a system equipped with a pressurized de-aeration reservoir, air is vented along the bleed hoses, which must be arranged to slope continuously upwards from the highest point on the engine and/or radiator to the header/
degas reservoir. Systems where the vent lines do not slope continuously upward have a low point in which liquid can be trapped. The trapped liquid may interfere with air venting during fill.
Although a cooling system may be equipped with air vent lines, there will still be components which must simultaneously fill and vent (such as the engine, and the degas reservoir outlet hose). Filling and venting can take place simultaneously through the same component ONLY if the following conditions are met:
• The component is large enough to accommodate both the incoming liquid stream and the outgoing air stream.
• The coolant enters the component and stays to one side throughout the fill process, never completely filling up the flow path (see figure 109).
Figure 114, Simultaneous fill and vent
The filling point should be the highest point of the system, and preferably at the same location as the pressure cap (to avoid the use of two caps). Once full, the fill level must be as high as the highest point in the system to ensure a complete fill.
Where a remote de-aerating header tank (degas reservoir) is used:
• The final fill level should be above the liquid level at the location(s) where the degas bleed hoses are tapped into the cooling system (This is required to provide a full fill even after some loss of coolant over time)
• The reservoir outlet tube should be relatively large in diameter, and should join the main system as close to the water pump as possible to ensure adequate pump priming and to maximize NPSHA.
• The reservoir outlet tube should be tilted at least 5 degrees from the vertical to force the incoming fluid to follow along one side of the tube, allowing air a path to exit from the tube.
Sometimes system designers provide venting locations with valves that must be manually opened during the fill process. When considering special tools or procedures for service filling, the system design engineer should always consider the consequences (and the likelihood) of the end user not having the special tools or procedures required.
An FMEA (Failure Mode and Effects Analysis) is a good way to quantify risk, and to provide insight into ways of mitigating risk.
For example, the end user or mechanic may not have access to a service manual when needed, so highly visible labels near the coolant fill location may be a good way to alert someone that there are manual valves that need to be opened during the refill process.
Outlet Mounted Thermostat Location
Systems with the thermostat in the outlet from the engine require bleeds on both sides of the thermostat, or a vent notch or jiggle pin in the thermostat or housing to allow transfer of air across the valve.
Inlet Mounted Thermostat Location
When the thermostat is at the inlet to the engine, filling is much more difficult because the thermostat effectively splits the cooling system into two separate parts. The first part is formed by components on the same side of the thermostat to which the reservoir is attached. This side fills normally, while the second part of the circuit remains empty. When coolant in the first part reaches the level of the radiator inlet hose, the second part starts to back fill through the hose. This frequently leads to air locks and false fills (see figure 115).
Refill Quantity
The refill quantity must be sufficient to allow vehicle operation without overheating. As a general guideline, a target of 95% of total system capacity on first fill is recommended.
The high risk of false fills in engines having a thermostat at the inlet, dictates that the degas reservoir outlet hose should always be connected to the system on the engine side of the thermostat. This is necessary to provide adequate NPSHA, to ensure that the water pump is always
1 2
3
Dwg 1: This configuration allows air to escape during fill.
Dwg 2: The fill velocity coming straight down the tube causes the coolant to block the air from escaping.
Dwg 3: The tube is too small for air and coolant at the same time.
primed, and to give the engine priority during filling (engine damage may occur in an incorrectly filled system, even during the engine idle period required to de-aerate the system).
Figure 115, False fill with inlet mounted thermostat
After a service fill, the volume of air that remains in the system must be less than the volume in the reservoir between the 'max' and 'min' fill lines. This ensures that the drop in reservoir level that will occur when the remaining air is eventually purged into the reservoir will not result in a liquid level that is inadequate for system performance.
In-Plant Filling
The design of the filler neck should allow adequate clearance for production filling tools, and the volume underneath the filler neck (generally in the degas reservoir) should be large enough to allow turbulence created during the filling process to dissipate. This prevents air from becoming entrained and causing an air lock.
Filling the cooling system in the vehicle assembly plant uses of a high level of vacuum assistance, typically on the order of 97% or greater. As a consequence, a 100% fill is nominally guaranteed. An overly restrictive system, however, may require an excessively long period of time for evacuation and fill.
In-Service Draining
When the cooling system is drained for the purpose of replacing the coolant, it is important that all (100%) of the old coolant is removed. This is important because any remaining old coolant may contaminate the new coolant,
dilute the new coolant or reduce the amount of available corrosion inhibitors provided by the new coolant.
