MACS Monthly Newsletter - 2022 - OCT3

is cooled. This cooled coolant is then pumped into a
so-called cooler which is a cooling heat exchanger
located in the HVAC module that cools and dehumidifies
the cabin supply air, matching the function
of a traditional evaporator. This form of the SLS is
applied when the goal is to isolate the refrigerant
from the cabin and is very beneficial in a two-evaporator
SUV style HVAC system where the refrigerant
charge is dramatically reduced. Also, in the chiller,
the refrigerant must be colder than the coolant to
transfer the heat resulting in lower evaporating refrigerant
temperatures and pressures than a DX system.
For
a single loop condensing system, a WCC is applied
to condense the refrigerant. This type of system
is generally used in vehicles that have cooling
systems with a low temperature coolant loop that is
present for another function, such as close-coupled
Charge Air cooling. In such systems, the condensing
heat is rejected to the ambient air via a Low Temperature
Radiator (LTR) along with the other cooling
loads. In this architecture, the low temperature
cooling system is in the vehicle for another, primary
purpose and the WCC is added to either reduce the
number of heat exchangers in the front end of the
vehicle or to reduce vehicle front end heat exchanger
packaging space. For condensing,
the refrigerant
must be at a higher temperature and pressure
than DX system to transfer the heat to the coolant.
The basic DX system is compared to the single loop
cooling in Figures 4 and 5. In this SLS setup, vehicle
heating needs may be coolant based in the case of
a Hybrid vehicle or electric resistance heaters for
full EVs.
In the dual loop set up, shown in Figure 6, a comFigure
4: Basic Direct Expansion (DX) System
pact base refrigerant system is linked to both the
cold and hot sources of the refrigerant. This is an
arrangement applied to today's EVs as it provides
multiple functions of both cooling and heating
where heating is a very significant challenge without
the historical source of excess heat from an ICE
based vehicle. Counter to expectation, Hybrid vehicles
with a defined electric-only range generally
carry an electrical heater and the full dual secondary
loop to enable use of the excess IC engine heat
when it is operating. In many cases, during both the
heating and cooling of a hybrid vehicle, the ICE engine
is started only to meet the demands of passenger
thermal comfort.
Along with the need to thermally manage an EVs
propulsion system's motors, power
electronics and energy storage battery,
passenger
thermal
comfort
Figure 5: Single Loop SLS, Cabin Cooling
October 2022
3
management creates a significant
vehicle driving range reducing energy
demand, particularly in the heating
mode. One benefit of the dual
loop SLS arrangement, a basic feature
of all heat pumps, is that heating
mode performance can be up to four
times the efficiency of direct electrical
resistance heating. This efficiency
benefit, called the Coefficient of
Performance or COP, is present in
both the air conditioning and heating
operating modes and stems from
the basic thermodynamics of the DX
system.
MACS Service Reports
Tim Craig
Tim Craig
MACS Monthly Newsletter - 2022

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