Discharge Gas Temperature Protection; Excessive Discharge Gas Temperatures; Internal Thermo-Disc; Advanced Scroll Temperature Protection (Astp) - Emerson Copeland Scroll ZR24KRE Application Manuallines

Discharge gas temperature protection

3.8
IMPORTANT
Inadequate lubrication! Scroll set damage! All ZR*KRE compressors must
be equipped with a discharge gas temperature protection.
A good system control shall prevent the system from operating outside the published operating
envelope and acceptable superheat range, whatever the climatic conditions and the capacity
demand. However, under some extreme operating conditions such as loss of charge or improper
control operation, the internal discharge gas temperature reached can cause compressor damage.
In order to guarantee positive compressor protection, discharge gas temperature protection is
required for any application with Copeland brand compressors.
The maximum discharge gas temperature is 130 °C for all ZR*KRE models.
Discharge gas temperature protection is the "fall-back" for failure of the system control. It is essential
that proper control of both the evaporating and condensing pressures and the superheat is
maintained and has the ability to cope with all likely conditions and high loads. Reliance on protectors
will cause inadequate system performance and short cycling.
NOTE: The maximum discharge gas temperatures indicated in this chapter are valid for safe
operation within the approved application envelope. The discharge line thermostat has the
function of a compressor safety device; it is not designed to control the operating envelope.
For compressor envelope control, an additional control device or regulation must be used.

3.8.1 Excessive discharge gas temperatures

A few of the possible consequences of excessive discharge gas temperatures are listed below:
▪ Since the oil circulates in the system with the refrigerant, it is subjected to high discharge gas
temperatures. If the discharge gas temperature becomes too high, the so-called "cooking" effect
will occur (heating of oil under exclusion of air). Carbon deposits can form at points of high
temperature, for example on the valves, oil channels, oil filters, etc. The oil lubricity will be reduced
and a progressive wear process will occur which will prematurely damage the compressor.
▪ The stability of the refrigerant can also be affected, particularly if traces of contaminant are
present.
The problems listed under the first 2 points frequently occur simultaneously, particularly since the
chemical reaction speed approximately doubles with every 10 K temperature rise. This directly leads
to chemical reactions of the oil with the refrigerant and the compounds extracted from sealants and
insulation material. As a consequence, contaminants of various types, among them acids, will form
inside the system.

3.8.2 Internal thermo-disc

ZR24KRE to ZR81KRE compressors have an internal thermo-disc discharge gas temperature
protection. This thermo-disc opens a gas passage from the discharge port to the suction side near
the motor protector when the discharged gas reaches a critical temperature. The hot gas then causes
the motor protector to trip shutting down the compressor. It opens at 146 °C ± 4 K and closes at
91 °C ± 7 K.

3.8.3 Advanced Scroll Temperature Protection (ASTP)

ZR108KRE to ZR190KRE compressors are equipped with an Advanced Scroll Temperature
Protection (ASTP). ASTP is a temperature sensitive thermo-disc that acts to protect the compressor
from discharge gas overheating. Once the discharge gas reaches a critical temperature, the ASTP
feature will cause the scrolls to separate and stop pumping although the motor continues to run. After
running for some time without pumping gas, the motor protector will open.
NOTE: ASTP was developed to protect the compressor, not for system envelope control.
If the system engineer wants to prevent ASTP trips and to limit the maximum compressor discharge
temperature to a lower value, a discharge sensor can be used. The recommended setpoint is 130 °C.
This value should be determined and verified according to the application. Any protector attached to
the discharge line must be well insulated with good quality material that will last for the unit lifetime.
A label has been added above the terminal box to identify compressors with Advanced Scroll
Temperature Protection – see Figure 13 below.
12 AGL_AC_ST_ZRKRE_E_Rev01