.

Notes:

Slide valve or capacity control is outlined in the Operating Principles section of the IOM.

For minimum load information on CGAM scroll chillers, see What is the minimum unloading point for this chiller?

Changes:

Other Question/Problem/Cause Statements:

• How low can this chiller unload?
• What is the minimum step of unloading for this chiller?
• How does this chiller unload?
• Do we have any information on compressor capacity control?
• How do we control capacity with this chiller?
• Where can I find information on slide valve operation and compressor unloading?
• What is this minimum load capability of this chiller?
• What is the minimum unloading capacity for this chiller?
• How far will this chiller unload?
• What is the minimum capacity of the chiller?
• What is the minimum unloading capacity of this chiller?
• What's the min load?
• What is the turndown for this chiller?
• What is the minimum turndown for this chiller?
• What is the maximum turndown for this chiller?

Facts:

• Air-Cooled Chiller (Pueblo) (RTAC)
• Water-Cooled Chiller (Pueblo) (RTWD)
• Compressor Chiller (Pueblo) (RTUD 70-200 Ton)
• Air-Cooled Chiller (Pueblo) (RTAA 130-400 Ton)
• Air-Cooled Chiller (Pueblo) (RTAA 70-125 Ton)
• Compressor Chiller (Pueblo) (RTUA)
• Water-Cooled Chiller (Pueblo) (RTHA)
• Water-Cooled Chiller (Pueblo) (RTHB)
• Water-Cooled Chiller (Pueblo) (RTHC)
• Water-Cooled Chiller (Pueblo) (RTHD)
• Water-Cooled Chiller (Pueblo) (RTWA)
• Air-Cooled Chiller (Pueblo) (RTAE)