Twin Radiators

Need for 2 Radiators

The major reason to go for a 2-radiator design is space constraint. Usually, in commercially sold passenger vehicles, there is sufficient space available for a sufficiently large radiator to be fitted. But in formula student vehicles or in the Racecar applications where space is a major constraint and compact packaging is a prime concern, this issue may arise.

Usually, in Formula Student Vehicles the radiators are placed in the side pods. This is done so as to achieve maximum airflow to the radiator and this also helps to direct the flow over the rear tires so as to reduce the tire induced drag. But usually, the space available in the side is not sufficient to fit a radiator large enough to meet our cooling requirements. Hence we chose to make two smaller radiators.

This also serves another purpose of aerodynamics. Since the radiator is a large porous medium in the path of the flowing air, it disturbs the air flowing to the rear wing. Hence when the radiator is placed only on one side of the vehicle this creates an aerodynamic imbalance between the left and right side of the vehicle and at high-speed corners, this can add on to the yaw momentum of the car. Hence it is advisable to have an asymmetric design about the central axis of the vehicle to avoid an imbalance in the aerodynamic forces of the car.

Now once you have decided to split your radiator requirements and is going to be fulfilled with 2 radiators, now the question arises that should these be connected in series or should the be connected in parallel. The easiest way to explain the two methods of connection is that in series connection the outlet of 1st radiator is connected to the inlet of the 2nd radiator. In the parallel connection, the inlets of both the radiators are connected and similarly, the outlets of the radiators are connected. Now we shall look into both the situations and decide which is suiting our needs.

We should firstly recollect the fundamental that the amount of heat dissipated by the radiator is directionally proportional to the temperature difference or the thermal gradient in the system. Hence larger the temperature difference, more effective the system is.

So moving on we shall analyse both the situations as follows.

To understand the situation, we use hypothetical numbers to visualise the 2 different cases. They are as follows –

Series Connection –

  Inlet Temperature Outlet Temperature
Radiator S1 50 ⁰C 35 ⁰C
Radiator S2 35 ⁰C 30 ⁰C

(Radiator one is dissipating more heat than radiator 2)

Parallel Connection –

  Inlet Temperature Outlet Temperature
Radiator P1 50 ⁰C 50 ⁰C
Radiator P2 30 ⁰C 30 ⁰C

(Both radiators are functioning in a similar manner)

  • Both P1 and P2 are exhausting the same heat. Both will exhaust less heat than Radiator S1 but more than Radiator S2. The net of both systems will be the same.
  • Radiator P1 gives the same temperature drop as Radiator S1 & S2 combined because it has half the flow and half the cooling surface area.
  • The temperature gradient efficiency difference that is made obvious by the less efficient Radiator S2 and more efficient Radiator S1 is present in both P1 & P2 radiators. If we were able to sample the centre of P1 or P2we would get the same temperature as between S1 and S2.

The efficiency of any radiator (heat exchanger) is a function of the temperature difference between the two fluids in question. All else being equal, a heat exchanger with a greater temperature differential will transfer more heat.

Each radiator will have a temperature gradient across it. (Here I’m talking about how much the temperature of each fluid changes as it passes through the heat exchanger.) If you hook them in parallel, each one will be getting 1/N of the flow, but they will all have the same temperature gradient from input to output.

If you hook them in series, all of the flow will be going through all of them, but each one will have only roughly 1/N of the overall temperature difference across it — with the hottest one also having the highest differential because it’s transferring more heat to the other fluid.

Note that you can make this “series or parallel” decision independently for each of the two fluids. There is a total of four different ways you can configure them.

Overall, I don’t think it really makes any practical difference in terms of the thermodynamics. Personally, I would be inclined to hook them in parallel+parallel (i.e., parallel paths for both fluids) — partly because I like that kind of symmetry, and partly because of secondary considerations such as maintenance. When you have a parallel connection with individual shutoff valves, you can repair or replace one radiator without shutting down the system altogether. You can either run at reduced capacity or design the radiator system to have N+1 redundancy in the first place.

Series benefits

  1. The primary benefit of series radiators is that you can guarantee that the flow through each radiator is equal. This is necessary for optimum efficiency. In a parallel system, you can make all hose lengths identical and have all the same fittings (minor losses) for each path, but it is not a guarantee.
  2. The second benefit of the series is that increased flow velocity increases turbulence inside the radiator. It may cause a measurable increase in total heat transfer if the fluid is not as good of thermal conductor like oil.
  3. It takes fewer fittings to plumb radiators in series. This means less install labour and less potential leak locations.

Parallel benefits

  1. Increased flow velocity in the series configuration also increases pressure drop, pumping energy requirements, and heat added to the fluid from this pumping energy input (it all has to go somewhere).
  2. Parallel has the ability to isolate a radiator for service during operation as Dave Tweed mentioned. But this is a minor benefit, because, with a few more plumbing additions, series can be isolated and serviced during operation as well.
  3. It is easier to compare radiator efficiencies when running in parallel. When one radiator has fouled from internal or external contamination it is easy to see that it has less of a differential than the other without doing any math.

Hence we need to balance out the pros and cons of both the arrangements of the 2 radiator configuration before we finalise one of them.

Reference

  1. https://engineering.stackexchange.com/questions/11493/radiators-in-series-or-parallel#:~:text=Each%20radiator%20will%20have%20a,gradient%20from%20input%20to%20output.
  2. http://www.fsae.com/forums/archive/index.php/t-4173.html?s=2f4ac3793225a1cc9fb6f05198249449
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