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A vapor recovery unit that performs well in spring can appear to lose efficiency in summer without anything having failed. The cause is usually the ambient temperature itself. Higher ambient temperature raises the temperature of the vapor arriving at the unit, reduces the temperature difference available in the condenser and increases the refrigeration duty needed to reach the same outlet condition. The unit has not degraded; the duty has changed around it.
The three effects of higher ambient temperature
First, vapor generated during transfer is warmer and carries a higher hydrocarbon partial pressure, so the same volume contains more to recover. Second, the condenser has less temperature difference to work with, both because the inlet is warmer and because cooling medium temperatures rise. Third, the refrigeration system itself works harder, with lower efficiency at higher condensing temperatures, so the available cooling capacity is reduced exactly when the demand is highest.
What changes in practice
| Effect | What it does to the unit | What can be adjusted |
|---|---|---|
| Warmer inlet vapor | Raises the load for the same flow | Confirm condenser duty is adequate |
| Smaller temperature difference | Outlet temperature drifts up | Review cooling medium and settings |
| Lower refrigeration efficiency | Less cooling capacity available | Check condenser cooling on the refrigeration side |
| Higher vapor pressure | More hydrocarbon per unit volume | Review duty assumptions for summer |
| Frost behaving differently | Defrost timing may need review | Adjust defrost control to conditions |
| Longer run times | More hours at high duty | Check the unit is not overloaded on peak days |
Distinguishing seasonal behaviour from a fault
The test is whether the effect reverses. If outlet temperature and recovery improve again as ambient temperature falls, the unit is behaving as expected for its design conditions and the question becomes whether the design allows enough margin for the summer period. If performance does not recover when conditions improve, then something has changed in the unit and the usual checks apply.
What can reasonably be done
- Record ambient temperature alongside unit readings so seasonal effects become visible;
- Confirm the condenser duty matches the summer duty, not just the annual average;
- Review cooling medium temperature and confirm it is as low as it can reasonably be;
- Check heat transfer surfaces are clean before the hot period rather than during it;
- Review defrost control so it suits the conditions actually being experienced;
- Revisit the duty assumptions if summer performance is consistently below expectation.
Frequently asked questions
Does this mean the unit was undersized?
Not necessarily. It may mean the original duty assumptions did not reflect the hottest conditions at the site. Revisiting those assumptions is the first step before considering equipment changes.
Would more refrigeration solve it?
Adding cooling capacity addresses one of the three effects but not all of them. Warmer inlet vapor and higher vapor pressure still raise the load, so the duty should be reviewed as a whole before adding capacity.
Should the unit be logged differently in summer?
Yes. Recording ambient temperature alongside the usual readings makes it possible to separate seasonal behaviour from genuine deterioration, which in turn avoids unnecessary maintenance and unnecessary equipment changes.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units, membrane separation skids and condensers for fuel depots, terminals and retail sites. Contact us for duty assessment and equipment selection support.