 Brinestein - Refrigerator Calculating System
Submitted to Parallax by Larry M. Chambers
Chambers Engineering: Specialized Industrial Controls
I've been in the industrial/commercial refrigeration business for some 30 years and have seen many PC based application for calculating refrigeration systems, but it seems that most authors are impressed with their on ability and lose track of simplicity. Many of these applications are fine for engineers or technicians who understand the refrigeration process but usually it's the salesperson that is put before the customer and rarely do they have the skills to calculate a system properly since their expertise is in sales. I felt that writing a straightforward PC application would be like reinventing the wheel, with so many calculator application on the market, so I decided the fix for this problem would be a handheld device. The "Brinestein" Capacity and Volume  Calculator (I thought the name was fitting) prompts the user for information, such as desired process temperature, evaporator differential temperature (delta T), brine type to be used, evaporator loop volume, etc.. Most of these prompts allow the user to branch to subroutines to help them calculate this information if it is not known. The device will calculate the proper tonnage chiller or chillers required at the given load demand, the estimated volume of the chilled brine circuit, and the volume of brining chemical if required. The calculator produces warning messages if an entry or a calculation may cause problems in the final installation. Values are entered as it would be received from the customer, deration factors and safety margins are automatically calculated and the entered values are adjusted accordingly, relieving the user of this burden. Chiller requirements are returned in "Tons of refrigeration", system volume is returned in "U.S. Gallons" and brine information is returned in either "U.S. Pounds" or "U.S. Gallons" depending on the brining chemical chosen for the calculation (calcium chloride and sodium chloride are dry chemicals, all other choices are liquids).
 About the Circuit:
The circuit is based upon the Basic Stamp BS2p24 microcontroller. Peripheral devices controlled by the BS2p24 include a 4x20 LCD display (Truly MTC-C204DPLY-1N), a 4x4 matrix keypad which was constructed from 16 tactile switches connected in a 4x4 array (I couldn't find a membrane pad that would fit my design), and a 64k serial EEProm (Fairchild NM24C65U ). The BS2p24 interfaces with the keypad using a 16 key encoder (Fairchild MM74C922 ). A 9-pin D-Sub connector is provided on the side of the device for programming and debugging ( the device itself could be used for any number of other applications, personal organizer, any type of calculator, text quiz game, just by changing code).
 About the Code:
The required information for this project is stored in the 64k EEProm and is loaded by running the EEPromWrite******.bsp files before loading the program code (see below). This data includes chiller specs, air handler specs, heat exchanger specs, pipe volume information, and brine data, arranged in "255" delimited lists. The EEProm also stores all the display text (prompts, error messages, etc.) arranged in "0" delimited lists.
The code for this project is broken up into reuseable building blocks, each block is loaded into a separate program slot of the BS2p24. For instance, one block controls  LCD display and keypad entry, another handles all math functions (the program performs 32 bit division for better accuracy), another calculates refrigeration load and so on. Each program slot (except for the math block) begins with identical variable definition tables, this allows values to be passed between program slots with the least amount of code. The math block receives and returns its variable values on the same variable for each math function which allows the values to be mapped to a specific variable in all the other slots. Program slot 0 acts as a traffic cop sending further execution to the proper program slot. |