The normal procedure to achieve 100% drain of used coolant is to drain and then flush with water. If the used coolant is heavily discolored and/or contains particulate, damage of the engine components may have occurred.
This may require chemical cleaning of the cooling system.
Equipment is available to perform 100% coolant evacuation (drain) of the old coolant, flush and complete fill with new coolant.
Most vehicle manufacturers have recommended practices for drain, flush and fill. An example of a typical drain procedure follows.
• Drain coolant from the radiator petcock.
• Place flush/fill tee in heater inlet hose and disconnect upper radiator hose from radiator.
• Flush with clean tap water until clear.
• Start engine while continuing to reverse flush with water.
• Run engine ten minutes.
• Disconnect upper radiator hose and fill cooling system with water. Reverse flush with clean tap water for five minutes.
• Drain water from cooling system and replace upper radiator hose.
• Blow out the heater core and block with 15 psi air introduced through the heater inlet hose.
• Install pressure cap and close radiator petcock.
• Using vacuum-fill tool, check system for integrity and install a quantity of concentrated coolant equal to 50%
of the total cooling system capacity.
• Top off the cooling system with clean tap water (distilled or deionized may be preferred).
• Turn on engine and check cooling system for leaks.
• When engine is warm, turn on the heater to make sure there is coolant flow through the heater.
• Turn off engine and allow it to cool.
• When the engine is cool, top off the coolant reservoir to the proper level.
Heater
Stat Pump
Reservoir
Radiator
Engine
This configuration will not allow the engine to fill
OVERVIEW
This section discusses the reasons for the de-aeration system, the causes of aeration, and sub-system design considerations.
DE-AERATION REQUIREMENTS
Figure 116: De-aeration P-Diagram
EFFECTS OF AERATION
Aeration of the coolant can lead to a number of performance and durability related problems such as:
• overheating due to reduced heat transfer coefficients in the engine and heat exchangers,
• overheating due to reduced coolant pump performance caused by the compressibility of the air entrained in the coolant,
• cooling system noises (e.g. heater gurgle),
• loss of coolant when combustion gases enter the coolant circuit, causing the total volume of the coolant (coolant with entrained air) to exceed the available volume.
Figure 117 shows a picture of a remote (from the radiator) de-aeration reservoir.
Figure 117: Remote header tank (viewed from radiator)
Sources of Entrained Air
Entrained air can come from an improper fill or a leak in the system. Air can also become entrained in the coolant when there is excessive turbulence in the degas reservoir. This can occur at high pump speeds with an improper degas system design if the air becomes re-entrained in the coolant. Also, an improper degas system design can result in the flow of air outward (rather than inward) through the degas reservoir inlets. Entrainment can also be due to the leakage of exhaust gases past the cylinder head gasket into the coolant. This can occur under normal operating conditions in diesel engines due to the high combustion pressures. Cooling systems for diesel engines must be The de-aeration system must remove entrained
exhaust gasses from the coolant at the same rate at which the exhaust gasses are entering the coolant (diesel engines). Also, when conducting a service fill, the de-aeration system must remove air remaining in the system at a rate that is consistent with an affordable service fill procedure.
Control Factors restrictor size and location
flow to reservoir reservoir design De-aeration
Sub-system Noise Factors restrictor size(V) coolant level(A) engine speed(U) vehicle attitude(U) thermostat position(E)
Air/gas
entrainment De-aeration
SC and FM
loss of heat to reservoir more hose joints re-aeration corrosion Legend: V Component Variability
A Aging
U Customer Usage E Environment FM Failure Modes SE Side Effects
Part 2 - Section 10
Deaeration Subsystem
designed to insure that the coolant is de-aerated at a rate faster then the rate at which exhaust gases enter the system.
Coolant Flow Rate To The Degas Reservoir
There is a limit to the flow rate that can be directed through the degas reservoir. Above this maximum flow rate, excessive turbulence in the reservoir causes air entrainment into the coolant. Size and design of the reservoir determine the limiting flow rate. System design must provide for a flow rate to the degas reservoir which is below the limiting flow rate.
You can determine this maximum flow rate by installing the degas reservoir on a flow stand and progressively increasing the flow rate through the reservoir. The flow rate at which re-aeration begins to occur can be detected by an increase in the fluid level (due to the increased overall coolant volume). To avoid creating turbulence when this flow enters the tank, restrictors should be placed upstream of the tank to allow the coolant to decelerate before entering. If the restrictor is molded into the tank, the connection should be angled to direct the coolant jet along the tank wall at an acute angle, thus allowing dissipation of the kinetic energy.
