ACtion Magazine - March 2015 - (Page 22)
System charge determination
By Ward Atkinson (Bill Hill, MACRAE LLC, contributed to this article.)
onversion vehicles which have additional cooling units
added to the OEM's production front end A/C systems
present a special challenge: establishing the "new" system's correct refrigerant and lubricant charge amounts.
Modification of a Production A/C system with
Vehicle manufacturers establish A/C system charges in an environmental test facility by running a charge determination test procedure at
high ambient load conditions.
from which the ﬁnal charge amount can be determined. Then the vehicle
is operated on the road (30 to 50 MPH under safe road conditions) at an
ambient of at least 95 degrees F and as much humidity as possible.
Instrumentation on the system requires high and low side system pressures, and tubing (pipe) surface temperatures at the compressor inlet and
discharge lines, the TXV inlet line(s) and outlet air (panel and/or auxiliary evaporator) temperature. "T" into the low side pressure gauge line a
charging hose attached to a small cylinder of refrigerant and an accurate
Since the original OEM front A/C system has an established charge
amount, this amount of refrigerant can be used as the
starting point in establishing the refrigerant charge curve.
Operate so all system(s) have a maximum load (controls set to outside air, or vehicle windows open and high
blower speed(s). Additional charge curves for various potential system
operating modes should be established. To assure system operation when
Environmental test facility
Since companies in the conversion industry normally do not have access to a test chamber, there is a method (detailed below) that can provide
some guidance by operating the modiﬁed vehicle under high load weather
conditions to develop an on-road system refrigerant charge curve.
Consideration for different system designs
There is a large variety of A/C system designs used in today's vehicles.
They can include systems with oriﬁce tube expansion devices with low
side accumulators, systems with thermal expansion devices, high side receivers (some with multiple receivers) systems. Others can have internal
heat exchangers, systems with condensers that have built-in receivers and
integrated sub-cooling loops, and systems which utilize both refrigerantto-water chillers as well as refrigerant-to-air evaporators.
Some of these systems have very low refrigerant charges approaching 350 grams, and the amount of reserve charge that is designed into the
system varies by vehicle manufacturer. This makes the conversion of an
OEM production A/C system more complex. Some vehicle conversions
also add an additional auxiliary condenser that will increase the amount of
refrigerant required for operation.
Establishing system charge
The procedure begins with evacuating the system and installing a
small amount of refrigerant as a starting point to develop a charge curve
ACTION * March 2015
Typical refrigerant charge curves
Table of Contents for the Digital Edition of ACtion Magazine - March 2015
Engine cooling systems: Electric cooling fan operating strategies
System Charge Determination
Heavy duty and off road
Letters to the Editor
By the numbers
ACtion Magazine - March 2015