My Back Room

This site is intended to provide a (hopefully) organized way to share with others in the commercial refrigeration service trade so they too can benefit from some of what I have accumulated.

I’m adding more content as time permits, but for a while my main focus has been to work on how its display, accessibility and to provide a workable search. Check out the site’s Google Search in the upper RH corner of each page. The Navigation Bar’s dropdown menus will help you drill down to what you’re looking for. There’s even an “old school” table of content page if you prefer.

If you have comments, questions or perhaps you need some help with a problem on the job, please go to the contact page where you can send me an email.   If you have some old manuals, wiring diagrams and other odds & ends, I now have the uploader working so you can send it in and I will put it up.

Whether you’re a tech or an owner in need of help (or a shoulder to cry on), by all means drop me a note and I will gladly assist you in any way I can.  I do this not only because I truly love this business, but also because want to share with you what I have been able to learn in my experience.

There is no charge for this, however if you can afford it, please consider a donation to help offset the recurring costs of running the site.  Any amount would be appreciated.

Thanks for visiting,

How are Filter-Driers Rated for Capacity?

Typically, filter-driers are rated per AHRI standards which require no more than a 1 psi pressure drop at its maximum flow capacity. That is the basis for how they are selected and so it should give you an idea of what is “normal”…ie, 1 psi or less. If the pressure drop exceeds that, it may be time to consider replacing it.

If you pick up a new replacement filter-drier and check the recommended maximum load or tons listed on the back of the box, you may see one rating for air conditioning and another, lower rating for commercial refrigeration.

One additional rating you don’t see on the box is a higher capacity rating for OEM equipment manufacturers…which explains why filter-driers on new equipment are usually quite small in comparison to what you would install in the field.

One reason for this may be because a factory installed filter drier is considered to be installed and a brand-new and therefore clean system. Field replacements on the other hand are installed on systems that have been in service for a long time and also have been opened to conditions less than ideal.

Another reason, of course, is cost.

…and the idea that a refrigeration system should have a cleaner filter-drier than an air conditioning system is debatable in my book. 😉

Evaporator TD and System Balance

This is taken from a past discussion in an HVAC-Talk thread about how to select a walk-in cooler evaporator so it will result in a suitable TD:


Could someone explain to me how you size an evaporator to get a specific TD? I tried to understand the explanation in the Heatcraft Engineering Manual, but I just can’t seem to get it.


1. Start with defining what your box temperature is.

2. Calculate the box load.

3. Determine what RH your product requires.

4. Find the proper coil TD range to achieve that RH from the Heatcraft Engineering Manual.

5. Select a condensing unit that will satisfy the box load at an evap temp equal to the box temp minus the TD.

6. Now…if you want to get real picky about it, you can plot out the system balance curve. This is a plot of Evap temp vs. Btuh which will show how the condensing unit capacity and evaporator capacity relate. Where the plots cross is where your system is balanced.

Clear as Mud?

Definitely muddy…LOL

So let’s say I have a 10,000 BTU condenser at 20° SST and my evaporator is also 10,000 BTU. Box temp of 35°.  That would be a 15° TD correct?  So if I wanted to change the humidity in the box to 90%, which is 7-9° TD, could I just get a larger evaporator or would I have to get a smaller condenser?


Evaporators are typically rated at a 10ºF TD so in your example, the evaporator would have a capacity of 10,000 Btuh at a 10ºF TD or only 7000 Btuh at a 7ºF TD. If the 35ºF box load requires 10,000 then you need a bigger evap for the 7º TD. You need to select a coil with a nominal capacity of 10/7 x 10,000 or about 14,300 Btuh.

Then you need to select a condensing unit that has a capacity of 10,000 Btuh at your design evap temperature of +28ºF. (35º – 7ºF TD = 28ºF). The unit in your example would probably do close to 12,000 Btuh at +28ºF SST, so it’s too big.

Get a condensing unit catalog with the full performance charts and an evaporator catalog to study. Heatcraft’s site will have both.

Here’s a simple graph which demonstrates a basic compressor/evaporator balance:

First plot the condensing unit capacity at various SST points taken straight from the catalog. Then plot the evaporator performance based on its specific capacity (Btuh/ºF TD) for a given design box temperature.

In this example the evaporator has a specific capacity of 900 Btuh/ºF TD, so at a 10ºF TD the SST would be 25ºF and the evap’s capacity would be 9000 Btuh. That’s one point on the graph.  Do the same for 15ºF TD and 5ºF TD and you have a nice straight-line plot.

So it’s clear that where the two lines representing condensing unit and evaporator capacity cross is where the two capacities are the same…ie, that’s where the system will balance.  The plot tells us the balance point is at 25.8°F SST.