Xace
Xace designs, rates, and simulates the performance of air coolers and heat recovery bundles. It also simulates air-cooled heat exchangers with the fans turned off. Fully incremental, Xace uses HTRI’s latest pointwise methods.
Methods in Xace are based on extensive research data taken in a wind tunnel, a full-scale air cooler, and a one-third scale model air cooler. Tubeside methods used by Xace are identical to those used by Xist.
Process Specifications
Xace provides multiple tubeside process options and a great deal of flexibility in outside process conditions.
- Air flow rate can be either volumetric or mass, or given as an approach or face velocity. Ambient pressure is specified at an altitude above sea level, air properties allow for humidity, and dehumidification is accounted for in the heat transfer calculations.
- For air-cooled heat exchangers, tubeside fluid can be condensing vapor composed of a single component or a multicomponent mixture with or without noncondensables. Fluid can enter the exchanger as superheated or saturated vapor or as a two-phase mixture and can exit as a two-phase mixture, a saturated liquid, or a subcooled liquid.
- For heat recovery bundles, tubeside fluid can be a pure component or multicomponent mixture boiling liquid. It can enter the reboiler subcooled, saturated, or partially vaporized and can contain nonboiling components and/or noncondensing components.
- A single-phase liquid or gas tubeside stream is allowed in both heat recovery bundles and air-cooled heat exchangers.
- In heat recovery bundles, hot fluid can be on the tube side, permitting the component to rate and simulate air preheater bundles.
- For heat recovery bundles, the outside fluid can be boiling for low-finned and plain tubes or condensing for high-finned, low-finned, and plain tubes.
Geometry Specifications
Xace provides for most air-cooled heat exchanger and heat recovery bundle geometries:
- forced- or induced-draft fans
- A-frame configurations
- air-cooled exchangers with the fans turned off
- one to 99 tuberows
- bundles-in-parallel, bays-in-parallel, bundles-in-series
Although the program does not handle bays-in-series, 99 tuberows per bundle allow direct simulation of most applications. - horizontal, inclined, or vertical tubes
- plain, low-finned, high-finned, continuous-finned, and stud-finned tubes with or without twisted tapes on staggered, inline, or equal-velocity layouts
- up to nine different tube types per bundle
- one to 2376 tubepasses
- split-pass header arrangements
One to 24 tubepasses can be specified in each row of the exchanger bundle. - multiple service air-coolers
Bundles of differing geometry and tubeside process fluids with the airside fluid in parallel can be specified.
Calculation Modes
Xace operates in rating, simulation, and design modes.
Rating
You define exchanger geometry and specify process conditions. Xace calculates expected heat transfer coefficients and pressure drops, compares these calculations to the required heat duty, and reports the difference as under- or overdesign.
Simulation
You define exchanger geometry and specify partial process conditions. Using an iterative procedure in the simulation mode, the program adjusts for omitted process conditions until predicted heat duty matches assumed process conditions. Two simulation options are available: calculating the exit temperatures of both streams, and calculating the flow rate and exit temperature of the cold fluid. The latter calculation option is normally used for air-cooled heat exchangers.
Design
Xace contains two distinct design options: program and user-specified.
In the program (or classic) mode, Xace determines the minimum number of bays in parallel with the smallest width bundles that meet specified process conditions and optional constraints on pressure drop and velocity. Xace uses a unique design procedure that determines optimal face velocities, number of rows in the exchanger, and number of tubepasses. You can enter bundle width constraints, or the program can set them. A cost estimate included in the design mode allows you to select a design other than the smallest based on cost. The design process is interactive, and you have control over the allowed ranges of each geometry parameter.
In the user-specified (or grid) design mode, you have complete control over which geometry is varied and over what range each geometric parameter is varied. The program runs all possible permutations and automatically selects the best available design.
Calculation Methods
The tubeside condensation and boiling methods use HTRI’s latest pointwise calculations for heat transfer and pressure drop. Airside methods reflect current wind tunnel and air-cooled heat exchanger research. With a single input code, you can access HTRI’s experimentally determined f- and j-factor curves reported in the Extended Surfaces Data Book, or you can specify f- and/or j-curves for a specific tube.
Xace employs a three-dimensional incrementation scheme. The exchanger (rows and header geometry) is divided into small increments, each of which contains a single row and one tubepass. Localized methods and fluid properties predict the heat transfer and pressure drop in each increment. These incremental techniques allow accurate prediction of exchangers with complex geometry and hydraulics.
Special Features
- Airside maldistribution profiles for investigating the effects of unequal air distribution, with a special code to access HTRI airside maldistribution profiles directly, simulating the effects of walls adjacent to the heat exchanger, ground clearance, and plenum height
- A calculation option that can predict exchanger performance with total phase separation in the return headers
- Direct links to vendor-supplied fan selection software, providing you with manufacturers’ fan specifications that satisfy heat exchanger requirements
- Special calculation methods that take into account airside pressure drop changes caused by fan guards, louvers, hail screens, ground clearance, and blockage under the fan
- Automatic flow quantity balancing down each tuberow of a bundle with more than one tuberow per tubepass
- A flow-regime map for all two-phase fluids in a horizontal exchanger
- Temperature maldistribution option to investigate effect of non-uniform inlet air temperature distribution
- Radiation calculations for high temperature gray gases