Pitfalls to Avoid When Calculating Performance of Coils With Interlaced Circuiting

Interlaced circuiting of heat exchanger tubes is an effective way to make heat transfer equipment more versatile and adaptable to variable conditions. But, in my experience, industry knowledge of interlaced circuiting’s impact on coil performance at different load conditions is often overly conservative or incomplete. This article will cover the following topics:

• Overview of interlaced circuiting – working principle and intended function
• Conventional industry knowledge of interlaced circuiting’s impact on performance
• Details of coil performance at full and half-circuit operation

Overview of Interlaced Circuiting

Interlaced circuiting, sometimes called intertwined circuiting, is a circuiting pattern in which two or more sets of non-connected tube circuits are interlaced together within a single coil.

The purpose behind this circuit arrangement is to allow better control of a heat exchanger’s output. Unlike coils with a single set of circuits, where your operation options are simply ‘on’ or ‘off,’ interlaced circuitry allows a coil to operate at partial load efficiently.

This helps offset the efficiency losses associated with low-load operation and affords better control over the coil’s output. This design involves multiple headers, the flow to one of which can be turned off for operation during low-load conditions. Doing so effectively removes a portion of the coil’s tubes from operation to meet a temporarily lessened performance requirement. Interlaced circuiting also reduces the frequency of cycling during low-load operation, which can reduce equipment wear and tear.

Oftentimes, interlaced circuits are used to reduce a coil by half, but some configurations – a split-face evaporator with interlaced circuiting, for example – could have four output capabilities.

Conventional Industry Knowledge of Interlaced Circuiting’s Impact on Performance

When calculating the performance of coils with interlaced circuitry, it’s common for engineers to factor in a derate of 1/2 to determine the coil’s output when half of its circuits are in operation. The rationale is understandable – it’s reasonable to assume that 50% less tubes in operation should translate to roughly 50% less performance.

However, in my experience, this method fails to comprehensively account for thermal conduction’s impact inside the coil, leading to overly conservative performance predictions.

Details of Coil Performance at Full and Half-Circuit Operation

As noted above, the reason for the performance discrepancy between the two half-circuit coils has to do with thermal conduction through the coil's fins. With half of the coil’s circuits operating, heat is still transferred out from the coil tubes to the entire fin bundle. It is not easy to calculate fins' efficiency under half-circuit conditions, but it's a key factor when calculating the effective heat transfer area and overall performance of a coil, and failing to accurately do so can lead to conservative estimates.

To show the extent to which this thermal load affects the coil’s performance, we will use the equation Q = h (A₁ + A₂) ΔT where:

• Q = total heat transfer load
• h = overall heat transfer coefficient
• A₁= effective heat transfer area for circuit set #1
• A₂= effective heat transfer area for circuit set #2 
• ΔT= temperature difference between fin-side fluid and tube-side fluid

A₁ = Atube, 1 ⋅ ηt1 + Afin, 1 ⋅ ηƒ1

A2 = Atube, 2 ⋅ ηt2 + Afin, 2 ⋅ ηƒ2

Atube, 1 = tube surface area for circuit set #1

Atube, 2 = tube surface area for circuit set #2

Afin, 1 = fin surface area for circuit set #1

Afin, 2 = fin surface area for circuit set #2

ηt1 = the efficiency of tube surface area for circuit set #1

ηƒ1 = the efficiency of fin surface area for circuit set #1

ηt2 = the efficiency of tube surface area for circuit set #2

ηƒ2 = the efficiency of fin surface area for circuit set #2

When both sets of circuits are operating together: ηt1 = ηt2 = 1, 0 < ηƒ1 = ηƒ2 < 1

When circuit set #1 is operating and circuit set #2 is not operating: ηt1 = 1, 0 < ηt2  < 1, 0 < ηƒ2 < ηƒ1 < 1

Using this formula, we can illustrate that the “50% circuits in operation = 50% output” method is overly conservative. The curve below shows the performance of a 4-row coil with all circuits in operation compared to its performance when half of the coil’s circuits are in use. Some important values have been omitted to preserve trade secrets.

As you can see, the curve illustrates that when half of an interlaced coil’s circuits are in operation, the 50% derate value used by many in the industry is overly conservative. We at SRC have developed a method to more accurately model the performance of coils with interlaced circuitry when half of the circuits are not operating. This method was used to generate the curve above, which we’ve validated through testing in our wind tunnel test lab.

If your system has coils with interlaced circuiting, it’s likely that your performance calculations are overly conservative and opportunities to increase efficiency could exist. If you’re interested in maximizing your coil performance, give us a call. We’d love the opportunity to help you get the most out of your system. We can even test your design in our test laboratories so you can be confident your equipment will perform as designed.

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