System design must also ensure that the flow through the reservoir always exits the reservoir at the bottom, where there is coolant, and never at the top, where the air collects.
The limiting flow rate varies with the level of coolant in the reservoir. The limiting flow rate decreases as coolant level decreases. The system design must specify a maximum allowable flow rate for the reservoir, and a minimum allowable fill level which must be maintained.
Exhaust Gas Leakage (diesel engines)
Where exhaust gas leakage into the coolant is expected as in diesel engines, the system requires a degas reservoir with adequate coolant flow to ensure that the exhaust gases are moved into the reservoir. A degas reservoir consists of a low flow region (beneath the pressure relief valve), where the buoyant forces of the entrained air are greater than the inertial forces in the coolant. Proper sub-system design must meet the following two requirements:
• Provide flow to the reservoir fast enough to move the incoming exhaust gases.
• After the flow reaches the reservoir, the velocity must be low enough to ensure de-aeration.
If the reservoir is large enough, the coolant will dissipate any turbulence before the coolant leaves the reservoir. A design that directs the incoming flow around a circular tank wall will take advantage of the wall friction to dissipate kinetic energy. The appropriate use of chambers can increase the residence time of the fluid in the reservoir, which also helps dissipate energy.
Loss of coolant through the pressure relief valve is an indication that the de-aeration system is not keeping up with the incoming exhaust gasses. To determine quantitatively how much air the de-aeration system can pass into the reservoir, the de-aeration reservoir and all attached hoses can be installed on a flow stand according to the following procedure:
1 Replace the reservoir outlet hose with a clear hose.
2 Inject a measured amount of air into the fluid entering the reservoir through the inlet hose.
3 Increase the amount of air in increments until the reser-voir outlet hose begins to show evidence of aeration.
This maximum air flow amount will vary with coolant flow rate.
OVERVIEW
This chapter discusses cooling system expansion volume.
TERMS USED IN THIS CHAPTER
Evaporative Cooling: The process of transferring heat to a liquid where the heat energy vaporizes the cooling fluid.
CONTAINMENT REQUIREMENTS
Figure 118: P-diagram for coolant containment
In general we rely on evaporative cooling after engine shut-down to prevent engine metal temperatures from exceeding temperature limits which could result in cylinder head warping and head gasket leakage. Evaporative cooling is the process of transferring heat to a liquid where the heat energy vaporizes the cooling fluid.
It is necessary to provide volume in the cooling system to contain the expansion of the coolant as it vaporizes after
shut-down, and as it expands due to thermal expansion during engine operation.
A cooling system designer can determine the required expansion volume due to vaporization after shut-down by doing the following:
1. Mark the level of the coolant in the degas reservoir before shut-down (following a high load condition) 2. Mark coolant peak level, which occurs in the first few
minutes after shutdown.
3. Measure the difference in volume between these two levels.
To determine the expansion due to the temperature increase of the coolant, 7% of the total volume of the coolant can be assumed if the system is filled at a coolant temperature of approximately 20°C, and the system does not operate above a coolant temperature of 124°C, and the coolant is a 50/50 mixture by volume of ethylene glycol and water. If the coolant temperature is lower when filling, or the peak temperature is higher, or a different coolant is used, then the expansion chart for the appropriate coolant must be used.
The amount of expansion volume required equals the sum of:
• the expansion volume required for thermal expansion, plus
• the amount required for vaporization after shut-down, plus
• some safety factor to account for differences in vehicles, and aging (which may increase engine temperatures).
The expansion volume is located in the de-aerating header tank above the liquid level, so that when the pressure relief valve is opened, vapor is vented before coolant. This location reduces the likelihood of coolant loss and is convenient because the air volume can easily be provided by stopping the fill process before the expansion volume begins to fill.
The cooling system must provide enough expansion volume to prevent loss of coolant when the coolant expands due to increasing temperatures or due to vaporization occurring after shut-down.
Noise factors Coolant level in reservoir (U)
Temperature (E)
Relief valve pressure Return valve pressure Coolant circulation after
shutdown Expansion volume Coolant temperature limit
Control Factors Containment
Subsystem
SE & FM Coolant vapor loss Reduced system pressure
Legend V = Component variability
A = Aging U = Customer usage
E = Environmental SE = Side effects FM = Failure modes