Friday, April 20, 2007

eQuest Sensible Heat Recovery

Topic: Parametrically simulating sensible heat recovery in eQuest requires specifying a number of items in coordination to obtain meaningful results.

$ This example models a sensible-only, in-line flat plate air-to-air heat
$ exchanger
(FP A2A HX) as the energy recovery equipment. Adjust the supply
$ and returns statics of the main unit to account for the increased pressure
$ drop, if the associated HX circuit not bypassed when off-line.
$ SUPPLY-STATIC = 2.5 (original, assume OSA not bypassed when HX is offline)
$ RETURN-STATIC = 1.6 (original, assume exhaust is bypassed when HX offline)
$ Parametric example, given the above initial statics:
$ Change the following keyword to YES in a parametric run; all other heat
$ recovery parameters are ignored when set to NO for the baseline case:

$ An FP A2A HX is a sensible-only heat recovery device:
$ Specify OA and EXH flows if only a fraction of the exhaust is recovered;
$ partial-recovery example is shown:
ERV-OA-FLOW = 22000
ERV-EXH-FLOW = 20000

$ Check manufactures data for effectiveness rating, efficiencies near 70% are
$ obtainable with FP A2A HX units:

$ Control on the basis of differential temperature between exhaust and OSA; the
$ default deadband is 5°F and as such may be left unspecified:
$ Heat recovery is usually economic in the Pacific NW, OA-HEAT/COOL also an option:
$ FLOAT gives maximum heat recovery but recovers even unwanted heat, so
be sure to use mixed air reset for temperature control:
$ Exhaust bypass control is typical for FP A2A HX:
$ This example uses main unit's fans for heat recovery:
$ Keywords STANDARD and PREMIUM may alternately be specified here, but
$ the motor class parameter is not applicable to an in-line FP A2A HX:
$ Set the ERV OA static to zero, since in this example it is 'unswitched'
$ in series with the main unit; set ERV EXH static to the design pressure drop
$ across the HX (pressure drop IS switched).
$ If left undefined, both will default to 1.5 inches:
$ This setting eliminates condensation and frost simulation warnings:

Generally, air-to-air and passive-refrigerant heat recovery work out favorably as an energy efficiency measure in the Pacific Northwest. Using runaround circuits that introduce operational parasitics for heat recovery often makes the investment unattractive from an energy savings standpoint.

In hot and humid climate, better results may be obtained using a heat (enthalpy) wheel with sensible and latent recovery capability. Sensible-only as shown in the example above is economical in relatively low humidity, cool-summer climates that don't provide much summertime heat recovery opportunity.

To determine if total enthalpy recovery may be attractive, first calculate the theoretical summertime opportunity value:

  • What are the estimated flow rates of the outdoor and exhaust airstreams?
  • What are the outdoor air expected summer average conditions in terms of temperature and humidity (i.e. enthalpy)?
  • What is the expected temperature and humidity of the exhaust airstream?
    • The exhaust airstream in general occupancy applications can be estimated at about 85°F and 35% relative humidity, corresponding to the absolute moisture content at 55°F saturation temperature from a typical cooling coil, plus 2% for people and processes.
  • Now the opportunity value of summertime heat recovery may be estimated:
    • Calculate the enthalpy difference between the outdoor summer average and the estimated exhaust airstream, and multiply by the ratio of the exhaust/OSA flow rates.
Without significant differential enthalpy between outdoor ambient average and the exhaust airstreams, there simply may not be much summertime heat recovery opportunity.

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