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:
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.
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.
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:
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: η