[BLDG-SIM] DOE2.1E Heat recovery question

[Tom Anderson]

Dominique:

I have a response but note that my response is *speculation*, but here goes.

My guess is that the difference between compressor load and total cooling effect
is actually the difference between evaporator load and total case load per
hour.  When I was in refrigeration school, over two decades ago, I was taught to
size the refrigerated case evaporator so that the daily heat gain can be met by
16 hours of evaporator run time per 24 hours.  The reason for this was to allow
sufficient time for off cycle defrost.  Have not kept up with modern
refrigerated display cases, so do not know if the schooled methodology is still
appropriate.

This brings up an important air conditioning sizing issue for supermarkets.
ASHRAE recommends HVAC designers take into account the heat absorbed by
refrigerated display cases when running heat gain/loss calcs.  Unfortunately,
ASHRAE offers no guidance for imperical hourly heat absorption by the display
case verses the evaporator load.

For a recent supermarket commissioning project, the HVAC designer assumed 16
hr/day evaporator sizing, thus the heat absorbed from the space was 2/3 of the
scheduled evaporated loads.  This was further multiplied by another fudge
factor, since the designer was hesitant to assume net heat absorption to space
as 2/3 of scheduled evaporator load, which seems reasonable to me.

Hope this helps.

     Thomas E. Anderson
     President
     Cx Associates, Ltd.
     Building Commissioning Specialists
     http://www.cx-assoc.com
     933 Road 101
     Jeffersonville, Vermont 05464 USA
     hvac at cx-assoc.com
     Tel: 802-644-5616 Fax: 802-644-6797




Dominique Michaud wrote:

> >From the DOE2.1E Supplement:
>
> REFG-ZONE-LOAD  is the total cooling effect...
>
>                         Manufacturers of supermarket cases usually do not
> list the
>                         total cooling effect of their cases directly.
> Instead, they list the
>                         compressor capacity at a standard suction
> temperature
>                         required per lineal foot of case work. The sensible
> cooling effect
>                         is typically 65% of this number, and the latent
> cooling effect is
>                         about 10%. The total cooling effect is then about
> 75% of the
>                         listed compressor capacity per lineal foot,
> multiplied by the
>                         lineal feet of case work.
>
> Does this mean that 75% of the compressor's cooling capacity cools the zone?
> Where does the other 25% disappear to?
> Do these figures vary depending on ambient conditions?
> Are there any references for these numbers (ASHRAE or otherwise)?
>
> Any and all help is greatly appreciated.
>
> Dominique Michaud
> Building Energy Technician
> Dessau-Soprin Inc.
> 375 blvd Rolland-Therrien, Suite 400
> Longueuil, Quebec, J4H 4A6
> Tel.: (514) 281-1033 ext 2568
> Fax: (450) 442-9996
>
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